Merge branch 'main' into bayandin/replace-cachepot-with-sccache

## Problem
We use `build-tools` image as a base image to build other images, and it
has a pretty old `libpq-dev` installed (v13; it wasn't that old until I
removed system Postgres 14 from `build-tools` image in
https://github.com/neondatabase/neon/pull/6540)

## Summary of changes
- Remove `libpq-dev` from `build-tools` image
- Set `LD_LIBRARY_PATH` for tests (for different Postgres binaries that
we use, like psql and pgbench)
- Set `PQ_LIB_DIR` to build Storage Controller
- Set `LD_LIBRARY_PATH`/`DYLD_LIBRARY_PATH` in the Storage Controller
where it calls Postgres binaries

.github/actions/run-python-test-set/action.yml

@@ -114,6 +114,7 @@ runs:
         export PLATFORM=${PLATFORM:-github-actions-selfhosted}
         export POSTGRES_DISTRIB_DIR=${POSTGRES_DISTRIB_DIR:-/tmp/neon/pg_install}
         export DEFAULT_PG_VERSION=${PG_VERSION#v}
+        export LD_LIBRARY_PATH=${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/lib
 
         if [ "${BUILD_TYPE}" = "remote" ]; then
           export REMOTE_ENV=1

.github/workflows/benchmarking.yml

@@ -379,6 +379,10 @@ jobs:
 
     - name: Add Postgres binaries to PATH
       run: |
+        LD_LIBRARY_PATH="${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/lib"
+        export LD_LIBRARY_PATH
+        echo "LD_LIBRARY_PATH=${LD_LIBRARY_PATH}" >> $GITHUB_ENV
+
         ${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/bin/pgbench --version
         echo "${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/bin" >> $GITHUB_PATH
 

.github/workflows/build-build-tools-image.yml

@@ -82,6 +82,7 @@ jobs:
           tags: neondatabase/build-tools:${{ inputs.image-tag }}-${{ matrix.arch }}
 
       - name: Remove custom docker config directory
+        if: always()
         run: |
           rm -rf /tmp/.docker-custom
 

.github/workflows/build_and_test.yml

@@ -133,6 +133,11 @@ jobs:
         password: ${{ secrets.NEON_DOCKERHUB_PASSWORD }}
       options: --init
 
+    env:
+      SCCACHE_BUCKET: neon-github-dev
+      SCCACHE_REGION: eu-central-1
+      SCCACHE_S3_KEY_PREFIX: sccache
+
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -193,6 +198,8 @@ jobs:
         if: ${{ !cancelled() }}
         run: cargo deny check --hide-inclusion-graph
 
+      - run: sccache --show-stats
+
   build-neon:
     needs: [ check-permissions, tag, build-build-tools-image ]
     runs-on: [ self-hosted, gen3, large ]
@@ -214,6 +221,9 @@ jobs:
       BUILD_TYPE: ${{ matrix.build_type }}
       GIT_VERSION: ${{ github.event.pull_request.head.sha || github.sha }}
       BUILD_TAG: ${{ needs.tag.outputs.build-tag }}
+      SCCACHE_BUCKET: neon-github-dev
+      SCCACHE_REGION: eu-central-1
+      SCCACHE_S3_KEY_PREFIX: sccache
 
     steps:
       - name: Fix git ownership
@@ -335,8 +345,12 @@ jobs:
 
       - name: Run cargo build
         run: |
+          PQ_LIB_DIR=$(pwd)/pg_install/v16/lib
+          export PQ_LIB_DIR
           ${cov_prefix} mold -run cargo build $CARGO_FLAGS $CARGO_FEATURES --bins --tests
 
+      - run: sccache --show-stats
+
       # Do install *before* running rust tests because they might recompile the
       # binaries with different features/flags.
       - name: Install rust binaries
@@ -383,6 +397,11 @@ jobs:
         env:
           NEXTEST_RETRIES: 3
         run: |
+          PQ_LIB_DIR=$(pwd)/pg_install/v16/lib
+          export PQ_LIB_DIR
+          LD_LIBRARY_PATH=$(pwd)/pg_install/v16/lib
+          export LD_LIBRARY_PATH
+
           #nextest does not yet support running doctests
           cargo test --doc $CARGO_FLAGS $CARGO_FEATURES
 
@@ -405,6 +424,8 @@ jobs:
           export REMOTE_STORAGE_AZURE_REGION="${{ vars.REMOTE_STORAGE_AZURE_REGION }}"
           ${cov_prefix} cargo nextest run $CARGO_FLAGS $CARGO_FEATURES -E 'package(remote_storage)' -E 'test(test_real_azure)'
 
+      - run: sccache --show-stats
+
       - name: Install postgres binaries
         run: cp -a pg_install /tmp/neon/pg_install
 
@@ -758,6 +779,9 @@ jobs:
             GIT_VERSION=${{ github.event.pull_request.head.sha || github.sha }}
             BUILD_TAG=${{ needs.tag.outputs.build-tag }}
             TAG=${{ needs.build-build-tools-image.outputs.image-tag }}
+            RUSTC_WRAPPER=sccache
+            AWS_ACCESS_KEY_ID=${{ secrets.AWS_ACCESS_KEY_DEV }}
+            AWS_SECRET_ACCESS_KEY=${{ secrets.AWS_SECRET_KEY_DEV }}
           provenance: false
           push: true
           pull: true
@@ -850,6 +874,9 @@ jobs:
             PG_VERSION=${{ matrix.version }}
             BUILD_TAG=${{ needs.tag.outputs.build-tag }}
             TAG=${{ needs.build-build-tools-image.outputs.image-tag }}
+            RUSTC_WRAPPER=sccache
+            AWS_ACCESS_KEY_ID=${{ secrets.AWS_ACCESS_KEY_DEV }}
+            AWS_SECRET_ACCESS_KEY=${{ secrets.AWS_SECRET_KEY_DEV }}
           provenance: false
           push: true
           pull: true
@@ -890,6 +917,9 @@ jobs:
             GIT_VERSION=${{ github.event.pull_request.head.sha || github.sha }}
             BUILD_TAG=${{ needs.tag.outputs.build-tag }}
             TAG=${{ needs.build-build-tools-image.outputs.image-tag }}
+            RUSTC_WRAPPER=sccache
+            AWS_ACCESS_KEY_ID=${{ secrets.AWS_ACCESS_KEY_DEV }}
+            AWS_SECRET_ACCESS_KEY=${{ secrets.AWS_SECRET_KEY_DEV }}
           provenance: false
           push: true
           pull: true

.github/workflows/neon_extra_builds.yml

@@ -232,12 +232,19 @@ jobs:
 
       - name: Run cargo build
         run: |
+          PQ_LIB_DIR=$(pwd)/pg_install/v16/lib
+          export PQ_LIB_DIR
           mold -run cargo build --locked $CARGO_FLAGS $CARGO_FEATURES --bins --tests -j$(nproc)
 
       - name: Run cargo test
         env:
           NEXTEST_RETRIES: 3
         run: |
+          PQ_LIB_DIR=$(pwd)/pg_install/v16/lib
+          export PQ_LIB_DIR
+          LD_LIBRARY_PATH=$(pwd)/pg_install/v16/lib
+          export LD_LIBRARY_PATH
+
           cargo nextest run $CARGO_FEATURES -j$(nproc)
 
           # Run separate tests for real S3
@@ -357,7 +364,7 @@ jobs:
 
     env:
       BUILD_TYPE: release
-      # remove the cachepot wrapper and build without crate caches
+      # remove the sccache wrapper and build without crate caches
       RUSTC_WRAPPER: ""
       # build with incremental compilation produce partial results
       # so do not attempt to cache this build, also disable the incremental compilation

Dockerfile

@@ -29,25 +29,25 @@ WORKDIR /home/nonroot
 ARG GIT_VERSION=local
 ARG BUILD_TAG
 
-# Enable https://github.com/paritytech/cachepot to cache Rust crates' compilation results in Docker builds.
-# Set up cachepot to use an AWS S3 bucket for cache results, to reuse it between `docker build` invocations.
-# cachepot falls back to local filesystem if S3 is misconfigured, not failing the build
-ARG RUSTC_WRAPPER=cachepot
-ENV AWS_REGION=eu-central-1
-ENV CACHEPOT_S3_KEY_PREFIX=cachepot
-ARG CACHEPOT_BUCKET=neon-github-dev
-#ARG AWS_ACCESS_KEY_ID
-#ARG AWS_SECRET_ACCESS_KEY
+# Enable https://github.com/mozilla/sccache to cache Rust crates' compilation results in Docker builds.
+# Set up sccache to use an AWS S3 bucket for cache results, to reuse it between `docker build` invocations.
+ARG RUSTC_WRAPPER
+ARG SCCACHE_BUCKET=neon-github-dev
+ARG SCCACHE_REGION=eu-central-1
+ARG SCCACHE_S3_KEY_PREFIX=sccache
+ARG AWS_ACCESS_KEY_ID
+ARG AWS_SECRET_ACCESS_KEY
 
 COPY --from=pg-build /home/nonroot/pg_install/v14/include/postgresql/server pg_install/v14/include/postgresql/server
 COPY --from=pg-build /home/nonroot/pg_install/v15/include/postgresql/server pg_install/v15/include/postgresql/server
 COPY --from=pg-build /home/nonroot/pg_install/v16/include/postgresql/server pg_install/v16/include/postgresql/server
+COPY --from=pg-build /home/nonroot/pg_install/v16/lib                       pg_install/v16/lib
 COPY --chown=nonroot . .
 
 # Show build caching stats to check if it was used in the end.
-# Has to be the part of the same RUN since cachepot daemon is killed in the end of this RUN, losing the compilation stats.
+# Has to be the part of the same RUN since sccache daemon is killed in the end of this RUN, losing the compilation stats.
 RUN set -e \
-    && RUSTFLAGS="-Clinker=clang -Clink-arg=-fuse-ld=mold -Clink-arg=-Wl,--no-rosegment" cargo build  \
+    && PQ_LIB_DIR=$(pwd)/pg_install/v16/lib RUSTFLAGS="-Clinker=clang -Clink-arg=-fuse-ld=mold -Clink-arg=-Wl,--no-rosegment" cargo build \
       --bin pg_sni_router  \
       --bin pageserver  \
       --bin pagectl  \
@@ -57,7 +57,7 @@ RUN set -e \
       --bin proxy  \
       --bin neon_local \
       --locked --release \
-    && cachepot -s
+    && sccache --show-stats
 
 # Build final image
 #

Dockerfile.build-tools

@@ -26,7 +26,6 @@ RUN set -e \
         liblzma-dev \
         libncurses5-dev \
         libncursesw5-dev \
-        libpq-dev \
         libreadline-dev \
         libseccomp-dev \
         libsqlite3-dev \
@@ -184,7 +183,7 @@ RUN curl -sSO https://static.rust-lang.org/rustup/dist/$(uname -m)-unknown-linux
     . "$HOME/.cargo/env" && \
     cargo --version && rustup --version && \
     rustup component add llvm-tools-preview rustfmt clippy && \
-    cargo install --git https://github.com/paritytech/cachepot && \
+    cargo install sccache && \
     cargo install rustfilt && \
     cargo install cargo-hakari && \
     cargo install cargo-deny --locked && \
@@ -192,7 +191,9 @@ RUN curl -sSO https://static.rust-lang.org/rustup/dist/$(uname -m)-unknown-linux
     cargo install cargo-nextest && \
     rm -rf /home/nonroot/.cargo/registry && \
     rm -rf /home/nonroot/.cargo/git
-ENV RUSTC_WRAPPER=cachepot
+ENV RUSTC_WRAPPER=sccache
+# Let sscache to fall back to local filesystem if S3 is misconfigured, not failing the build
+ENV SCCACHE_IGNORE_SERVER_IO_ERROR=1
 
