pageserver: lift ancestor timeline logic from read path (#6543)

When the read path needs to follow a key into the ancestor timeline, it
needs to wait for said ancestor to become active and aware of it's
branching lsn. The logic is lifted into a separate function with it's
own new error type.

This is done because the vectored read path needs the same logic. It's
also the reason for the newly introduced error type.

When we'll switch the read path to proxy into `get_vectored`, we can
remove the duplicated variants from `PageReconstructError`.

pageserver/src/tenant/timeline.rs

@@ -457,6 +457,21 @@ pub(crate) enum GetVectoredError {
     InvalidLsn(Lsn),
 }
 
+#[derive(thiserror::Error, Debug)]
+pub(crate) enum GetReadyAncestorError {
+    #[error("ancestor timeline {0} is being stopped")]
+    AncestorStopping(TimelineId),
+
+    #[error("Ancestor LSN wait error: {0}")]
+    AncestorLsnTimeout(#[from] WaitLsnError),
+
+    #[error("Cancelled")]
+    Cancelled,
+
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}
+
 #[derive(Clone, Copy)]
 pub enum LogicalSizeCalculationCause {
     Initial,
@@ -535,6 +550,18 @@ impl From<GetVectoredError> for CreateImageLayersError {
     }
 }
 
+impl From<GetReadyAncestorError> for PageReconstructError {
+    fn from(e: GetReadyAncestorError) -> Self {
+        use GetReadyAncestorError::*;
+        match e {
+            AncestorStopping(tid) => PageReconstructError::AncestorStopping(tid),
+            AncestorLsnTimeout(wait_err) => PageReconstructError::AncestorLsnTimeout(wait_err),
+            Cancelled => PageReconstructError::Cancelled,
+            Other(other) => PageReconstructError::Other(other),
+        }
+    }
+}
+
 /// Public interface functions
 impl Timeline {
     /// Get the LSN where this branch was created
@@ -2400,60 +2427,8 @@ impl Timeline {
                     timeline.ancestor_lsn,
                     cont_lsn
                 );
-                let ancestor = match timeline.get_ancestor_timeline() {
-                    Ok(timeline) => timeline,
-                    Err(e) => return Err(PageReconstructError::from(e)),
-                };
 
-                // It's possible that the ancestor timeline isn't active yet, or
-                // is active but hasn't yet caught up to the branch point. Wait
-                // for it.
-                //
-                // This cannot happen while the pageserver is running normally,
-                // because you cannot create a branch from a point that isn't
-                // present in the pageserver yet. However, we don't wait for the
-                // branch point to be uploaded to cloud storage before creating
-                // a branch. I.e., the branch LSN need not be remote consistent
-                // for the branching operation to succeed.
-                //
-                // Hence, if we try to load a tenant in such a state where
-                // 1. the existence of the branch was persisted (in IndexPart and/or locally)
-                // 2. but the ancestor state is behind branch_lsn because it was not yet persisted
-                // then we will need to wait for the ancestor timeline to
-                // re-stream WAL up to branch_lsn before we access it.
-                //
-                // How can a tenant get in such a state?
-                // - ungraceful pageserver process exit
-                // - detach+attach => this is a bug, https://github.com/neondatabase/neon/issues/4219
-                //
-                // NB: this could be avoided by requiring
-                //   branch_lsn >= remote_consistent_lsn
-                // during branch creation.
-                match ancestor.wait_to_become_active(ctx).await {
-                    Ok(()) => {}
-                    Err(TimelineState::Stopping) => {
-                        return Err(PageReconstructError::AncestorStopping(ancestor.timeline_id));
-                    }
-                    Err(state) => {
-                        return Err(PageReconstructError::Other(anyhow::anyhow!(
-                            "Timeline {} will not become active. Current state: {:?}",
-                            ancestor.timeline_id,
-                            &state,
-                        )));
-                    }
-                }
-                ancestor
-                    .wait_lsn(timeline.ancestor_lsn, ctx)
-                    .await
-                    .map_err(|e| match e {
-                        e @ WaitLsnError::Timeout(_) => PageReconstructError::AncestorLsnTimeout(e),
-                        WaitLsnError::Shutdown => PageReconstructError::Cancelled,
-                        e @ WaitLsnError::BadState => {
-                            PageReconstructError::Other(anyhow::anyhow!(e))
-                        }
-                    })?;
-
-                timeline_owned = ancestor;
+                timeline_owned = timeline.get_ready_ancestor_timeline(ctx).await?;
                 timeline = &*timeline_owned;
                 prev_lsn = Lsn(u64::MAX);
                 continue 'outer;
@@ -2583,6 +2558,66 @@ impl Timeline {
         Some((lsn, img))
     }
 
+    async fn get_ready_ancestor_timeline(
+        &self,
+        ctx: &RequestContext,
+    ) -> Result<Arc<Timeline>, GetReadyAncestorError> {
+        let ancestor = match self.get_ancestor_timeline() {
+            Ok(timeline) => timeline,
+            Err(e) => return Err(GetReadyAncestorError::from(e)),
+        };
+
+        // It's possible that the ancestor timeline isn't active yet, or
+        // is active but hasn't yet caught up to the branch point. Wait
+        // for it.
+        //
+        // This cannot happen while the pageserver is running normally,
+        // because you cannot create a branch from a point that isn't
+        // present in the pageserver yet. However, we don't wait for the
+        // branch point to be uploaded to cloud storage before creating
+        // a branch. I.e., the branch LSN need not be remote consistent
+        // for the branching operation to succeed.
+        //
+        // Hence, if we try to load a tenant in such a state where
+        // 1. the existence of the branch was persisted (in IndexPart and/or locally)
+        // 2. but the ancestor state is behind branch_lsn because it was not yet persisted
+        // then we will need to wait for the ancestor timeline to
+        // re-stream WAL up to branch_lsn before we access it.
+        //
+        // How can a tenant get in such a state?
+        // - ungraceful pageserver process exit
+        // - detach+attach => this is a bug, https://github.com/neondatabase/neon/issues/4219
+        //
+        // NB: this could be avoided by requiring
+        //   branch_lsn >= remote_consistent_lsn
+        // during branch creation.
+        match ancestor.wait_to_become_active(ctx).await {
+            Ok(()) => {}
+            Err(TimelineState::Stopping) => {
+                return Err(GetReadyAncestorError::AncestorStopping(
+                    ancestor.timeline_id,
+                ));
+            }
+            Err(state) => {
+                return Err(GetReadyAncestorError::Other(anyhow::anyhow!(
+                    "Timeline {} will not become active. Current state: {:?}",
+                    ancestor.timeline_id,
+                    &state,
+                )));
+            }
+        }
+        ancestor
+            .wait_lsn(self.ancestor_lsn, ctx)
+            .await
+            .map_err(|e| match e {
+                e @ WaitLsnError::Timeout(_) => GetReadyAncestorError::AncestorLsnTimeout(e),
+                WaitLsnError::Shutdown => GetReadyAncestorError::Cancelled,
+                e @ WaitLsnError::BadState => GetReadyAncestorError::Other(anyhow::anyhow!(e)),
+            })?;
+
+        Ok(ancestor)
+    }
+
     fn get_ancestor_timeline(&self) -> anyhow::Result<Arc<Timeline>> {
         let ancestor = self.ancestor_timeline.as_ref().with_context(|| {
             format!(