Clarify what is protected by WaitLSNLock

Not just WaitLSNState.waitersHeap, but also WaitLSNState.procInfos and
updating of WaitLSNState.minWaitedLSN is protected by WaitLSNLock.  There
is one now documented exclusion on fast-path checking of
WaitLSNProcInfo.inHeap flag.

Discussion: https://postgr.es/m/202404030658.hhj3vfxeyhft%40alvherre.pgsql

1 files changed, 5 insertions, 2 deletions

diff --git a/src/include/commands/waitlsn.h b/src/include/commands/waitlsn.h
index b3d9eed64d8..da17b8be6f9 100644
--- a/src/include/commands/waitlsn.h
+++ b/src/include/commands/waitlsn.h
@@ -49,7 +49,7 @@ typedef struct WaitLSNState
 	/*
 	 * The minimum LSN value some process is waiting for.  Used for the
 	 * fast-path checking if we need to wake up any waiters after replaying a
-	 * WAL record.
+	 * WAL record.  Could be read lock-less.  Update protected by WaitLSNLock.
 	 */
 	pg_atomic_uint64 minWaitedLSN;
 
@@ -59,7 +59,10 @@ typedef struct WaitLSNState
 	 */
 	pairingheap waitersHeap;
 
-	/* An array with per-process information, indexed by the process number */
+	/*
+	 * An array with per-process information, indexed by the process number.
+	 * Protected by WaitLSNLock.
+	 */
 	WaitLSNProcInfo procInfos[FLEXIBLE_ARRAY_MEMBER];
 } WaitLSNState;