pageserver: tighten compaction failure detection (#10502)

## Problem

If compaction fails, we disable L0 flush stalls to avoid persistent
stalls. However, the logic would unset the failure marker on offload
failures or shutdown. This can lead to sudden L0 flush stalls if we try
and fail to offload a timeline with compaction failures, or if there is
some kind of shutdown race.

Touches #10405.

## Summary of changes

Don't touch the compaction failure marker on offload failures or
shutdown.

pageserver/src/tenant/timeline.rs

@@ -1704,14 +1704,16 @@ impl Timeline {
         };
 
         // Signal compaction failure to avoid L0 flush stalls when it's broken.
-        let compaction_failed = match result {
-            Ok(_) => false,
-            Err(CompactionError::Offload(_)) => false, // doesn't halt compaction
-            Err(CompactionError::ShuttingDown) => false, // not a failure
-            Err(CompactionError::Other(_)) => true,
+        match result {
+            Ok(_) => self.compaction_failed.store(false, AtomicOrdering::Relaxed),
+            Err(CompactionError::Other(_)) => {
+                self.compaction_failed.store(true, AtomicOrdering::Relaxed)
+            }
+            // Don't change the current value on offload failure or shutdown. We don't want to
+            // abruptly stall nor resume L0 flushes in these cases.
+            Err(CompactionError::Offload(_)) => {}
+            Err(CompactionError::ShuttingDown) => {}
         };
-        self.compaction_failed
-            .store(compaction_failed, AtomicOrdering::Relaxed);
 
         result
     }