 # Show versions
 RUN whoami \

Dockerfile.compute-node

@@ -868,10 +868,22 @@ FROM $REPOSITORY/$IMAGE:$TAG AS compute-tools
 ARG BUILD_TAG
 ENV BUILD_TAG=$BUILD_TAG
 
+ARG RUSTC_WRAPPER
+ARG SCCACHE_BUCKET=neon-github-dev
+ARG SCCACHE_REGION=eu-central-1
+ARG SCCACHE_S3_KEY_PREFIX=sccache
+ARG AWS_ACCESS_KEY_ID
+ARG AWS_SECRET_ACCESS_KEY
+
 USER nonroot
 # Copy entire project to get Cargo.* files with proper dependencies for the whole project
 COPY --chown=nonroot . .
-RUN cd compute_tools && mold -run cargo build --locked --profile release-line-debug-size-lto
+RUN set -e \
+    && RUSTFLAGS="-Clinker=clang -Clink-arg=-fuse-ld=mold -Clink-arg=-Wl,--no-rosegment" cargo build  \
+      --bin compute_ctl  \
+      --locked \
+      --profile release-line-debug-size-lto \
+    && sccache --show-stats
 
 #########################################################################################
 #

control_plane/src/local_env.rs

@@ -325,11 +325,16 @@ impl LocalEnv {
         }
     }
 
-    pub fn pg_bin_dir(&self, pg_version: u32) -> anyhow::Result<PathBuf> {
-        Ok(self.pg_distrib_dir(pg_version)?.join("bin"))
+    pub fn pg_dir(&self, pg_version: u32, dir_name: &str) -> anyhow::Result<PathBuf> {
+        Ok(self.pg_distrib_dir(pg_version)?.join(dir_name))
     }
+
+    pub fn pg_bin_dir(&self, pg_version: u32) -> anyhow::Result<PathBuf> {
+        self.pg_dir(pg_version, "bin")
+    }
+
     pub fn pg_lib_dir(&self, pg_version: u32) -> anyhow::Result<PathBuf> {
-        Ok(self.pg_distrib_dir(pg_version)?.join("lib"))
+        self.pg_dir(pg_version, "lib")
     }
 
     pub fn pageserver_bin(&self) -> PathBuf {

control_plane/src/storage_controller.rs

@@ -155,16 +155,16 @@ impl StorageController {
         .expect("non-Unicode path")
     }
 
-    /// Find the directory containing postgres binaries, such as `initdb` and `pg_ctl`
+    /// Find the directory containing postgres subdirectories, such `bin` and `lib`
     ///
     /// This usually uses STORAGE_CONTROLLER_POSTGRES_VERSION of postgres, but will fall back
     /// to other versions if that one isn't found.  Some automated tests create circumstances
     /// where only one version is available in pg_distrib_dir, such as `test_remote_extensions`.
-    pub async fn get_pg_bin_dir(&self) -> anyhow::Result<Utf8PathBuf> {
+    async fn get_pg_dir(&self, dir_name: &str) -> anyhow::Result<Utf8PathBuf> {
         let prefer_versions = [STORAGE_CONTROLLER_POSTGRES_VERSION, 15, 14];
 
         for v in prefer_versions {
-            let path = Utf8PathBuf::from_path_buf(self.env.pg_bin_dir(v)?).unwrap();
+            let path = Utf8PathBuf::from_path_buf(self.env.pg_dir(v, dir_name)?).unwrap();
             if tokio::fs::try_exists(&path).await? {
                 return Ok(path);
             }
@@ -172,11 +172,20 @@ impl StorageController {
 
         // Fall through
         anyhow::bail!(
-            "Postgres binaries not found in {}",
-            self.env.pg_distrib_dir.display()
+            "Postgres directory '{}' not found in {}",
+            dir_name,
+            self.env.pg_distrib_dir.display(),
         );
     }
 
+    pub async fn get_pg_bin_dir(&self) -> anyhow::Result<Utf8PathBuf> {
+        self.get_pg_dir("bin").await
+    }
+
+    pub async fn get_pg_lib_dir(&self) -> anyhow::Result<Utf8PathBuf> {
+        self.get_pg_dir("lib").await
+    }
+
     /// Readiness check for our postgres process
     async fn pg_isready(&self, pg_bin_dir: &Utf8Path) -> anyhow::Result<bool> {
         let bin_path = pg_bin_dir.join("pg_isready");
@@ -229,12 +238,17 @@ impl StorageController {
             .unwrap()
             .join("storage_controller_db");
         let pg_bin_dir = self.get_pg_bin_dir().await?;
+        let pg_lib_dir = self.get_pg_lib_dir().await?;
         let pg_log_path = pg_data_path.join("postgres.log");
 
         if !tokio::fs::try_exists(&pg_data_path).await? {
             // Initialize empty database
             let initdb_path = pg_bin_dir.join("initdb");
             let mut child = Command::new(&initdb_path)
+                .envs(vec![
+                    ("LD_LIBRARY_PATH".to_owned(), pg_lib_dir.to_string()),
+                    ("DYLD_LIBRARY_PATH".to_owned(), pg_lib_dir.to_string()),
+                ])
                 .args(["-D", pg_data_path.as_ref()])
                 .spawn()
                 .expect("Failed to spawn initdb");
@@ -269,7 +283,10 @@ impl StorageController {
             &self.env.base_data_dir,
             pg_bin_dir.join("pg_ctl").as_std_path(),
             db_start_args,
-            [],
+            vec![
+                ("LD_LIBRARY_PATH".to_owned(), pg_lib_dir.to_string()),
+                ("DYLD_LIBRARY_PATH".to_owned(), pg_lib_dir.to_string()),
+            ],
             background_process::InitialPidFile::Create(self.postgres_pid_file()),
             retry_timeout,
             || self.pg_isready(&pg_bin_dir),
@@ -324,7 +341,10 @@ impl StorageController {
             &self.env.base_data_dir,
             &self.env.storage_controller_bin(),
             args,
-            [],
+            vec![
+                ("LD_LIBRARY_PATH".to_owned(), pg_lib_dir.to_string()),
+                ("DYLD_LIBRARY_PATH".to_owned(), pg_lib_dir.to_string()),
+            ],
             background_process::InitialPidFile::Create(self.pid_file()),
             retry_timeout,
             || async {

libs/postgres_ffi/src/xlog_utils.rs

@@ -356,6 +356,28 @@ impl CheckPoint {
         }
         false
     }
+
+    /// Advance next multi-XID/offset to those given in arguments.
+    ///
+    /// It's important that this handles wraparound correctly. This should match the
+    /// MultiXactAdvanceNextMXact() logic in PostgreSQL's xlog_redo() function.
+    ///
+    /// Returns 'true' if the Checkpoint was updated.
+    pub fn update_next_multixid(&mut self, multi_xid: u32, multi_offset: u32) -> bool {
+        let mut modified = false;
+
+        if multi_xid.wrapping_sub(self.nextMulti) as i32 > 0 {
+            self.nextMulti = multi_xid;
+            modified = true;
+        }
+
+        if multi_offset.wrapping_sub(self.nextMultiOffset) as i32 > 0 {
+            self.nextMultiOffset = multi_offset;
+            modified = true;
+        }
+
+        modified
+    }
 }
 
 /// Generate new, empty WAL segment, with correct block headers at the first

libs/postgres_ffi/wal_craft/src/xlog_utils_test.rs

@@ -202,6 +202,53 @@ pub fn test_update_next_xid() {
     assert_eq!(checkpoint.nextXid.value, 2048);
 }
 
+#[test]
+pub fn test_update_next_multixid() {
+    let checkpoint_buf = [0u8; std::mem::size_of::<CheckPoint>()];
+    let mut checkpoint = CheckPoint::decode(&checkpoint_buf).unwrap();
+
+    // simple case
+    checkpoint.nextMulti = 20;
+    checkpoint.nextMultiOffset = 20;
+    checkpoint.update_next_multixid(1000, 2000);
+    assert_eq!(checkpoint.nextMulti, 1000);
+    assert_eq!(checkpoint.nextMultiOffset, 2000);
+
+    // No change
+    checkpoint.update_next_multixid(500, 900);
+    assert_eq!(checkpoint.nextMulti, 1000);
+    assert_eq!(checkpoint.nextMultiOffset, 2000);
+
+    // Close to wraparound, but not wrapped around yet
+    checkpoint.nextMulti = 0xffff0000;
+    checkpoint.nextMultiOffset = 0xfffe0000;
+    checkpoint.update_next_multixid(0xffff00ff, 0xfffe00ff);
+    assert_eq!(checkpoint.nextMulti, 0xffff00ff);
+    assert_eq!(checkpoint.nextMultiOffset, 0xfffe00ff);
+
+    // Wraparound
+    checkpoint.update_next_multixid(1, 900);
+    assert_eq!(checkpoint.nextMulti, 1);
+    assert_eq!(checkpoint.nextMultiOffset, 900);
+
+    // Wraparound nextMulti to 0.
+    //
+    // It's a bit surprising that nextMulti can be 0, because that's a special value
+    // (InvalidMultiXactId). However, that's how Postgres does it at multi-xid wraparound:
+    // nextMulti wraps around to 0, but then when the next multi-xid is assigned, it skips
+    // the 0 and the next multi-xid actually assigned is 1.
+    checkpoint.nextMulti = 0xffff0000;
+    checkpoint.nextMultiOffset = 0xfffe0000;
+    checkpoint.update_next_multixid(0, 0xfffe00ff);
+    assert_eq!(checkpoint.nextMulti, 0);
+    assert_eq!(checkpoint.nextMultiOffset, 0xfffe00ff);
+
+    // Wraparound nextMultiOffset to 0
+    checkpoint.update_next_multixid(0, 0);
+    assert_eq!(checkpoint.nextMulti, 0);
+    assert_eq!(checkpoint.nextMultiOffset, 0);
+}
+
 #[test]
 pub fn test_encode_logical_message() {
     let expected = [

libs/utils/src/lib.rs

@@ -124,9 +124,9 @@ pub mod poison;
 ///
 /// #############################################################################################
 /// TODO this macro is not the way the library is intended to be used, see <https://github.com/neondatabase/neon/issues/1565> for details.
-/// We use `cachepot` to reduce our current CI build times: <https://github.com/neondatabase/cloud/pull/1033#issuecomment-1100935036>
+/// We use `sccache` to reduce our current CI build times: <https://github.com/neondatabase/cloud/pull/1033#issuecomment-1100935036>
 /// Yet, it seems to ignore the GIT_VERSION env variable, passed to Docker build, even with build.rs that contains
-/// `println!("cargo:rerun-if-env-changed=GIT_VERSION");` code for cachepot cache invalidation.
+/// `println!("cargo:rerun-if-env-changed=GIT_VERSION");` code for sccache cache invalidation.
 /// The problem needs further investigation and regular `const` declaration instead of a macro.
 #[macro_export]
 macro_rules! project_git_version {

pageserver/src/metrics.rs

@@ -476,7 +476,7 @@ static STANDBY_HORIZON: Lazy<IntGaugeVec> = Lazy::new(|| {
 static RESIDENT_PHYSICAL_SIZE: Lazy<UIntGaugeVec> = Lazy::new(|| {
     register_uint_gauge_vec!(
         "pageserver_resident_physical_size",
-        "The size of the layer files present in the pageserver's filesystem.",
+        "The size of the layer files present in the pageserver's filesystem, for attached locations.",
         &["tenant_id", "shard_id", "timeline_id"]
     )
     .expect("failed to define a metric")
@@ -1691,6 +1691,15 @@ pub(crate) static SECONDARY_MODE: Lazy<SecondaryModeMetrics> = Lazy::new(|| {
 }
 });
 
+pub(crate) static SECONDARY_RESIDENT_PHYSICAL_SIZE: Lazy<UIntGaugeVec> = Lazy::new(|| {
+    register_uint_gauge_vec!(
+        "pageserver_secondary_resident_physical_size",
+        "The size of the layer files present in the pageserver's filesystem, for secondary locations.",
+        &["tenant_id", "shard_id"]
+    )
+    .expect("failed to define a metric")
+});
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum RemoteOpKind {
     Upload,

pageserver/src/tenant/secondary.rs

@@ -23,6 +23,8 @@ use super::{
     storage_layer::LayerName,
 };
 
+use crate::metrics::SECONDARY_RESIDENT_PHYSICAL_SIZE;
+use metrics::UIntGauge;
 use pageserver_api::{
     models,
     shard::{ShardIdentity, TenantShardId},
@@ -99,6 +101,17 @@ pub(crate) struct SecondaryTenant {
 
