Fix waiting in RegisterSyncRequest().

If we run out of space in the checkpointer sync request queue (which is
hopefully rare on real systems, but common with very small buffer pool),
we wait for it to drain.  While waiting, we should report that as a wait
event so that users know what is going on, and also handle postmaster
death, since otherwise the loop might never terminate if the
checkpointer has exited.

Back-patch to 12.  Although the problem exists in earlier releases too,
the code is structured differently before 12 so I haven't gone any
further for now, in the absence of field complaints.

Reported-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220226213942.nb7uvb2pamyu26dj%40alap3.anarazel.de

doc/src/sgml/monitoring.sgml

@@ -1483,7 +1483,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry>Waiting for confirmation from remote server during synchronous replication.</entry>
         </row>
         <row>
-         <entry morerows="2"><literal>Timeout</literal></entry>
+         <entry morerows="3"><literal>Timeout</literal></entry>
          <entry><literal>BaseBackupThrottle</literal></entry>
          <entry>Waiting during base backup when throttling activity.</entry>
         </row>
@@ -1495,6 +1495,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>RecoveryApplyDelay</literal></entry>
          <entry>Waiting to apply WAL at recovery because it is delayed.</entry>
         </row>
+        <row>
+         <entry><literal>RegisterSyncRequest</literal></entry>
+         <entry>Waiting while sending synchronization requests to the
+         checkpointer, because the request queue is full.</entry>
+        </row>
         <row>
          <entry morerows="67"><literal>IO</literal></entry>
          <entry><literal>BufFileRead</literal></entry>

src/backend/postmaster/pgstat.c

@@ -3889,6 +3889,9 @@ pgstat_get_wait_timeout(WaitEventTimeout w)
 		case WAIT_EVENT_RECOVERY_APPLY_DELAY:
 			event_name = "RecoveryApplyDelay";
 			break;
+		case WAIT_EVENT_REGISTER_SYNC_REQUEST:
+			event_name = "RegisterSyncRequest";
+			break;
 			/* no default case, so that compiler will warn */
 	}
 

src/backend/storage/sync/sync.c

@@ -27,6 +27,7 @@
 #include "postmaster/bgwriter.h"
 #include "storage/bufmgr.h"
 #include "storage/ipc.h"
+#include "storage/latch.h"
 #include "storage/md.h"
 #include "utils/hsearch.h"
 #include "utils/memutils.h"
@@ -569,7 +570,8 @@ RegisterSyncRequest(const FileTag *ftag, SyncRequestType type,
 		if (ret || (!ret && !retryOnError))
 			break;
 
-		pg_usleep(10000L);
+		WaitLatch(NULL, WL_EXIT_ON_PM_DEATH | WL_TIMEOUT, 10,
+				  WAIT_EVENT_REGISTER_SYNC_REQUEST);
 	}
 
 	return ret;

src/include/pgstat.h

@@ -867,7 +867,8 @@ typedef enum
 {
 	WAIT_EVENT_BASE_BACKUP_THROTTLE = PG_WAIT_TIMEOUT,
 	WAIT_EVENT_PG_SLEEP,
-	WAIT_EVENT_RECOVERY_APPLY_DELAY
+	WAIT_EVENT_RECOVERY_APPLY_DELAY,
+	WAIT_EVENT_REGISTER_SYNC_REQUEST
 } WaitEventTimeout;
 
 /* ----------