propagate lock guard to background deletion task (#4495)

## Problem

1. During the rollout we got a panic: "timeline that we were deleting
was concurrently removed from 'timelines' map" that was caused by lock
guard not being propagated to the background part of the deletion.
Existing test didnt catch it because failpoint that was used for
verification was placed earlier prior to background task spawning.
2. When looking at surrounding code one more bug was detected. We
removed timeline from the map before deletion is finished, which breaks
client retry logic, because it will indicate 404 before actual deletion
is completed which can lead to client stopping its retry poll earlier.

## Summary of changes

1. Carry the lock guard over to background deletion. Ensure existing
test case fails without applied patch (second deletion becomes stuck
without it, which eventually leads to a test failure).
2. Move delete_all call earlier so timeline is removed from the map is
the last thing done during deletion.

Additionally I've added timeline_id to the `update_gc_info` span,
because `debug_assert_current_span_has_tenant_and_timeline_id` in
`download_remote_layer` was firing when `update_gc_info` lead to
on-demand downloads via `find_lsn_for_timestamp` (caught by @problame).
This is not directly related to the PR but fixes possible flakiness.

Another smaller set of changes involves deletion wrapper used in python
tests. Now there is a simpler wrapper that waits for deletions to
complete `timeline_delete_wait_completed`. Most of the
test_delete_timeline.py tests make negative tests, i.e., "does
ps_http.timeline_delete() fail in this and that scenario".
These can be left alone. Other places when we actually do the deletions,
we need to use the helper that polls for completion.

Discussion
https://neondb.slack.com/archives/C03F5SM1N02/p1686668007396639

resolves #4496

---------

Co-authored-by: Christian Schwarz <christian@neon.tech>

pageserver/src/tenant.rs

@@ -473,6 +473,14 @@ pub(crate) enum ShutdownError {
     AlreadyStopping,
 }
 
+struct DeletionGuard(OwnedMutexGuard<bool>);
+
+impl DeletionGuard {
+    fn is_deleted(&self) -> bool {
+        *self.0
+    }
+}
+
 impl Tenant {
     /// Yet another helper for timeline initialization.
     /// Contains the common part of `load_local_timeline` and `load_remote_timeline`.
@@ -1138,7 +1146,11 @@ impl Tenant {
                                 )
                                 .context("create_timeline_struct")?;
 
-                            let guard = Arc::clone(&timeline.delete_lock).lock_owned().await;
+                            let guard = DeletionGuard(
+                                Arc::clone(&timeline.delete_lock)
+                                    .try_lock_owned()
+                                    .expect("cannot happen because we're the only owner"),
+                            );
 
                             // Note: here we even skip populating layer map. Timeline is essentially uninitialized.
                             // RemoteTimelineClient is the only functioning part.
@@ -1549,6 +1561,7 @@ impl Tenant {
         &self,
         timeline_id: TimelineId,
         timeline: Arc<Timeline>,
+        guard: DeletionGuard,
     ) -> anyhow::Result<()> {
         {
             // Grab the layer_removal_cs lock, and actually perform the deletion.
@@ -1621,6 +1634,25 @@ impl Tenant {
             Err(anyhow::anyhow!("failpoint: timeline-delete-after-rm"))?
         });
 
+        if let Some(remote_client) = &timeline.remote_client {
+            remote_client.delete_all().await.context("delete_all")?
+        };
+
+        // Have a failpoint that can use the `pause` failpoint action.
+        // We don't want to block the executor thread, hence, spawn_blocking + await.
+        if cfg!(feature = "testing") {
+            tokio::task::spawn_blocking({
+                let current = tracing::Span::current();
+                move || {
+                    let _entered = current.entered();
+                    tracing::info!("at failpoint in_progress_delete");
+                    fail::fail_point!("in_progress_delete");
+                }
+            })
+            .await
+            .expect("spawn_blocking");
+        }
+
         {
             // Remove the timeline from the map.
             let mut timelines = self.timelines.lock().unwrap();
@@ -1641,12 +1673,7 @@ impl Tenant {
             drop(timelines);
         }
 
-        let remote_client = match &timeline.remote_client {
-            Some(remote_client) => remote_client,
-            None => return Ok(()),
-        };
-
-        remote_client.delete_all().await?;
+        drop(guard);
 
         Ok(())
     }
@@ -1694,23 +1721,18 @@ impl Tenant {
             timeline = Arc::clone(timeline_entry.get());
 
             // Prevent two tasks from trying to delete the timeline at the same time.
-            //
-            // XXX: We should perhaps return an HTTP "202 Accepted" to signal that the caller
-            // needs to poll until the operation has finished. But for now, we return an
-            // error, because the control plane knows to retry errors.
-
             delete_lock_guard =
-                Arc::clone(&timeline.delete_lock)
-                    .try_lock_owned()
-                    .map_err(|_| {
+                DeletionGuard(Arc::clone(&timeline.delete_lock).try_lock_owned().map_err(
+                    |_| {
                         DeleteTimelineError::Other(anyhow::anyhow!(
                             "timeline deletion is already in progress"
                         ))
-                    })?;
+                    },
+                )?);
 
             // If another task finished the deletion just before we acquired the lock,
             // return success.
-            if *delete_lock_guard {
+            if delete_lock_guard.is_deleted() {
                 return Ok(());
             }
 
@@ -1784,7 +1806,7 @@ impl Tenant {
         self: Arc<Self>,
         timeline_id: TimelineId,
         timeline: Arc<Timeline>,
-        _guard: OwnedMutexGuard<bool>,
+        guard: DeletionGuard,
     ) {
         let tenant_id = self.tenant_id;
         let timeline_clone = Arc::clone(&timeline);
@@ -1797,7 +1819,7 @@ impl Tenant {
             "timeline_delete",
             false,
             async move {
-                if let Err(err) = self.delete_timeline(timeline_id, timeline).await {
+                if let Err(err) = self.delete_timeline(timeline_id, timeline, guard).await {
                     error!("Error: {err:#}");
                     timeline_clone.set_broken(err.to_string())
                 };

pageserver/src/tenant/remote_timeline_client.rs

@@ -753,22 +753,18 @@ impl RemoteTimelineClient {
 
         // Have a failpoint that can use the `pause` failpoint action.
         // We don't want to block the executor thread, hence, spawn_blocking + await.
-        #[cfg(feature = "testing")]
-        tokio::task::spawn_blocking({
-            let current = tracing::Span::current();
-            move || {
-                let _entered = current.entered();
-                tracing::info!(
-                    "at failpoint persist_index_part_with_deleted_flag_after_set_before_upload_pause"
-                );
-                fail::fail_point!(
-                    "persist_index_part_with_deleted_flag_after_set_before_upload_pause"
-                );
-            }
-        })
-        .await
-        .expect("spawn_blocking");
-
+        if cfg!(feature = "testing") {
+            tokio::task::spawn_blocking({
+                let current = tracing::Span::current();
+                move || {
+                    let _entered = current.entered();
+                    tracing::info!("at failpoint persist_deleted_index_part");
+                    fail::fail_point!("persist_deleted_index_part");
+                }
+            })
+            .await
+            .expect("spawn_blocking");
+        }
         upload::upload_index_part(
             self.conf,
             &self.storage_impl,

pageserver/src/tenant/timeline.rs

@@ -3791,6 +3791,7 @@ impl Timeline {
     /// for example. The caller should hold `Tenant::gc_cs` lock to ensure
     /// that.
     ///
+    #[instrument(skip_all, fields(timline_id=%self.timeline_id))]
     pub(super) async fn update_gc_info(
         &self,
         retain_lsns: Vec<Lsn>,