     // Public state indicating overall progress of downloads relative to the last heatmap seen
     pub(crate) progress: std::sync::Mutex<models::SecondaryProgress>,
+
+    // Sum of layer sizes on local disk
+    pub(super) resident_size_metric: UIntGauge,
+}
+
+impl Drop for SecondaryTenant {
+    fn drop(&mut self) {
+        let tenant_id = self.tenant_shard_id.tenant_id.to_string();
+        let shard_id = format!("{}", self.tenant_shard_id.shard_slug());
+        let _ = SECONDARY_RESIDENT_PHYSICAL_SIZE.remove_label_values(&[&tenant_id, &shard_id]);
+    }
 }
 
 impl SecondaryTenant {
@@ -108,6 +121,12 @@ impl SecondaryTenant {
         tenant_conf: TenantConfOpt,
         config: &SecondaryLocationConfig,
     ) -> Arc<Self> {
+        let tenant_id = tenant_shard_id.tenant_id.to_string();
+        let shard_id = format!("{}", tenant_shard_id.shard_slug());
+        let resident_size_metric = SECONDARY_RESIDENT_PHYSICAL_SIZE
+            .get_metric_with_label_values(&[&tenant_id, &shard_id])
+            .unwrap();
+
         Arc::new(Self {
             tenant_shard_id,
             // todo: shall we make this a descendent of the
@@ -123,6 +142,8 @@ impl SecondaryTenant {
             detail: std::sync::Mutex::new(SecondaryDetail::new(config.clone())),
 
             progress: std::sync::Mutex::default(),
+
+            resident_size_metric,
         })
     }
 
@@ -211,16 +232,12 @@ impl SecondaryTenant {
             // have to 100% match what is on disk, because it's a best-effort warming
             // of the cache.
             let mut detail = this.detail.lock().unwrap();
-            if let Some(timeline_detail) = detail.timelines.get_mut(&timeline_id) {
-                let removed = timeline_detail.on_disk_layers.remove(&name);
-
-                // We might race with removal of the same layer during downloads, if it was removed
-                // from the heatmap.  If we see that the OnDiskState is gone, then no need to
-                // do a physical deletion or store in evicted_at.
-                if let Some(removed) = removed {
-                    removed.remove_blocking();
-                    timeline_detail.evicted_at.insert(name, now);
-                }
+            if let Some(removed) =
+                detail.evict_layer(name, &timeline_id, now, &this.resident_size_metric)
+            {
+                // We might race with removal of the same layer during downloads, so finding the layer we
+                // were trying to remove is optional.  Only issue the disk I/O to remove it if we found it.
+                removed.remove_blocking();
             }
         })
         .await

pageserver/src/tenant/secondary/downloader.rs

@@ -46,6 +46,7 @@ use crate::tenant::{
 use camino::Utf8PathBuf;
 use chrono::format::{DelayedFormat, StrftimeItems};
 use futures::Future;
+use metrics::UIntGauge;
 use pageserver_api::models::SecondaryProgress;
 use pageserver_api::shard::TenantShardId;
 use remote_storage::{DownloadError, Etag, GenericRemoteStorage};
@@ -131,16 +132,66 @@ impl OnDiskState {
             .or_else(fs_ext::ignore_not_found)
             .fatal_err("Deleting secondary layer")
     }
+
+    pub(crate) fn file_size(&self) -> u64 {
+        self.metadata.file_size
+    }
 }
 
 #[derive(Debug, Clone, Default)]
 pub(super) struct SecondaryDetailTimeline {
-    pub(super) on_disk_layers: HashMap<LayerName, OnDiskState>,
+    on_disk_layers: HashMap<LayerName, OnDiskState>,
 
     /// We remember when layers were evicted, to prevent re-downloading them.
     pub(super) evicted_at: HashMap<LayerName, SystemTime>,
 }
 
+impl SecondaryDetailTimeline {
+    pub(super) fn remove_layer(
+        &mut self,
+        name: &LayerName,
+        resident_metric: &UIntGauge,
+    ) -> Option<OnDiskState> {
+        let removed = self.on_disk_layers.remove(name);
+        if let Some(removed) = &removed {
+            resident_metric.sub(removed.file_size());
+        }
+        removed
+    }
+
+    /// `local_path`
+    fn touch_layer<F>(
+        &mut self,
+        conf: &'static PageServerConf,
+        tenant_shard_id: &TenantShardId,
+        timeline_id: &TimelineId,
+        touched: &HeatMapLayer,
+        resident_metric: &UIntGauge,
+        local_path: F,
+    ) where
+        F: FnOnce() -> Utf8PathBuf,
+    {
+        use std::collections::hash_map::Entry;
+        match self.on_disk_layers.entry(touched.name.clone()) {
+            Entry::Occupied(mut v) => {
+                v.get_mut().access_time = touched.access_time;
+            }
+            Entry::Vacant(e) => {
+                e.insert(OnDiskState::new(
+                    conf,
+                    tenant_shard_id,
+                    timeline_id,
+                    touched.name.clone(),
+                    touched.metadata.clone(),
+                    touched.access_time,
+                    local_path(),
+                ));
+                resident_metric.add(touched.metadata.file_size);
+            }
+        }
+    }
+}
+
 // Aspects of a heatmap that we remember after downloading it
 #[derive(Clone, Debug)]
 struct DownloadSummary {
@@ -158,7 +209,7 @@ pub(super) struct SecondaryDetail {
 
     last_download: Option<DownloadSummary>,
     next_download: Option<Instant>,
-    pub(super) timelines: HashMap<TimelineId, SecondaryDetailTimeline>,
+    timelines: HashMap<TimelineId, SecondaryDetailTimeline>,
 }
 
 /// Helper for logging SystemTime
@@ -191,6 +242,38 @@ impl SecondaryDetail {
         }
     }
 
+    pub(super) fn evict_layer(
+        &mut self,
+        name: LayerName,
+        timeline_id: &TimelineId,
+        now: SystemTime,
+        resident_metric: &UIntGauge,
+    ) -> Option<OnDiskState> {
+        let timeline = self.timelines.get_mut(timeline_id)?;
+        let removed = timeline.remove_layer(&name, resident_metric);
+        if removed.is_some() {
+            timeline.evicted_at.insert(name, now);
+        }
+        removed
+    }
+
+    pub(super) fn remove_timeline(
+        &mut self,
+        timeline_id: &TimelineId,
+        resident_metric: &UIntGauge,
+    ) {
+        let removed = self.timelines.remove(timeline_id);
+        if let Some(removed) = removed {
+            resident_metric.sub(
+                removed
+                    .on_disk_layers
+                    .values()
+                    .map(|l| l.metadata.file_size)
+                    .sum(),
+            );
+        }
+    }
+
     /// Additionally returns the total number of layers, used for more stable relative access time
     /// based eviction.
     pub(super) fn get_layers_for_eviction(
@@ -601,8 +684,13 @@ impl<'a> TenantDownloader<'a> {
                 Some(t) => t,
                 None => {
                     // We have no existing state: need to scan local disk for layers first.
-                    let timeline_state =
-                        init_timeline_state(self.conf, tenant_shard_id, timeline).await;
+                    let timeline_state = init_timeline_state(
+                        self.conf,
+                        tenant_shard_id,
+                        timeline,
+                        &self.secondary_state.resident_size_metric,
+                    )
+                    .await;
 
                     // Re-acquire detail lock now that we're done with async load from local FS
                     self.secondary_state
@@ -671,6 +759,25 @@ impl<'a> TenantDownloader<'a> {
                 .await?;
         }
 
+        // Metrics consistency check in testing builds
+        if cfg!(feature = "testing") {
+            let detail = self.secondary_state.detail.lock().unwrap();
+            let resident_size = detail
+                .timelines
+                .values()
+                .map(|tl| {
+                    tl.on_disk_layers
+                        .values()
+                        .map(|v| v.metadata.file_size)
+                        .sum::<u64>()
+                })
+                .sum::<u64>();
+            assert_eq!(
+                resident_size,
+                self.secondary_state.resident_size_metric.get()
+            );
+        }
+
         // Only update last_etag after a full successful download: this way will not skip
         // the next download, even if the heatmap's actual etag is unchanged.
         self.secondary_state.detail.lock().unwrap().last_download = Some(DownloadSummary {
@@ -783,7 +890,7 @@ impl<'a> TenantDownloader<'a> {
             for delete_timeline in &delete_timelines {
                 // We haven't removed from disk yet, but optimistically remove from in-memory state: if removal
                 // from disk fails that will be a fatal error.
-                detail.timelines.remove(delete_timeline);
+                detail.remove_timeline(delete_timeline, &self.secondary_state.resident_size_metric);
             }
         }
 
@@ -801,7 +908,7 @@ impl<'a> TenantDownloader<'a> {
             let Some(timeline_state) = detail.timelines.get_mut(&timeline_id) else {
                 continue;
             };
-            timeline_state.on_disk_layers.remove(&layer_name);
+            timeline_state.remove_layer(&layer_name, &self.secondary_state.resident_size_metric);
         }
 
         for timeline_id in delete_timelines {
@@ -1000,33 +1107,24 @@ impl<'a> TenantDownloader<'a> {
             let timeline_detail = detail.timelines.entry(timeline_id).or_default();
 
             tracing::info!("Wrote timeline_detail for {} touched layers", touched.len());
-
-            for t in touched {
-                use std::collections::hash_map::Entry;
-                match timeline_detail.on_disk_layers.entry(t.name.clone()) {
-                    Entry::Occupied(mut v) => {
-                        v.get_mut().access_time = t.access_time;
-                    }
-                    Entry::Vacant(e) => {
-                        let local_path = local_layer_path(
+            touched.into_iter().for_each(|t| {
+                timeline_detail.touch_layer(
+                    self.conf,
+                    tenant_shard_id,
+                    &timeline_id,
+                    &t,
+                    &self.secondary_state.resident_size_metric,
+                    || {
+                        local_layer_path(
                             self.conf,
                             tenant_shard_id,
                             &timeline_id,
                             &t.name,
                             &t.metadata.generation,
-                        );
-                        e.insert(OnDiskState::new(
-                            self.conf,
-                            tenant_shard_id,
-                            &timeline_id,
-                            t.name,
-                            t.metadata.clone(),
-                            t.access_time,
-                            local_path,
-                        ));
-                    }
-                }
-            }
+                        )
+                    },
+                )
+            });
         }
 
         result
@@ -1135,6 +1233,7 @@ async fn init_timeline_state(
     conf: &'static PageServerConf,
     tenant_shard_id: &TenantShardId,
     heatmap: &HeatMapTimeline,
+    resident_metric: &UIntGauge,
 ) -> SecondaryDetailTimeline {
     let timeline_path = conf.timeline_path(tenant_shard_id, &heatmap.timeline_id);
     let mut detail = SecondaryDetailTimeline::default();
@@ -1210,17 +1309,13 @@ async fn init_timeline_state(
                         } else {
                             // We expect the access time to be initialized immediately afterwards, when
                             // the latest heatmap is applied to the state.
-                            detail.on_disk_layers.insert(
-                                name.clone(),
-                                OnDiskState::new(
-                                    conf,
-                                    tenant_shard_id,
-                                    &heatmap.timeline_id,
-                                    name,
-                                    remote_meta.metadata.clone(),
-                                    remote_meta.access_time,
-                                    file_path,
-                                ),
+                            detail.touch_layer(
+                                conf,
+                                tenant_shard_id,
+                                &heatmap.timeline_id,
+                                remote_meta,
+                                resident_metric,
+                                || file_path,
                             );
                         }
                     }

pageserver/src/walingest.rs

@@ -343,7 +343,33 @@ impl WalIngest {
                         xlog_checkpoint.oldestActiveXid,
                         self.checkpoint.oldestActiveXid
                     );
-                    self.checkpoint.oldestActiveXid = xlog_checkpoint.oldestActiveXid;
+
+                    // A shutdown checkpoint has `oldestActiveXid == InvalidTransactionid`,
+                    // because at shutdown, all in-progress transactions will implicitly
+                    // end. Postgres startup code knows that, and allows hot standby to start
+                    // immediately from a shutdown checkpoint.
+                    //
+                    // In Neon, Postgres hot standby startup always behaves as if starting from
+                    // an online checkpoint. It needs a valid `oldestActiveXid` value, so
+                    // instead of overwriting self.checkpoint.oldestActiveXid with
+                    // InvalidTransactionid from the checkpoint WAL record, update it to a
+                    // proper value, knowing that there are no in-progress transactions at this
+                    // point, except for prepared transactions.
+                    //
+                    // See also the neon code changes in the InitWalRecovery() function.
+                    if xlog_checkpoint.oldestActiveXid == pg_constants::INVALID_TRANSACTION_ID
+                        && info == pg_constants::XLOG_CHECKPOINT_SHUTDOWN
+                    {
+                        let mut oldest_active_xid = self.checkpoint.nextXid.value as u32;
+                        for xid in modification.tline.list_twophase_files(lsn, ctx).await? {
+                            if (xid.wrapping_sub(oldest_active_xid) as i32) < 0 {
+                                oldest_active_xid = xid;
+                            }
+                        }
+                        self.checkpoint.oldestActiveXid = oldest_active_xid;
+                    } else {
+                        self.checkpoint.oldestActiveXid = xlog_checkpoint.oldestActiveXid;
+                    }
 
