fix(pageserver): consider tombstones in replorigin (#11752)

## Problem

We didn't consider tombstones in replorigin read path in the past. This
was fine because tombstones are stored as LSN::Invalid before we
universally define what the tombstone is for sparse keyspaces.

Now we remove non-inherited keys during detach ancestor and write the
universal tombstone "empty image". So we need to consider it across all
the read paths.

related: https://github.com/neondatabase/neon/pull/11299

## Summary of changes

Empty value gets ignored for replorigin scans.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/pgdatadir_mapping.rs

@@ -1084,8 +1084,17 @@ impl Timeline {
         let mut result = HashMap::new();
         for (k, v) in kv {
             let v = v?;
+            if v.is_empty() {
+                // This is a tombstone -- we can skip it.
+                // Originally, the replorigin code uses `Lsn::INVALID` to represent a tombstone. However, as it part of
+                // the sparse keyspace and the sparse keyspace uses an empty image to universally represent a tombstone,
+                // we also need to consider that. Such tombstones might be written on the detach ancestor code path to
+                // avoid the value going into the child branch. (See [`crate::tenant::timeline::detach_ancestor::generate_tombstone_image_layer`] for more details.)
+                continue;
+            }
             let origin_id = k.field6 as RepOriginId;
-            let origin_lsn = Lsn::des(&v).unwrap();
+            let origin_lsn = Lsn::des(&v)
+                .with_context(|| format!("decode replorigin value for {}: {v:?}", origin_id))?;
             if origin_lsn != Lsn::INVALID {
                 result.insert(origin_id, origin_lsn);
             }
@@ -2578,6 +2587,11 @@ impl DatadirModification<'_> {
         }
     }
 
+    #[cfg(test)]
+    pub fn put_for_unit_test(&mut self, key: Key, val: Value) {
+        self.put(key, val);
+    }
+
     fn put(&mut self, key: Key, val: Value) {
         if Self::is_data_key(&key) {
             self.put_data(key.to_compact(), val)

pageserver/src/tenant.rs

@@ -5931,7 +5931,9 @@ mod tests {
     use itertools::Itertools;
     #[cfg(feature = "testing")]
     use models::CompactLsnRange;
-    use pageserver_api::key::{AUX_KEY_PREFIX, Key, NON_INHERITED_RANGE, RELATION_SIZE_PREFIX};
+    use pageserver_api::key::{
+        AUX_KEY_PREFIX, Key, NON_INHERITED_RANGE, RELATION_SIZE_PREFIX, repl_origin_key,
+    };
     use pageserver_api::keyspace::KeySpace;
     #[cfg(feature = "testing")]
     use pageserver_api::keyspace::KeySpaceRandomAccum;
@@ -8167,6 +8169,54 @@ mod tests {
         assert_eq!(files.get("pg_logical/mappings/test2"), None);
     }
 
+    #[tokio::test]
+    async fn test_repl_origin_tombstones() {
+        let harness = TenantHarness::create("test_repl_origin_tombstones")
+            .await
+            .unwrap();
+
+        let (tenant, ctx) = harness.load().await;
+        let io_concurrency = IoConcurrency::spawn_for_test();
+
+        let mut lsn = Lsn(0x08);
+
+        let tline: Arc<Timeline> = tenant
+            .create_test_timeline(TIMELINE_ID, lsn, DEFAULT_PG_VERSION, &ctx)
+            .await
+            .unwrap();
+
+        let repl_lsn = Lsn(0x10);
+        {
+            lsn += 8;
+            let mut modification = tline.begin_modification(lsn);
+            modification.put_for_unit_test(repl_origin_key(2), Value::Image(Bytes::new()));
+            modification.set_replorigin(1, repl_lsn).await.unwrap();
+            modification.commit(&ctx).await.unwrap();
+        }
+
+        // we can read everything from the storage
+        let repl_origins = tline
+            .get_replorigins(lsn, &ctx, io_concurrency.clone())
+            .await
+            .unwrap();
+        assert_eq!(repl_origins.len(), 1);
+        assert_eq!(repl_origins[&1], lsn);
+
+        {
+            lsn += 8;
+            let mut modification = tline.begin_modification(lsn);
+            modification.put_for_unit_test(
+                repl_origin_key(3),
+                Value::Image(Bytes::copy_from_slice(b"cannot_decode_this")),
+            );
+            modification.commit(&ctx).await.unwrap();
+        }
+        let result = tline
+            .get_replorigins(lsn, &ctx, io_concurrency.clone())
+            .await;
+        assert!(result.is_err());
+    }
+
     #[tokio::test]
     async fn test_metadata_image_creation() -> anyhow::Result<()> {
         let harness = TenantHarness::create("test_metadata_image_creation").await?;

pageserver/src/tenant/timeline/detach_ancestor.rs

@@ -178,7 +178,7 @@ impl Attempt {
     }
 }
 
-async fn generate_tombstone_image_layer(
+pub(crate) async fn generate_tombstone_image_layer(
     detached: &Arc<Timeline>,
     ancestor: &Arc<Timeline>,
     ancestor_lsn: Lsn,