pgserver: add immutable layer map manager

Signed-off-by: Alex Chi <iskyzh@gmail.com>

Cargo.lock

@@ -110,6 +110,12 @@ dependencies = [
  "backtrace",
 ]
 
+[[package]]
+name = "arc-swap"
+version = "1.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bddcadddf5e9015d310179a59bb28c4d4b9920ad0f11e8e14dbadf654890c9a6"
+
 [[package]]
 name = "archery"
 version = "0.5.0"
@@ -2542,6 +2548,7 @@ name = "pageserver"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "arc-swap",
  "async-stream",
  "async-trait",
  "byteorder",

Cargo.toml

@@ -32,6 +32,7 @@ license = "Apache-2.0"
 ## All dependency versions, used in the project
 [workspace.dependencies]
 anyhow = { version = "1.0", features = ["backtrace"] }
+arc-swap = "1.6"
 async-stream = "0.3"
 async-trait = "0.1"
 atty = "0.2.14"

pageserver/Cargo.toml

@@ -12,6 +12,7 @@ testing = ["fail/failpoints"]
 
 [dependencies]
 anyhow.workspace = true
+arc-swap.workspace = true
 async-stream.workspace = true
 async-trait.workspace = true
 byteorder.workspace = true

pageserver/src/tenant.rs

@@ -87,6 +87,7 @@ pub mod disk_btree;
 pub(crate) mod ephemeral_file;
 pub mod layer_map;
 pub mod manifest;
+pub mod layer_map_mgr;
 
 pub mod metadata;
 mod par_fsync;

pageserver/src/tenant/layer_map.rs

@@ -66,7 +66,7 @@ use super::storage_layer::PersistentLayerDesc;
 ///
 /// LayerMap tracks what layers exist on a timeline.
 ///
-#[derive(Default)]
+#[derive(Default, Clone)]
 pub struct LayerMap {
     //
     // 'open_layer' holds the current InMemoryLayer that is accepting new

pageserver/src/tenant/layer_map/historic_layer_coverage.rs

@@ -72,6 +72,7 @@ impl From<&PersistentLayerDesc> for LayerKey {
 /// Allows answering layer map queries very efficiently,
 /// but doesn't allow retroactive insertion, which is
 /// sometimes necessary. See BufferedHistoricLayerCoverage.
+#[derive(Clone)]
 pub struct HistoricLayerCoverage<Value> {
     /// The latest state
     head: LayerCoverageTuple<Value>,
@@ -425,6 +426,7 @@ fn test_persistent_overlapping() {
 ///
 /// See this for more on persistent and retroactive techniques:
 /// https://www.youtube.com/watch?v=WqCWghETNDc&t=581s
+#[derive(Clone)]
 pub struct BufferedHistoricLayerCoverage<Value> {
     /// A persistent layer map that we rebuild when we need to retroactively update
     historic_coverage: HistoricLayerCoverage<Value>,

pageserver/src/tenant/layer_map/layer_coverage.rs

@@ -15,6 +15,7 @@ use rpds::RedBlackTreeMapSync;
 ///
 /// NOTE The struct is parameterized over Value for easier
 ///      testing, but in practice it's some sort of layer.
+#[derive(Clone)]
 pub struct LayerCoverage<Value> {
     /// For every change in coverage (as we sweep the key space)
     /// we store (lsn.end, value).
@@ -139,6 +140,7 @@ impl<Value: Clone> LayerCoverage<Value> {
 }
 
 /// Image and delta coverage at a specific LSN.
+#[derive(Clone)]
 pub struct LayerCoverageTuple<Value> {
     pub image_coverage: LayerCoverage<Value>,
     pub delta_coverage: LayerCoverage<Value>,