                     // Write a new checkpoint key-value pair on every checkpoint record, even
                     // if nothing really changed. Not strictly required, but it seems nice to
@@ -375,6 +401,7 @@ impl WalIngest {
                 if info == pg_constants::XLOG_RUNNING_XACTS {
                     let xlrec = crate::walrecord::XlRunningXacts::decode(&mut buf);
                     self.checkpoint.oldestActiveXid = xlrec.oldest_running_xid;
+                    self.checkpoint_modified = true;
                 }
             }
             pg_constants::RM_REPLORIGIN_ID => {
@@ -1277,13 +1304,10 @@ impl WalIngest {
             xlrec.pageno, xlrec.oldest_xid, xlrec.oldest_xid_db
         );
 
-        // Here we treat oldestXid and oldestXidDB
-        // differently from postgres redo routines.
-        // In postgres checkpoint.oldestXid lags behind xlrec.oldest_xid
-        // until checkpoint happens and updates the value.
-        // Here we can use the most recent value.
-        // It's just an optimization, though and can be deleted.
-        // TODO Figure out if there will be any issues with replica.
+        // In Postgres, oldestXid and oldestXidDB are updated in memory when the CLOG is
+        // truncated, but a checkpoint record with the updated values isn't written until
+        // later. In Neon, a server can start at any LSN, not just on a checkpoint record,
+        // so we keep the oldestXid and oldestXidDB up-to-date.
         self.checkpoint.oldestXid = xlrec.oldest_xid;
         self.checkpoint.oldestXidDB = xlrec.oldest_xid_db;
         self.checkpoint_modified = true;
@@ -1384,14 +1408,31 @@ impl WalIngest {
             // Note: The multixact members can wrap around, even within one WAL record.
             offset = offset.wrapping_add(n_this_page as u32);
         }
-        if xlrec.mid >= self.checkpoint.nextMulti {
-            self.checkpoint.nextMulti = xlrec.mid + 1;
-            self.checkpoint_modified = true;
-        }
-        if xlrec.moff + xlrec.nmembers > self.checkpoint.nextMultiOffset {
-            self.checkpoint.nextMultiOffset = xlrec.moff + xlrec.nmembers;
+        let next_offset = offset;
+        assert!(xlrec.moff.wrapping_add(xlrec.nmembers) == next_offset);
+
+        // Update next-multi-xid and next-offset
+        //
+        // NB: In PostgreSQL, the next-multi-xid stored in the control file is allowed to
+        // go to 0, and it's fixed up by skipping to FirstMultiXactId in functions that
+        // read it, like GetNewMultiXactId(). This is different from how nextXid is
+        // incremented! nextXid skips over < FirstNormalTransactionId when the the value
+        // is stored, so it's never 0 in a checkpoint.
+        //
+        // I don't know why it's done that way, it seems less error-prone to skip over 0
+        // when the value is stored rather than when it's read. But let's do it the same
+        // way here.
+        let next_multi_xid = xlrec.mid.wrapping_add(1);
+
+        if self
+            .checkpoint
+            .update_next_multixid(next_multi_xid, next_offset)
+        {
             self.checkpoint_modified = true;
         }
+
+        // Also update the next-xid with the highest member. According to the comments in
+        // multixact_redo(), this shouldn't be necessary, but let's do the same here.
         let max_mbr_xid = xlrec.members.iter().fold(None, |acc, mbr| {
             if let Some(max_xid) = acc {
                 if mbr.xid.wrapping_sub(max_xid) as i32 > 0 {

pgxn/neon/neon.c

@@ -12,6 +12,8 @@
 #include "fmgr.h"
 
 #include "miscadmin.h"
+#include "access/subtrans.h"
+#include "access/twophase.h"
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "storage/buf_internals.h"
@@ -22,10 +24,12 @@
 #include "replication/logical.h"
 #include "replication/slot.h"
 #include "replication/walsender.h"
+#include "storage/proc.h"
 #include "storage/procsignal.h"
 #include "tcop/tcopprot.h"
 #include "funcapi.h"
 #include "access/htup_details.h"
+#include "utils/builtins.h"
 #include "utils/pg_lsn.h"
 #include "utils/guc.h"
 #include "utils/wait_event.h"
@@ -266,6 +270,293 @@ LogicalSlotsMonitorMain(Datum main_arg)
 	}
 }
 
+/*
+ * XXX: These private to procarray.c, but we need them here.
+ */
+#define PROCARRAY_MAXPROCS	(MaxBackends + max_prepared_xacts)
+#define TOTAL_MAX_CACHED_SUBXIDS \
+	((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
+
+/*
+ * Restore running-xact information by scanning the CLOG at startup.
+ *
+ * In PostgreSQL, a standby always has to wait for a running-xacts WAL record
+ * to arrive before it can start accepting queries. Furthermore, if there are
+ * transactions with too many subxids (> 64) open to fit in the in-memory
+ * subxids cache, the running-xacts record will be marked as "suboverflowed",
+ * and the standby will need to also wait for the currently in-progress
+ * transactions to finish.
+ *
+ * That's not great in PostgreSQL, because a hot standby does not necessary
+ * open up for queries immediately as you might expect. But it's worse in
+ * Neon: A standby in Neon doesn't need to start WAL replay from a checkpoint
+ * record; it can start at any LSN. Postgres arranges things so that there is
+ * a running-xacts record soon after every checkpoint record, but when you
+ * start from an arbitrary LSN, that doesn't help. If the primary is idle, or
+ * not running at all, it might never write a new running-xacts record,
+ * leaving the replica in a limbo where it can never start accepting queries.
+ *
+ * To mitigate that, we have an additional mechanism to find the running-xacts
+ * information: we scan the CLOG, making note of any XIDs not marked as
+ * committed or aborted. They are added to the Postgres known-assigned XIDs
+ * array by calling ProcArrayApplyRecoveryInfo() in the caller of this
+ * function.
+ *
+ * There is one big limitation with that mechanism: The size of the
+ * known-assigned XIDs is limited, so if there are a lot of in-progress XIDs,
+ * we have to give up. Furthermore, we don't know how many of the in-progress
+ * XIDs are subtransactions, and if we use up all the space in the
+ * known-assigned XIDs array for subtransactions, we might run out of space in
+ * the array later during WAL replay, causing the replica to shut down with
+ * "ERROR: too many KnownAssignedXids". The safe # of XIDs that we can add to
+ * the known-assigned array without risking that error later is very low,
+ * merely PGPROC_MAX_CACHED_SUBXIDS == 64, so we take our chances and use up
+ * to half of the known-assigned XIDs array for the subtransactions, even
+ * though that risks getting the error later.
+ *
+ * Note: It's OK if the recovered list of XIDs includes some transactions that
+ * have crashed in the primary, and hence will never commit. They will be seen
+ * as in-progress, until we see a new next running-acts record with an
+ * oldestActiveXid that invalidates them. That's how the known-assigned XIDs
+ * array always works.
+ *
+ * If scraping the CLOG doesn't succeed for some reason, like the subxid
+ * overflow, Postgres will fall back to waiting for a running-xacts record
+ * like usual.
+ *
+ * Returns true if a complete list of in-progress XIDs was scraped.
+ */
+static bool
+RestoreRunningXactsFromClog(CheckPoint *checkpoint, TransactionId **xids, int *nxids)
+{
+	TransactionId from;
+	TransactionId till;
+	int			max_xcnt;
+	TransactionId *prepared_xids = NULL;
+	int			n_prepared_xids;
+	TransactionId *restored_xids = NULL;
+	int			n_restored_xids;
+	int			next_prepared_idx;
+
+	Assert(*xids == NULL);
+
+	/*
+	 * If the checkpoint doesn't have a valid oldestActiveXid, bail out. We
+	 * don't know where to start the scan.
+	 *
+	 * This shouldn't happen, because the pageserver always maintains a valid
+	 * oldestActiveXid nowadays. Except when starting at an old point in time
+	 * that was ingested before the pageserver was taught to do that.
+	 */
+	if (!TransactionIdIsValid(checkpoint->oldestActiveXid))
+	{
+		elog(LOG, "cannot restore running-xacts from CLOG because oldestActiveXid is not set");
+		goto fail;
+	}
+
+	/*
+	 * We will scan the CLOG starting from the oldest active XID.
+	 *
+	 * In some corner cases, the oldestActiveXid from the last checkpoint
+	 * might already have been truncated from the CLOG. That is,
+	 * oldestActiveXid might be older than oldestXid. That's possible because
+	 * oldestActiveXid is only updated at checkpoints. After the last
+	 * checkpoint, the oldest transaction might have committed, and the CLOG
+	 * might also have been already truncated. So if oldestActiveXid is older
+	 * than oldestXid, start at oldestXid instead. (Otherwise we'd try to
+	 * access CLOG segments that have already been truncated away.)
+	 */
+	from = TransactionIdPrecedes(checkpoint->oldestXid, checkpoint->oldestActiveXid)
+		? checkpoint->oldestActiveXid : checkpoint->oldestXid;
+	till = XidFromFullTransactionId(checkpoint->nextXid);
+
+	/*
+	 * To avoid "too many KnownAssignedXids" error later during replay, we
+	 * limit number of collected transactions. This is a tradeoff: if we are
+	 * willing to consume more of the KnownAssignedXids space for the XIDs
+	 * now, that allows us to start up, but we might run out of space later.
+	 *
+	 * The size of the KnownAssignedXids array is TOTAL_MAX_CACHED_SUBXIDS,
+	 * which is (PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS). In
+	 * PostgreSQL, that's always enough because the primary will always write
+	 * an XLOG_XACT_ASSIGNMENT record if a transaction has more than
+	 * PGPROC_MAX_CACHED_SUBXIDS subtransactions. Seeing that record allows
+	 * the standby to mark the XIDs in pg_subtrans and removing them from the
+	 * KnowingAssignedXids array.
+	 *
+	 * Here, we don't know which XIDs belong to subtransactions that have
+	 * already been WAL-logged with an XLOG_XACT_ASSIGNMENT record. If we
+	 * wanted to be totally safe and avoid the possibility of getting a "too
+	 * many KnownAssignedXids" error later, we would have to limit ourselves
+	 * to PGPROC_MAX_CACHED_SUBXIDS, which is not much. And that includes top
+	 * transaction IDs too, because we cannot distinguish between top
+	 * transaction IDs and subtransactions here.
+	 *
+	 * Somewhat arbitrarily, we use up to half of KnownAssignedXids. That
+	 * strikes a sensible balance between being useful, and risking a "too
+	 * many KnownAssignedXids" error later.
+	 */
+	max_xcnt = TOTAL_MAX_CACHED_SUBXIDS / 2;
+
+	/*
+	 * Collect XIDs of prepared transactions in an array. This includes only
+	 * their top-level XIDs. We assume that StandbyRecoverPreparedTransactions
+	 * has already been called, so we can find all the sub-transactions in
+	 * pg_subtrans.
+	 */
+	PrescanPreparedTransactions(&prepared_xids, &n_prepared_xids);
+	qsort(prepared_xids, n_prepared_xids, sizeof(TransactionId), xidLogicalComparator);
+
+	/*
+	 * Scan the CLOG, collecting in-progress XIDs into 'restored_xids'.
+	 */
+	elog(DEBUG1, "scanning CLOG between %u and %u for in-progress XIDs", from, till);
+	restored_xids = (TransactionId *) palloc(max_xcnt * sizeof(TransactionId));
+	n_restored_xids = 0;
+	next_prepared_idx = 0;
+	for (TransactionId xid = from; xid != till;)
+	{
+		XLogRecPtr	xidlsn;
+		XidStatus	xidstatus;
+
+		xidstatus = TransactionIdGetStatus(xid, &xidlsn);
+
+		/*
+		 * "Merge" the prepared transactions into the restored_xids array as
+		 * we go.  The prepared transactions array is sorted. This is mostly
+		 * a sanity check to ensure that all the prpeared transactions are
+		 * seen as in-progress. (There is a check after the loop that we didn't
+		 * miss any.)
+		 */
+		if (next_prepared_idx < n_prepared_xids && xid == prepared_xids[next_prepared_idx])
+		{
+			/*
+			 * This is a top-level transaction ID of a prepared transaction.
+			 * Include it in the array.
+			 */
+
+			/* sanity check */
+			if (xidstatus != TRANSACTION_STATUS_IN_PROGRESS)
+			{
+				elog(LOG, "prepared transaction %u has unexpected status %X, cannot restore running-xacts from CLOG",
+					 xid, xidstatus);
+				Assert(false);
+				goto fail;
+			}
+
+			elog(DEBUG1, "XID %u: was next prepared xact (%d / %d)", xid, next_prepared_idx, n_prepared_xids);
+			next_prepared_idx++;
+		}
+		else if (xidstatus == TRANSACTION_STATUS_COMMITTED)
+		{
+			elog(DEBUG1, "XID %u: was committed", xid);
+			goto skip;
+		}
+		else if (xidstatus == TRANSACTION_STATUS_ABORTED)
+		{
+			elog(DEBUG1, "XID %u: was aborted", xid);
+			goto skip;
+		}
+		else if (xidstatus == TRANSACTION_STATUS_IN_PROGRESS)
+		{
+			/*
+			 * In-progress transactions are included in the array.
+			 *
+			 * Except subtransactions of the prepared transactions. They are
+			 * already set in pg_subtrans, and hence don't need to be tracked
+			 * in the known-assigned XIDs array.
+			 */
+			if (n_prepared_xids > 0)
+			{
+				TransactionId parent = SubTransGetParent(xid);
+
+				if (TransactionIdIsValid(parent))
+				{
+					/*
+					 * This is a subtransaction belonging to a prepared
+					 * transaction.
+					 *
+					 * Sanity check that it is in the prepared XIDs array. It
+					 * should be, because StandbyRecoverPreparedTransactions
+					 * populated pg_subtrans, and no other XID should be set
+					 * in it yet. (This also relies on the fact that
+					 * StandbyRecoverPreparedTransactions sets the parent of
+					 * each subxid to point directly to the top-level XID,
+					 * rather than restoring the original subtransaction
+					 * hierarchy.)
+					 */
+					if (bsearch(&parent, prepared_xids, next_prepared_idx,
+								sizeof(TransactionId), xidLogicalComparator) == NULL)
+					{
+						elog(LOG, "sub-XID %u has unexpected parent %u, cannot restore running-xacts from CLOG",
+							 xid, parent);
+						Assert(false);
+						goto fail;
+					}
+					elog(DEBUG1, "XID %u: was a subtransaction of prepared xid %u", xid, parent);
+					goto skip;
+				}
+			}
+
+			/* include it in the array */
+			elog(DEBUG1, "XID %u: is in progress", xid);
+		}
+		else
+		{
+			/*
+			 * SUB_COMMITTED is a transient state used at commit. We don't
+			 * expect to see that here.
+			 */
+			elog(LOG, "XID %u has unexpected status %X in pg_xact, cannot restore running-xacts from CLOG",
+				 xid, xidstatus);
+			Assert(false);
+			goto fail;
+		}
+
+		if (n_restored_xids >= max_xcnt)
+		{
+			/*
+			 * Overflowed. We won't be able to install the RunningTransactions
+			 * snapshot.
+			 */
+			elog(LOG, "too many running xacts to restore from the CLOG; oldestXid=%u oldestActiveXid=%u nextXid %u",
+				 checkpoint->oldestXid, checkpoint->oldestActiveXid,
+				 XidFromFullTransactionId(checkpoint->nextXid));
+			goto fail;
+		}
+
+		restored_xids[n_restored_xids++] = xid;
+
+	skip:
+		TransactionIdAdvance(xid);
+		continue;
+	}
+
+	/* sanity check */
+	if (next_prepared_idx != n_prepared_xids)
+	{
+		elog(LOG, "prepared transaction ID %u was not visited in the CLOG scan, cannot restore running-xacts from CLOG",
+			 prepared_xids[next_prepared_idx]);
+		Assert(false);
+		goto fail;
+	}
+
+	elog(LOG, "restored %d running xacts by scanning the CLOG; oldestXid=%u oldestActiveXid=%u nextXid %u",
+		 n_restored_xids, checkpoint->oldestXid, checkpoint->oldestActiveXid, XidFromFullTransactionId(checkpoint->nextXid));
+	*nxids = n_restored_xids;
+	*xids = restored_xids;
+	return true;
+
+ fail:
+	*nxids = 0;
+	*xids = NULL;
+	if (restored_xids)
+		pfree(restored_xids);
+	if (prepared_xids)
+		pfree(prepared_xids);
+	return false;
+}
+
 void
 _PG_init(void)
 {
@@ -288,6 +579,8 @@ _PG_init(void)
 
