Re: pgsql: Implement WAIT FOR command

[Heikki Linnakangas]

On 05/11/2025 11:44, Alexander Korotkov wrote:
Implement WAIT FOR command

Happened to just spot this:

@@ -1831,20 +1832,30 @@ PerformWalRecovery(void)
                         */
                        ApplyWalRecord(xlogreader, record, &replayTLI);
 
 			/* Exit loop if we reached inclusive recovery target */
                        if (recoveryStopsAfter(xlogreader))
                        {
                                reachedRecoveryTarget = true;
                                break;
                        }
 
+			/*
+                        * If we replayed an LSN that someone was waiting for 
then walk
+                        * over the shared memory array and set latches to 
notify the
+                        * waiters.
+                        */
+                       if (waitLSNState &&
+                               (XLogRecoveryCtl->lastReplayedEndRecPtr >=
+                                
pg_atomic_read_u64(&waitLSNState->minWaitedLSN[WAIT_LSN_TYPE_REPLAY])))
+                               WaitLSNWakeup(WAIT_LSN_TYPE_REPLAY, 
XLogRecoveryCtl->lastReplayedEndRecPtr);
+
                        /* Else, try to fetch the next WAL record */
                        record = ReadRecord(xlogprefetcher, LOG, false, 
replayTLI);
                } while (record != NULL);
 
 		/*
                 * end of main redo apply loop
                 */
 
 		if (reachedRecoveryTarget)
                {

I think that WaitLSNWakeup() call is a little misplaced. It's after the 
recoveryStopsAfter() call, so if recovery is paused right after the 
record, the waiters won't be woken up until recovery is continued. And I 
suppose if recovery is stopped and the server is promoted, the waiters 
will never be woken up, although I didn't verify that.

To fix, that should be moved to between the ApplyWalRecord() and 
recoveryStopsHere() calls.

- Heikki