fix: resubscribe while holding the lock

pageserver/src/tenant.rs

@@ -1450,7 +1450,7 @@ impl Tenant {
         // need to synchronize and cannot be original semi-lockless algorithm for "upload
         // indexpart" part, we create a single task to delete the timeline.
 
-        let rx = {
+        let mut rx = {
             let mut g = timeline.delete_self.lock().await;
             let maybe_rx = if let Some(maybe_done) = g.as_ref() {
                 use timeline::MaybeDone;
@@ -1523,12 +1523,15 @@ impl Tenant {
                 *g = Some(timeline::MaybeDone::Pending(Arc::downgrade(&rx)));
                 rx
             }
+            // subscribe while the mutex is held, so we are guaranteed to receive the message,
+            // unless a panic comes.
+            .resubscribe()
         };
 
         {
             use tokio::sync::broadcast::error::RecvError;
 
-            match rx.resubscribe().recv().await {
+            match rx.recv().await {
                 Ok(Ok(())) => Ok(()),
                 Ok(Err(e)) => Err(DeleteTimelineError::from(&e)),
                 // lagged doesn't mean anything with 1 send, but whatever, handle it the same