 	pg_init_extension_server();
 
+	restore_running_xacts_callback = RestoreRunningXactsFromClog;
+
 	/*
 	 * Important: This must happen after other parts of the extension are
 	 * loaded, otherwise any settings to GUCs that were set before the

pgxn/neon_test_utils/Makefile

@@ -7,7 +7,7 @@ OBJS = \
 	neontest.o
 
 EXTENSION = neon_test_utils
-DATA = neon_test_utils--1.1.sql
+DATA = neon_test_utils--1.2.sql
 PGFILEDESC = "neon_test_utils - helpers for neon testing and debugging"
 
 PG_CONFIG = pg_config

pgxn/neon_test_utils/neon_test_utils--1.2.sql

@@ -41,7 +41,7 @@ RETURNS bytea
 AS 'MODULE_PATHNAME', 'get_raw_page_at_lsn_ex'
 LANGUAGE C PARALLEL UNSAFE;
 
-CREATE FUNCTION neon_xlogflush(lsn pg_lsn)
+CREATE FUNCTION neon_xlogflush(lsn pg_lsn DEFAULT NULL)
 RETURNS VOID
 AS 'MODULE_PATHNAME', 'neon_xlogflush'
 LANGUAGE C PARALLEL UNSAFE;

pgxn/neon_test_utils/neon_test_utils.control

@@ -1,6 +1,6 @@
 # neon_test_utils extension
 comment = 'helpers for neon testing and debugging'
-default_version = '1.1'
+default_version = '1.2'
 module_pathname = '$libdir/neon_test_utils'
 relocatable = true
 trusted = true

pgxn/neon_test_utils/neontest.c

@@ -15,6 +15,7 @@
 #include "access/relation.h"
 #include "access/xact.h"
 #include "access/xlog.h"
+#include "access/xlog_internal.h"
 #include "catalog/namespace.h"
 #include "fmgr.h"
 #include "funcapi.h"
@@ -444,11 +445,46 @@ get_raw_page_at_lsn_ex(PG_FUNCTION_ARGS)
 
 /*
  * Directly calls XLogFlush(lsn) to flush WAL buffers.
+ *
+ * If 'lsn' is not specified (is NULL), flush all generated WAL.
  */
 Datum
 neon_xlogflush(PG_FUNCTION_ARGS)
 {
-	XLogRecPtr	lsn = PG_GETARG_LSN(0);
+	XLogRecPtr	lsn;
+
+	if (RecoveryInProgress())
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("recovery is in progress"),
+				 errhint("cannot flush WAL during recovery.")));
+
+	if (!PG_ARGISNULL(0))
+		lsn = PG_GETARG_LSN(0);
+	else
+	{
+		lsn = GetXLogInsertRecPtr();
+
+		/*---
+		 * The LSN returned by GetXLogInsertRecPtr() is the position where the
+		 * next inserted record would begin. If the last record ended just at
+		 * the page boundary, the next record will begin after the page header
+		 * on the next page, and that's what GetXLogInsertRecPtr().returns,
+		 * but the page header has not been written yet. If we tried to flush
+		 * it, XLogFlush() would throw an error:
+		 *
+		 * ERROR : xlog flush request %X/%X is not satisfied --- flushed only to %X/%X
+		 *
+		 * To avoid that, if the insert position points to just after the page
+		 * header, back off to page boundary.
+		 */
+		if (lsn % XLOG_BLCKSZ == SizeOfXLogShortPHD &&
+			XLogSegmentOffset(lsn, wal_segment_size) > XLOG_BLCKSZ)
+			lsn -= SizeOfXLogShortPHD;
+		else if (lsn % XLOG_BLCKSZ == SizeOfXLogLongPHD &&
+				 XLogSegmentOffset(lsn, wal_segment_size) < XLOG_BLCKSZ)
+			lsn -= SizeOfXLogLongPHD;
+	}
 
 	XLogFlush(lsn);
 	PG_RETURN_VOID();

storage_broker/build.rs

@@ -2,7 +2,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     // Generate rust code from .proto protobuf.
     //
     // Note: we previously tried to use deterministic location at proto/ for
-    // easy location, but apparently interference with cachepot sometimes fails
+    // easy location, but apparently interference with sccache sometimes fails
     // the build then. Anyway, per cargo docs build script shouldn't output to
     // anywhere but $OUT_DIR.
     tonic_build::compile_protos("proto/broker.proto")

test_runner/fixtures/neon_fixtures.py

@@ -3491,7 +3491,6 @@ class Endpoint(PgProtocol, LogUtils):
     ):
         super().__init__(host="localhost", port=pg_port, user="cloud_admin", dbname="postgres")
         self.env = env
-        self.running = False
         self.branch_name: Optional[str] = None  # dubious
         self.endpoint_id: Optional[str] = None  # dubious, see asserts below
         self.pgdata_dir: Optional[str] = None  # Path to computenode PGDATA
@@ -3857,7 +3856,9 @@ class EndpointFactory:
 
         return self
 
-    def new_replica(self, origin: Endpoint, endpoint_id: str, config_lines: Optional[List[str]]):
+    def new_replica(
+        self, origin: Endpoint, endpoint_id: str, config_lines: Optional[List[str]] = None
+    ):
         branch_name = origin.branch_name
         assert origin in self.endpoints
         assert branch_name is not None

test_runner/fixtures/pageserver/utils.py

@@ -198,7 +198,7 @@ def wait_for_last_record_lsn(
     lsn: Lsn,
 ) -> Lsn:
     """waits for pageserver to catch up to a certain lsn, returns the last observed lsn."""
-    for i in range(100):
+    for i in range(1000):
         current_lsn = last_record_lsn(pageserver_http, tenant, timeline)
         if current_lsn >= lsn:
             return current_lsn