pageserver/src/tenant/layer_map_mgr.rs

@@ -0,0 +1,146 @@
+//! This module implements `LayerMapMgr`, which manages a layer map object and provides lock-free access to the state.
+//!
+//! A common usage pattern is as follows:
+//!
+//! ```ignore
+//! async fn compaction(&self) {
+//!     // Get the current state.
+//!     let state = self.layer_map_mgr.read();
+//!     // No lock held at this point. Do compaction based on the state. This part usually incurs I/O operations and may
+//!     // take a long time.
+//!     let compaction_result = self.do_compaction(&state).await?;
+//!     // Update the state.
+//!     self.layer_map_mgr.update(|mut state| async move {
+//!         // do updates to the state, return it.
+//!         Ok(state)
+//!     }).await?;
+//! }
+//! ```
+use anyhow::Result;
+use arc_swap::ArcSwap;
+use futures::Future;
+use std::sync::Arc;
+
+use super::layer_map::LayerMap;
+
+/// Manages the storage state. Provide utility functions to modify the layer map and get an immutable reference to the
+/// layer map.
+pub struct LayerMapMgr {
+    layer_map: ArcSwap<LayerMap>,
+    state_lock: tokio::sync::Mutex<()>,
+}
+
+impl LayerMapMgr {
+    /// Get the current state of the layer map.
+    pub fn read(&self) -> Arc<LayerMap> {
+        // TODO: it is possible to use `load` to reduce the overhead of cloning the Arc, but read path usually involves
+        // disk reads and layer mapping fetching, and therefore it's not a big deal to use a more optimized version
+        // here.
+        self.layer_map.load_full()
+    }
+
+    /// Clone the layer map for modification.
+    fn clone_for_write(&self, _state_lock_witness: &tokio::sync::MutexGuard<'_, ()>) -> LayerMap {
+        (**self.layer_map.load()).clone()
+    }
+
+    pub fn new(layer_map: LayerMap) -> Self {
+        Self {
+            layer_map: ArcSwap::new(Arc::new(layer_map)),
+            state_lock: tokio::sync::Mutex::new(()),
+        }
+    }
+
+    /// Update the layer map.
+    pub async fn update<O, F>(&self, operation: O) -> Result<()>
+    where
+        O: FnOnce(LayerMap) -> F,
+        F: Future<Output = Result<LayerMap>>,
+    {
+        let state_lock = self.state_lock.lock().await;
+        let state = self.clone_for_write(&state_lock);
+        let new_state = operation(state).await?;
+        self.layer_map.store(Arc::new(new_state));
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use utils::{
+        id::{TenantId, TimelineId},
+        lsn::Lsn,
+    };
+
+    use crate::{repository::Key, tenant::storage_layer::PersistentLayerDesc};
+
+    use super::*;
+
+    #[tokio::test]
+    async fn test_layer_map_manage() -> Result<()> {
+        let mgr = LayerMapMgr::new(Default::default());
+        mgr.update(|mut map| async move {
+            let mut updates = map.batch_update();
+            updates.insert_historic(PersistentLayerDesc::new_img(
+                TenantId::generate(),
+                TimelineId::generate(),
+                Key::from_i128(0)..Key::from_i128(1),
+                Lsn(0),
+                false,
+                0,
+            ));
+            updates.flush();
+            Ok(map)
+        })
+        .await?;
+
+        let ref_1 = mgr.read();
+
+        mgr.update(|mut map| async move {
+            let mut updates = map.batch_update();
+            updates.insert_historic(PersistentLayerDesc::new_img(
+                TenantId::generate(),
+                TimelineId::generate(),
+                Key::from_i128(1)..Key::from_i128(2),
+                Lsn(0),
+                false,
+                0,
+            ));
+            updates.flush();
+            Ok(map)
+        })
+        .await?;
+
+        let ref_2 = mgr.read();
+
+        // Modification should not be visible to the old reference.
+        assert_eq!(
+            ref_1
+                .search(Key::from_i128(0), Lsn(1))
+                .unwrap()
+                .layer
+                .key_range,
+            Key::from_i128(0)..Key::from_i128(1)
+        );
+        assert!(ref_1.search(Key::from_i128(1), Lsn(1)).is_none());
+
+        // Modification should be visible to the new reference.
+        assert_eq!(
+            ref_2
+                .search(Key::from_i128(0), Lsn(1))
+                .unwrap()
+                .layer
+                .key_range,
+            Key::from_i128(0)..Key::from_i128(1)
+        );
+        assert_eq!(
+            ref_2
+                .search(Key::from_i128(1), Lsn(1))
+                .unwrap()
+                .layer
+                .key_range,
+            Key::from_i128(1)..Key::from_i128(2)
+        );
+        Ok(())
+    }
+}