test_runner/regress/test_next_xid.py

@@ -7,6 +7,7 @@ from fixtures.log_helper import log
 from fixtures.neon_fixtures import (
     NeonEnvBuilder,
     PgBin,
+    VanillaPostgres,
     import_timeline_from_vanilla_postgres,
     wait_for_wal_insert_lsn,
 )
@@ -182,3 +183,275 @@ def test_import_at_2bil(
     cur = conn.cursor()
     cur.execute("SELECT count(*) from t")
     assert cur.fetchone() == (10000 + 1 + 1,)
+
+
+# Constants and macros copied from PostgreSQL multixact.c and headers. These are needed to
+# calculate the SLRU segments that a particular multixid or multixid-offsets falls into.
+BLCKSZ = 8192
+MULTIXACT_OFFSETS_PER_PAGE = int(BLCKSZ / 4)
+SLRU_PAGES_PER_SEGMENT = int(32)
+MXACT_MEMBER_BITS_PER_XACT = 8
+MXACT_MEMBER_FLAGS_PER_BYTE = 1
+MULTIXACT_FLAGBYTES_PER_GROUP = 4
+MULTIXACT_MEMBERS_PER_MEMBERGROUP = MULTIXACT_FLAGBYTES_PER_GROUP * MXACT_MEMBER_FLAGS_PER_BYTE
+MULTIXACT_MEMBERGROUP_SIZE = 4 * MULTIXACT_MEMBERS_PER_MEMBERGROUP + MULTIXACT_FLAGBYTES_PER_GROUP
+MULTIXACT_MEMBERGROUPS_PER_PAGE = int(BLCKSZ / MULTIXACT_MEMBERGROUP_SIZE)
+MULTIXACT_MEMBERS_PER_PAGE = MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP
+
+
+def MultiXactIdToOffsetSegment(xid: int):
+    return int(xid / (SLRU_PAGES_PER_SEGMENT * MULTIXACT_OFFSETS_PER_PAGE))
+
+
+def MXOffsetToMemberSegment(off: int):
+    return int(off / (SLRU_PAGES_PER_SEGMENT * MULTIXACT_MEMBERS_PER_PAGE))
+
+
+def advance_multixid_to(
+    pg_bin: PgBin, vanilla_pg: VanillaPostgres, next_multi_xid: int, next_multi_offset: int
+):
+    """
+    Use pg_resetwal to advance the nextMulti and nextMultiOffset values in a stand-alone
+    Postgres cluster. This is useful to get close to wraparound or some other interesting
+    value, without having to burn a lot of time consuming the (multi-)XIDs one by one.
+
+    The new values should be higher than the old ones, in a wraparound-aware sense.
+
+    On entry, the server should be running. It will be shut down and restarted.
+    """
+
+    # Read old values from the last checkpoint. We will pass the old oldestMultiXid value
+    # back to pg_resetwal, there's no option to leave it alone.
+    with vanilla_pg.connect() as conn:
+        with conn.cursor() as cur:
+            # Make sure the oldest-multi-xid value in the control file is up-to-date
+            cur.execute("checkpoint")
+            cur.execute("select oldest_multi_xid, next_multixact_id from pg_control_checkpoint()")
+            rec = cur.fetchone()
+            assert rec is not None
+            (ckpt_oldest_multi_xid, ckpt_next_multi_xid) = rec
+    log.info(f"oldestMultiXid was {ckpt_oldest_multi_xid}, nextMultiXid was {ckpt_next_multi_xid}")
+    log.info(f"Resetting to {next_multi_xid}")
+
+    # Use pg_resetwal to reset the next multiXid and multiOffset to given values.
+    vanilla_pg.stop()
+    pg_resetwal_path = os.path.join(pg_bin.pg_bin_path, "pg_resetwal")
+    cmd = [
+        pg_resetwal_path,
+        f"--multixact-ids={next_multi_xid},{ckpt_oldest_multi_xid}",
+        f"--multixact-offset={next_multi_offset}",
+        "-D",
+        str(vanilla_pg.pgdatadir),
+    ]
+    pg_bin.run_capture(cmd)
+
+    # Because we skip over a lot of values, Postgres hasn't created the SLRU segments for
+    # the new values yet. Create them manually, to allow Postgres to start up.
+    #
+    # This leaves "gaps" in the SLRU where segments between old value and new value are
+    # missing. That's OK for our purposes. Autovacuum will print some warnings about the
+    # missing segments, but will clean it up by truncating the SLRUs up to the new value,
+    # closing the gap.
+    segname = "%04X" % MultiXactIdToOffsetSegment(next_multi_xid)
+    log.info(f"Creating dummy segment pg_multixact/offsets/{segname}")
+    with open(vanilla_pg.pgdatadir / "pg_multixact" / "offsets" / segname, "w") as of:
+        of.write("\0" * SLRU_PAGES_PER_SEGMENT * BLCKSZ)
+        of.flush()
+
+    segname = "%04X" % MXOffsetToMemberSegment(next_multi_offset)
+    log.info(f"Creating dummy segment pg_multixact/members/{segname}")
+    with open(vanilla_pg.pgdatadir / "pg_multixact" / "members" / segname, "w") as of:
+        of.write("\0" * SLRU_PAGES_PER_SEGMENT * BLCKSZ)
+        of.flush()
+
+    # Start Postgres again and wait until autovacuum has processed all the databases
+    #
+    # This allows truncating the SLRUs, fixing the gaps with missing segments.
+    vanilla_pg.start()
+    with vanilla_pg.connect().cursor() as cur:
+        for _ in range(1000):
+            datminmxid = int(
+                query_scalar(cur, "select min(datminmxid::text::int8) from pg_database")
+            )
+            log.info(f"datminmxid {datminmxid}")
+            if next_multi_xid - datminmxid < 1_000_000:  # not wraparound-aware!
+                break
+            time.sleep(0.5)
+
+
+def test_multixid_wraparound_import(
+    neon_env_builder: NeonEnvBuilder,
+    test_output_dir: Path,
+    pg_bin: PgBin,
+    vanilla_pg,
+):
+    """
+    Test that the wraparound of the "next-multi-xid" counter is handled correctly in
+    pageserver, And multi-offsets as well
+    """
+    env = neon_env_builder.init_start()
+
+    # In order to to test multixid wraparound, we need to first advance the counter to
+    # within spitting distance of the wraparound, that is 2^32 multi-XIDs. We could simply
+    # run a workload that consumes a lot of multi-XIDs until we approach that, but that
+    # takes a very long time. So we cheat.
+    #
+    # Our strategy is to create a vanilla Postgres cluster, and use pg_resetwal to
+    # directly set the multi-xid counter a higher value. However, we cannot directly set
+    # it to just before 2^32 (~ 4 billion), because that would make the exisitng
+    # 'relminmxid' values to look like they're in the future. It's not clear how the
+    # system would behave in that situation. So instead, we bump it up ~ 1 billion
+    # multi-XIDs at a time, and let autovacuum to process all the relations and update
+    # 'relminmxid' between each run.
+    #
+    # XXX: For the multi-offsets, most of the bump is done in the last call.  This is
+    # because advancing it ~ 1 billion at a time hit a pathological case in the
+    # MultiXactMemberFreezeThreshold() function, causing autovacuum not trigger multixid
+    # freezing. See
+    # https://www.postgresql.org/message-id/85fb354c-f89f-4d47-b3a2-3cbd461c90a3%40iki.fi
+    # Multi-offsets don't have the same wraparound problems at 2 billion mark as
+    # multi-xids do, so one big jump is fine.
+    vanilla_pg.configure(
+        [
+            "log_autovacuum_min_duration = 0",
+            # Perform anti-wraparound vacuuming aggressively
+            "autovacuum_naptime='1 s'",
+            "autovacuum_freeze_max_age = 1000000",
+            "autovacuum_multixact_freeze_max_age = 1000000",
+        ],
+    )
+    vanilla_pg.start()
+    advance_multixid_to(pg_bin, vanilla_pg, 0x40000000, 0x10000000)
+    advance_multixid_to(pg_bin, vanilla_pg, 0x80000000, 0x20000000)
+    advance_multixid_to(pg_bin, vanilla_pg, 0xC0000000, 0x30000000)
+    advance_multixid_to(pg_bin, vanilla_pg, 0xFFFFFF00, 0xFFFFFF00)
+
+    vanilla_pg.safe_psql("create user cloud_admin with password 'postgres' superuser")
+    vanilla_pg.safe_psql("create table tt as select g as id from generate_series(1, 10) g")
+    vanilla_pg.safe_psql("CHECKPOINT")
+
+    # Import the cluster to the pageserver
+    tenant_id = TenantId.generate()
+    env.pageserver.tenant_create(tenant_id)
+    timeline_id = TimelineId.generate()
+    import_timeline_from_vanilla_postgres(
+        test_output_dir,
+        env,
+        pg_bin,
+        tenant_id,
+        timeline_id,
+        "imported_multixid_wraparound_test",
+        vanilla_pg.connstr(),
+    )
+    vanilla_pg.stop()
+
+    endpoint = env.endpoints.create_start(
+        "imported_multixid_wraparound_test",
+        tenant_id=tenant_id,
+        config_lines=[
+            "log_autovacuum_min_duration = 0",
+            "autovacuum_naptime='5 s'",
+            "autovacuum=off",
+        ],
+    )
+    conn = endpoint.connect()
+    cur = conn.cursor()
+    assert query_scalar(cur, "select count(*) from tt") == 10  # sanity check
+
+    # Install extension containing function needed for test
+    cur.execute("CREATE EXTENSION neon_test_utils")
+
+    # Consume a lot of XIDs, just to advance the XIDs to different range than the
+    # multi-xids. That avoids confusion while debugging
+    cur.execute("select test_consume_xids(100000)")
+    cur.execute("select pg_switch_wal()")
+    cur.execute("checkpoint")
+
+    # Use subtransactions so that each row in 'tt' is stamped with different XID. Leave
+    # the transaction open.
+    cur.execute("BEGIN")
+    cur.execute(
+        """
+do $$
+declare
+  idvar int;
+begin
+  for idvar in select id from tt loop
+    begin
+      update tt set id = idvar where id = idvar;
+    exception when others then
+      raise 'didn''t expect an error: %', sqlerrm;
+    end;
+  end loop;
+end;
+$$;
+"""
+    )
+
+    # In a different transaction, acquire a FOR KEY SHARE lock on each row. This generates
+    # a new multixid for each row, with the previous xmax and this transaction's XID as the
+    # members.
+    #
+    # Repeat this until the multi-xid counter wraps around.
+    conn3 = endpoint.connect()
+    cur3 = conn3.cursor()
+    next_multixact_id_before_restart = 0
+    observed_before_wraparound = False
+    while True:
+        cur3.execute("BEGIN")
+        cur3.execute("SELECT * FROM tt FOR KEY SHARE")
+
+        # Get the xmax of one of the rows we locked. It should be a multi-xid. It might
+        # not be the latest one, but close enough.
+        row_xmax = int(query_scalar(cur3, "SELECT xmax FROM tt LIMIT 1"))
+        cur3.execute("COMMIT")
+        log.info(f"observed a row with xmax {row_xmax}")
+
+        # High value means not wrapped around yet
+        if row_xmax >= 0xFFFFFF00:
+            observed_before_wraparound = True
+            continue
+
+        # xmax should not be a regular XID. (We bumped up the regular XID range earlier
+        # to around 100000 and above.)
+        assert row_xmax < 100
+
+        # xmax values < FirstNormalTransactionId (== 3) could be special XID values, or
+        # multixid values after wraparound. We don't know for sure which, so keep going to
+        # be sure we see value that's unambiguously a wrapped-around multixid
+        if row_xmax < 3:
+            continue
+
+        next_multixact_id_before_restart = row_xmax
+        log.info(
+            f"next_multixact_id is now at {next_multixact_id_before_restart} or a little higher"
+        )
+        break
+
+    # We should have observed the state before wraparound
+    assert observed_before_wraparound
+
+    cur.execute("COMMIT")
+
+    # Wait until pageserver has received all the data, and restart the endpoint
+    wait_for_wal_insert_lsn(env, endpoint, tenant_id, timeline_id)
+    endpoint.stop(mode="immediate")  # 'immediate' to avoid writing shutdown checkpoint
+    endpoint.start()
+
+    # Check that the next-multixid value wrapped around correctly
+    conn = endpoint.connect()
+    cur = conn.cursor()
+    cur.execute("select next_multixact_id from pg_control_checkpoint()")
+    next_multixact_id_after_restart = int(
+        query_scalar(cur, "select next_multixact_id from pg_control_checkpoint()")
+    )
+    log.info(f"next_multixact_id after restart: {next_multixact_id_after_restart}")
+    assert next_multixact_id_after_restart >= next_multixact_id_before_restart
+
+    # The multi-offset should wrap around as well
+    cur.execute("select next_multi_offset from pg_control_checkpoint()")
+    next_multi_offset_after_restart = int(
+        query_scalar(cur, "select next_multi_offset from pg_control_checkpoint()")
+    )
+    log.info(f"next_multi_offset after restart: {next_multi_offset_after_restart}")
+    assert next_multi_offset_after_restart < 100000

test_runner/regress/test_replica_start.py

@@ -0,0 +1,646 @@
+"""
+In PostgreSQL, a standby always has to wait for a running-xacts WAL record to
+arrive before it can start accepting queries. Furthermore, if there are
+transactions with too many subxids (> 64) open to fit in the in-memory subxids
+cache, the running-xacts record will be marked as "suboverflowed", and the
+standby will need to also wait for the currently in-progress transactions to
+finish.
+
+In Neon, we have an additional mechanism that scans the CLOG at server startup
+to determine the list of running transactions, so that the standby can start up
+immediately without waiting for the running-xacts record, but that mechanism
+only works if the # of active (sub-)transactions is reasonably small. Otherwise
+it falls back to waiting. Furthermore, it's somewhat optimistic in using up the
+known-assigned XIDs array: if too many transactions with subxids are started in
+the primary later, the replay in the replica will crash with "too many
+KnownAssignedXids" error.
+
+This module contains tests for those various cases at standby startup: starting
+from shutdown checkpoint, using the CLOG scanning mechanism, waiting for
+running-xacts record and for in-progress transactions to finish etc.
+"""
+
+import threading
+from contextlib import closing
+
+import psycopg2
+import pytest
+from fixtures.log_helper import log
+from fixtures.neon_fixtures import NeonEnv, wait_for_last_flush_lsn, wait_replica_caughtup
+from fixtures.pg_version import PgVersion
+from fixtures.utils import query_scalar, wait_until
+
+CREATE_SUBXACTS_FUNC = """
+create or replace function create_subxacts(n integer) returns void as $$
+declare
+   i integer;
+begin
+   for i in 1..n loop
+      begin
+         insert into t (payload) values (0);
+      exception
+         when others then
+            raise exception 'caught something: %', sqlerrm;
+      end;
+   end loop;
+end; $$ language plpgsql
+"""
+
+
+def test_replica_start_scan_clog(neon_simple_env: NeonEnv):
+    """
+    Test the CLOG-scanning mechanism at hot standby startup. There is one
+    transaction active in the primary when the standby is started. The primary
+    is killed before it has a chance to write a running-xacts record. The
+    CLOG-scanning at neon startup allows the standby to start up anyway.
+
+    See the module docstring for background.
+    """
+
+    # Initialize the primary, a test table, and a helper function to create lots
+    # of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("CREATE EXTENSION neon_test_utils")
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+    primary_cur.execute("select pg_switch_wal()")
+
+    # Start a transaction in the primary. Leave the transaction open.
+    #
+    # The transaction has some subtransactions, but not too many to cause the
+    # CLOG-scanning mechanism to give up.
+    primary_cur.execute("begin")
+    primary_cur.execute("select create_subxacts(50)")
+
+    # Wait for the WAL to be flushed, but then immediately kill the primary,
+    # before it has a chance to generate a running-xacts record.
+    primary_cur.execute("select neon_xlogflush()")
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+    primary.stop(mode="immediate")
+
+    # Create a replica. It should start up normally, thanks to the CLOG-scanning
+    # mechanism.
+    secondary = env.endpoints.new_replica_start(origin=primary, endpoint_id="secondary")
+
+    # The transaction did not commit, so it should not be visible in the secondary
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (0,)
+
+
+def test_replica_start_scan_clog_crashed_xids(neon_simple_env: NeonEnv):
+    """
+    Test the CLOG-scanning mechanism at hot standby startup, after
+    leaving behind crashed transactions.
+
+    See the module docstring for background.
+    """
+
+    # Initialize the primary, a test table, and a helper function to create lots
+    # of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+    primary_cur.execute("select pg_switch_wal()")
+
+    # Consume a lot of XIDs, then kill Postgres without giving it a
+    # chance to write abort records for them.
+    primary_cur.execute("begin")
+    primary_cur.execute("select create_subxacts(100000)")
+    primary.stop(mode="immediate")
+
+    # Restart the primary. Do some light work, and shut it down cleanly
+    primary.start()
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("insert into t (payload) values (0)")
+    primary.stop(mode="fast")
+
+    # Create a replica. It should start up normally, thanks to the CLOG-scanning
+    # mechanism. (Restarting the primary writes a checkpoint and/or running-xacts
+    # record, which allows the standby to know that the crashed XIDs are aborted)
+    secondary = env.endpoints.new_replica_start(origin=primary, endpoint_id="secondary")
+
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (1,)
+
+
+def test_replica_start_at_running_xacts(neon_simple_env: NeonEnv, pg_version):
+    """
+    Test that starting a replica works right after the primary has
+    created a running-xacts record. This may seem like a trivial case,
+    but during development, we had a bug that was triggered by having
+    oldestActiveXid == nextXid. Starting right after a running-xacts
+    record is one way to test that case.
+
+    See the module docstring for background.
+    """
+    env = neon_simple_env
+
+    if env.pg_version == PgVersion.V14 or env.pg_version == PgVersion.V15:
+        pytest.skip("pg_log_standby_snapshot() function is available only in PG16")
+
+    primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+
+    primary_cur.execute("CREATE EXTENSION neon_test_utils")
+    primary_cur.execute("select pg_log_standby_snapshot()")
+    primary_cur.execute("select neon_xlogflush()")
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+
+    secondary = env.endpoints.new_replica_start(origin=primary, endpoint_id="secondary")
+
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select 123")
+    assert secondary_cur.fetchone() == (123,)
+
+
+def test_replica_start_wait_subxids_finish(neon_simple_env: NeonEnv):
+    """
+    Test replica startup when there are a lot of (sub)transactions active in the
+    primary. That's too many for the CLOG-scanning mechanism to handle, so the
+    replica has to wait for the large transaction to finish before it starts to
+    accept queries.
+
+    After replica startup, test MVCC with transactions that were in-progress
+    when the replica was started.
+
+    See the module docstring for background.
+    """
+
+    # Initialize the primary, a test table, and a helper function to create
+    # lots of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+
+    # Start a transaction with 100000 subtransactions, and leave it open. That's
+    # too many to fit in the "known-assigned XIDs array" in the replica, and
+    # also too many to fit in the subxid caches so the running-xacts record will
+    # also overflow.
+    primary_cur.execute("begin")
+    primary_cur.execute("select create_subxacts(100000)")
+
+    # Start another, smaller transaction in the primary. We'll come back to this
+    # later.
+    primary_conn2 = primary.connect()
+    primary_cur2 = primary_conn2.cursor()
+    primary_cur2.execute("begin")
+    primary_cur2.execute("insert into t (payload) values (0)")
+
+    # Create a replica. but before that, wait for the wal to be flushed to
+    # safekeepers, so that the replica is started at a point where the large
+    # transaction is already active. (The whole transaction might not be flushed
+    # yet, but that's OK.)
+    #
+    # Start it in a separate thread, so that we can do other stuff while it's
+    # blocked waiting for the startup to finish.
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+    secondary = env.endpoints.new_replica(origin=primary, endpoint_id="secondary")
+    start_secondary_thread = threading.Thread(target=secondary.start)
+    start_secondary_thread.start()
+
+    # Verify that the replica has otherwise started up, but cannot start
+    # accepting queries yet.
+    log.info("Waiting 5 s to verify that the secondary does not start")
+    start_secondary_thread.join(5)
+    assert secondary.log_contains("consistent recovery state reached")
+    assert secondary.log_contains("started streaming WAL from primary")
+    # The "redo starts" message is printed when the first WAL record is
+    # received. It might or might not be present in the log depending on how
+    # far exactly the WAL was flushed when the replica was started, and whether
+    # background activity caused any more WAL records to be flushed on the
+    # primary afterwards.
+    #
+    # assert secondary.log_contains("redo # starts")
+
+    # should not be open for connections yet
+    assert start_secondary_thread.is_alive()
+    assert not secondary.is_running()
+    assert not secondary.log_contains("database system is ready to accept read-only connections")
+
+    # Commit the large transaction in the primary.
+    #
+    # Within the next 15 s, the primary should write a new running-xacts record
+    # to the WAL which shows the transaction as completed. Once the replica
+    # replays that record, it will start accepting queries.
+    primary_cur.execute("commit")
+    start_secondary_thread.join()
+
+    # Verify that the large transaction is correctly visible in the secondary
+    # (but not the second, small transaction, which is still in-progress!)
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (100000,)
+
+    # Perform some more MVCC testing using the second transaction that was
+    # started in the primary before the replica was created
+    primary_cur2.execute("select create_subxacts(10000)")
+
+    # The second transaction still hasn't committed
+    wait_replica_caughtup(primary, secondary)
+    secondary_cur.execute("BEGIN ISOLATION LEVEL REPEATABLE READ")
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (100000,)
+
+    # Commit the second transaction in the primary
+    primary_cur2.execute("commit")
+
+    # Should still be invisible to the old snapshot
+    wait_replica_caughtup(primary, secondary)
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (100000,)
+
+    # Commit the REPEATABLE READ transaction in the replica. Both
+    # primary transactions should now be visible to a new snapshot.
+    secondary_cur.execute("commit")
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (110001,)
+
+
+def test_replica_too_many_known_assigned_xids(neon_simple_env: NeonEnv):
+    """
+    The CLOG-scanning mechanism fills the known-assigned XIDs array
+    optimistically at standby startup, betting that it can still fit
+    upcoming transactions replayed later from the WAL in the
+    array. This test tests what happens when that bet fails and the
+    known-assigned XID array fills up after the standby has already
+    been started. The WAL redo will fail with an error:
+
+    FATAL:  too many KnownAssignedXids
+    CONTEXT:  WAL redo at 0/1895CB0 for neon/INSERT: off: 25, flags: 0x08; blkref #0: rel 1663/5/16385, blk 64
+
+    which causes the standby to shut down.
+
+    See the module docstring for background.
+    """
+
+    # Initialize the primary, a test table, and a helper function to create lots
+    # of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("CREATE EXTENSION neon_test_utils")
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+
+    # Determine how many connections we can use
+    primary_cur.execute("show max_connections")
+    max_connections = int(primary_cur.fetchall()[0][0])
+    primary_cur.execute("show superuser_reserved_connections")
+    superuser_reserved_connections = int(primary_cur.fetchall()[0][0])
+    n_connections = max_connections - superuser_reserved_connections
+    n_subxids = 200
+
+    # Start one top transaction in primary, with lots of subtransactions. This
+    # uses up much of the known-assigned XIDs space in the standby, but doesn't
+    # cause it to overflow.
+    large_p_conn = primary.connect()
+    large_p_cur = large_p_conn.cursor()
+    large_p_cur.execute("begin")
+    large_p_cur.execute(f"select create_subxacts({max_connections} * 30)")
+
+    with closing(primary.connect()) as small_p_conn:
+        with small_p_conn.cursor() as small_p_cur:
+            small_p_cur.execute("select create_subxacts(1)")
+
+    # Create a replica at this LSN
+    primary_cur.execute("select neon_xlogflush()")
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+    secondary = env.endpoints.new_replica_start(origin=primary, endpoint_id="secondary")
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+
+    # The transaction in primary has not committed yet.
+    wait_replica_caughtup(primary, secondary)
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (1,)
+
+    # Start max number of top transactions in primary, with a lot of
+    # subtransactions each. We add the subtransactions to each top transaction
+    # in a round-robin fashion, instead of adding a lot of subtransactions to
+    # one top transaction at a time. This way, we will have the max number of
+    # subtransactions in the in-memory subxid cache of each top transaction,
+    # until they all overflow.
+    #
+    # Currently, PGPROC_MAX_CACHED_SUBXIDS == 64, so this will overflow the all
+    # the subxid caches after creating 64 subxids in each top transaction. The
+    # point just before the caches have overflowed is the most interesting point
+    # in time, but we'll keep going beyond that, to ensure that this test is
+    # robust even if PGPROC_MAX_CACHED_SUBXIDS changes.
+    p_curs = []
+    for _ in range(0, n_connections):
+        p_cur = primary.connect().cursor()
+        p_cur.execute("begin")
+        p_curs.append(p_cur)
+
+    for _subxid in range(0, n_subxids):
+        for i in range(0, n_connections):
+            p_curs[i].execute("select create_subxacts(1)")
+
+    # Commit all the transactions in the primary
+    for i in range(0, n_connections):
+        p_curs[i].execute("commit")
+    large_p_cur.execute("commit")
+
+    # Wait until the replica crashes with "too many KnownAssignedXids" error.
+    def check_replica_crashed():
+        try:
+            secondary.connect()
+        except psycopg2.Error:
+            # Once the connection fails, return success
+            return None
+        raise RuntimeError("connection succeeded")
+
+    wait_until(20, 0.5, check_replica_crashed)
+    assert secondary.log_contains("too many KnownAssignedXids")
+
+    # Replica is crashed, so ignore stop result
+    secondary.check_stop_result = False
+
+
+def test_replica_start_repro_visibility_bug(neon_simple_env: NeonEnv):
+    """
+    Before PR #7288, a hot standby in neon incorrectly started up
+    immediately, before it had received a running-xacts record. That
+    led to visibility bugs if there were active transactions in the
+    primary. This test reproduces the incorrect query results and
+    incorrectly set hint bits, before that was fixed.
+    """
+    env = neon_simple_env
+
+    primary = env.endpoints.create_start(branch_name="main", endpoint_id="primary")
+    p_cur = primary.connect().cursor()
+
+    p_cur.execute("begin")
+    p_cur.execute("create table t(pk integer primary key, payload integer)")
+    p_cur.execute("insert into t values (generate_series(1,100000), 0)")
+
+    secondary = env.endpoints.new_replica_start(origin=primary, endpoint_id="secondary")
+    wait_replica_caughtup(primary, secondary)
+    s_cur = secondary.connect().cursor()
+
+    # Set hint bits for pg_class tuples. If primary's transaction is
+    # not marked as in-progress in MVCC snapshot, then XMIN_INVALID
+    # hint bit will be set for table's 't' tuple, making it invisible
+    # even after the commit record is replayed later.
+    s_cur.execute("select * from pg_class")
+
+    p_cur.execute("commit")
+    wait_replica_caughtup(primary, secondary)
+    s_cur.execute("select * from t where pk = 1")
+    assert s_cur.fetchone() == (1, 0)
+
+
+@pytest.mark.parametrize("shutdown", [True, False])
+def test_replica_start_with_prepared_xacts(neon_simple_env: NeonEnv, shutdown: bool):
+    """
+    Test the CLOG-scanning mechanism at hot standby startup in the presence of
+    prepared transactions.
+
+    This test is run in two variants: one where the primary server is shut down
+    before starting the secondary, or not.
+    """
+
+    # Initialize the primary, a test table, and a helper function to create lots
+    # of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(
+        branch_name="main", endpoint_id="primary", config_lines=["max_prepared_transactions=5"]
+    )
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("CREATE EXTENSION neon_test_utils")
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute("create table t1(pk integer primary key)")
+    primary_cur.execute("create table t2(pk integer primary key)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+
+    # Prepare a transaction for two-phase commit
+    primary_cur.execute("begin")
+    primary_cur.execute("insert into t1 values (1)")
+    primary_cur.execute("prepare transaction 't1'")
+
+    # Prepare another transaction for two-phase commit, with a subtransaction
+    primary_cur.execute("begin")
+    primary_cur.execute("insert into t2 values (2)")
+    primary_cur.execute("savepoint sp")
+    primary_cur.execute("insert into t2 values (3)")
+    primary_cur.execute("prepare transaction 't2'")
+
+    # Start a transaction in the primary. Leave the transaction open.
+    #
+    # The transaction has some subtransactions, but not too many to cause the
+    # CLOG-scanning mechanism to give up.
+    primary_cur.execute("begin")
+    primary_cur.execute("select create_subxacts(50)")
+
+    # Wait for the WAL to be flushed
+    primary_cur.execute("select neon_xlogflush()")
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+
+    if shutdown:
+        primary.stop(mode="fast")
+
+    # Create a replica. It should start up normally, thanks to the CLOG-scanning
+    # mechanism.
+    secondary = env.endpoints.new_replica_start(
+        origin=primary, endpoint_id="secondary", config_lines=["max_prepared_transactions=5"]
+    )
+
+    # The transaction did not commit, so it should not be visible in the secondary
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (0,)
+    secondary_cur.execute("select count(*) from t1")
+    assert secondary_cur.fetchone() == (0,)
+    secondary_cur.execute("select count(*) from t2")
+    assert secondary_cur.fetchone() == (0,)
+
+    if shutdown:
+        primary.start()
+        primary_conn = primary.connect()
+        primary_cur = primary_conn.cursor()
+    else:
+        primary_cur.execute("commit")
+    primary_cur.execute("commit prepared 't1'")
+    primary_cur.execute("commit prepared 't2'")
+
+    wait_replica_caughtup(primary, secondary)
+
+    secondary_cur.execute("select count(*) from t")
+    if shutdown:
+        assert secondary_cur.fetchone() == (0,)
+    else:
+        assert secondary_cur.fetchone() == (50,)
+    secondary_cur.execute("select * from t1")
+    assert secondary_cur.fetchall() == [(1,)]
+    secondary_cur.execute("select * from t2")
+    assert secondary_cur.fetchall() == [(2,), (3,)]
+
+
+def test_replica_start_with_prepared_xacts_with_subxacts(neon_simple_env: NeonEnv):
+    """
+    Test the CLOG-scanning mechanism at hot standby startup in the presence of
+    prepared transactions, with subtransactions.
+    """
+
+    # Initialize the primary, a test table, and a helper function to create lots
+    # of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(
+        branch_name="main", endpoint_id="primary", config_lines=["max_prepared_transactions=5"]
+    )
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+
+    # Install extension containing function needed for test
+    primary_cur.execute("CREATE EXTENSION neon_test_utils")
+
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+
+    # Advance nextXid close to the beginning of the next pg_subtrans segment (2^16 XIDs)
+    #
+    # This is interesting, because it tests that pg_subtrans is initialized correctly
+    # at standby startup. (We had a bug where it didn't at one point during development.)
+    while True:
+        xid = int(query_scalar(primary_cur, "SELECT txid_current()"))
+        log.info(f"xid now {xid}")
+        # Consume 500 transactions at a time until we get close
+        if xid < 65535 - 600:
+            primary_cur.execute("select test_consume_xids(500);")
+        else:
+            break
+    primary_cur.execute("checkpoint")
+
+    # Prepare a transaction for two-phase commit
+    primary_cur.execute("begin")
+    primary_cur.execute("select create_subxacts(1000)")
+    primary_cur.execute("prepare transaction 't1'")
+
+    # Wait for the WAL to be flushed, and stop the primary
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+    primary.stop(mode="fast")
+
+    # Create a replica. It should start up normally, thanks to the CLOG-scanning
+    # mechanism.
+    secondary = env.endpoints.new_replica_start(
+        origin=primary, endpoint_id="secondary", config_lines=["max_prepared_transactions=5"]
+    )
+
+    # The transaction did not commit, so it should not be visible in the secondary
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (0,)
+
+    primary.start()
+
+    # Open a lot of subtransactions in the primary, causing the subxids cache to overflow
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("select create_subxacts(100000)")
+
+    wait_replica_caughtup(primary, secondary)
+
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (100000,)
+
+    primary_cur.execute("commit prepared 't1'")
+
+    wait_replica_caughtup(primary, secondary)
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (101000,)
+
+
+def test_replica_start_with_prepared_xacts_with_many_subxacts(neon_simple_env: NeonEnv):
+    """
+    Test the CLOG-scanning mechanism at hot standby startup in the presence of
+    prepared transactions, with lots of subtransactions.
+
+    Like test_replica_start_with_prepared_xacts_with_subxacts, but with more
+    subxacts, to test that the prepared transaction's subxids don't consume
+    space in the known-assigned XIDs array. (They are set in pg_subtrans
+    instead)
+    """
+
+    # Initialize the primary, a test table, and a helper function to create lots
+    # of subtransactions.
+    env = neon_simple_env
+    primary = env.endpoints.create_start(
+        branch_name="main", endpoint_id="primary", config_lines=["max_prepared_transactions=5"]
+    )
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+
+    # Install extension containing function needed for test
+    primary_cur.execute("CREATE EXTENSION neon_test_utils")
+
+    primary_cur.execute("create table t(pk serial primary key, payload integer)")
+    primary_cur.execute(CREATE_SUBXACTS_FUNC)
+
+    # Prepare a transaction for two-phase commit, with lots of subxids
+    primary_cur.execute("begin")
+    primary_cur.execute("select create_subxacts(50000)")
+
+    # to make things a bit more varied, intersperse a few other XIDs in between
+    # the prepared transaction's sub-XIDs
+    with primary.connect().cursor() as primary_cur2:
+        primary_cur2.execute("insert into t (payload) values (123)")
+        primary_cur2.execute("begin; insert into t (payload) values (-1); rollback")
+
+    primary_cur.execute("select create_subxacts(50000)")
+    primary_cur.execute("prepare transaction 't1'")
+
+    # Wait for the WAL to be flushed
+    wait_for_last_flush_lsn(env, primary, env.initial_tenant, env.initial_timeline)
+
+    primary.stop(mode="fast")
+
+    # Create a replica. It should start up normally, thanks to the CLOG-scanning
+    # mechanism.
+    secondary = env.endpoints.new_replica_start(
+        origin=primary, endpoint_id="secondary", config_lines=["max_prepared_transactions=5"]
+    )
+
+    # The transaction did not commit, so it should not be visible in the secondary
+    secondary_conn = secondary.connect()
+    secondary_cur = secondary_conn.cursor()
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (1,)
+
+    primary.start()
+
+    # Open a lot of subtransactions in the primary, causing the subxids cache to overflow
+    primary_conn = primary.connect()
+    primary_cur = primary_conn.cursor()
+    primary_cur.execute("select create_subxacts(100000)")
+
+    wait_replica_caughtup(primary, secondary)
+
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (100001,)
+
+    primary_cur.execute("commit prepared 't1'")
+
+    wait_replica_caughtup(primary, secondary)
+    secondary_cur.execute("select count(*) from t")
+    assert secondary_cur.fetchone() == (200001,)

test_runner/regress/test_replication_start.py

@@ -1,32 +0,0 @@
-import pytest
-from fixtures.log_helper import log
-from fixtures.neon_fixtures import NeonEnv, wait_replica_caughtup
-
-
-@pytest.mark.xfail
-def test_replication_start(neon_simple_env: NeonEnv):
-    env = neon_simple_env
-
-    with env.endpoints.create_start(branch_name="main", endpoint_id="primary") as primary:
-        with primary.connect() as p_con:
-            with p_con.cursor() as p_cur:
-                p_cur.execute("begin")
-                p_cur.execute("create table t(pk integer primary key, payload integer)")
-                p_cur.execute("insert into t values (generate_series(1,100000), 0)")
-                p_cur.execute("select txid_current()")
-                xid = p_cur.fetchall()[0][0]
-                log.info(f"Master transaction {xid}")
-                with env.endpoints.new_replica_start(
-                    origin=primary, endpoint_id="secondary"
-                ) as secondary:
-                    wait_replica_caughtup(primary, secondary)
-                    with secondary.connect() as s_con:
-                        with s_con.cursor() as s_cur:
-                            # Enforce setting hint bits for pg_class tuples.
-                            # If master's transaction is not marked as in-progress in MVCC snapshot,
-                            # then XMIN_INVALID hint bit will be set for table's 't' tuple makeing it invisible.
-                            s_cur.execute("select * from pg_class")
-                            p_cur.execute("commit")
-                            wait_replica_caughtup(primary, secondary)
-                            s_cur.execute("select * from t where pk = 1")
-                            assert s_cur.fetchone() == (1, 0)

vendor/revisions.json

@@ -1,5 +1,5 @@
 {
-  "v16": ["16.3", "e06bebc75306b583e758b52c95946d41109239b2"],
-  "v15": ["15.7", "f54d7373eb0de5a54bce2becdb1c801026c7edff"],
-  "v14": ["14.12", "223dd925959f8124711dd3d867dc8ba6629d52c0"]
+  "v16": ["16.3", "b810fdfcbb59afea7ea7bbe0cf94eaccb55a2ea2"],
+  "v15": ["15.7", "4874c8e52ed349a9f8290bbdcd91eb92677a5d24"],
+  "v14": ["14.12", "ad73770c446ea361f43e4f0404798b7e5e7a62d8"]
 }