Add further debug info to help debug 019_replslot_limit.pl failures.

See also afdeff1052. Failures after that commit provided a few more hints,
but not yet enough to understand what's going on.

In 019_replslot_limit.pl shut down nodes with fast instead of immediate mode
if we observe the failure mode. That should tell us whether the failures we're
observing are just a timing issue under high load. PGCTLTIMEOUT should prevent
buildfarm animals from hanging endlessly.

Also adds a bit more logging to replication slot drop and ShutdownPostgres().

Discussion: https://postgr.es/m/20220225192941.hqnvefgdzaro6gzg@alap3.anarazel.de

src/backend/replication/slot.c

@@ -569,6 +569,10 @@ restart:
 		if (!s->in_use)
 			continue;
 
+		/* unlocked read of active_pid is ok for debugging purposes */
+		elog(DEBUG3, "temporary replication slot cleanup: %d in use, active_pid: %d",
+			 i, s->active_pid);
+
 		SpinLockAcquire(&s->mutex);
 		if (s->active_pid == MyProcPid)
 		{
@@ -629,6 +633,9 @@ ReplicationSlotDropPtr(ReplicationSlot *slot)
 	char		path[MAXPGPATH];
 	char		tmppath[MAXPGPATH];
 
+	/* temp debugging aid to analyze 019_replslot_limit failures */
+	elog(DEBUG3, "replication slot drop: %s: begin", NameStr(slot->data.name));
+
 	/*
 	 * If some other backend ran this code concurrently with us, we might try
 	 * to delete a slot with a certain name while someone else was trying to
@@ -679,6 +686,9 @@ ReplicationSlotDropPtr(ReplicationSlot *slot)
 						path, tmppath)));
 	}
 
+	elog(DEBUG3, "replication slot drop: %s: removed on-disk",
+		 NameStr(slot->data.name));
+
 	/*
 	 * The slot is definitely gone.  Lock out concurrent scans of the array
 	 * long enough to kill it.  It's OK to clear the active PID here without
@@ -734,6 +744,9 @@ ReplicationSlotDropPtr(ReplicationSlot *slot)
 	 * a slot while we're still cleaning up the detritus of the old one.
 	 */
 	LWLockRelease(ReplicationSlotAllocationLock);
+
+	elog(DEBUG3, "replication slot drop: %s: done",
+		 NameStr(slot->data.name));
 }
 
 /*

src/backend/storage/lmgr/lwlock.c

@@ -1945,3 +1945,10 @@ LWLockHeldByMeInMode(LWLock *l, LWLockMode mode)
 	}
 	return false;
 }
+
+/* temp debugging aid to analyze 019_replslot_limit failures */
+int
+LWLockHeldCount(void)
+{
+	return num_held_lwlocks;
+}

src/backend/utils/init/postinit.c

@@ -1262,6 +1262,23 @@ ShutdownPostgres(int code, Datum arg)
 	 * them explicitly.
 	 */
 	LockReleaseAll(USER_LOCKMETHOD, true);
+
+	/*
+	 * temp debugging aid to analyze 019_replslot_limit failures
+	 *
+	 * If an error were thrown outside of a transaction nothing up to now
+	 * would have released lwlocks. We probably will add an
+	 * LWLockReleaseAll(). But for now make it easier to understand such cases
+	 * by warning if any lwlocks are held.
+	 */
+#ifdef USE_ASSERT_CHECKING
+	{
+		int held_lwlocks = LWLockHeldCount();
+		if (held_lwlocks)
+			elog(WARNING, "holding %d lwlocks at the end of ShutdownPostgres()",
+				 held_lwlocks);
+	}
+#endif
 }
 
 

src/include/storage/lwlock.h

@@ -121,6 +121,7 @@ extern void LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val);
 extern void LWLockReleaseAll(void);
 extern bool LWLockHeldByMe(LWLock *lock);
 extern bool LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode);
+extern int LWLockHeldCount(void);
 
 extern bool LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval);
 extern void LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 value);

src/test/recovery/t/019_replslot_limit.pl

@@ -335,7 +335,23 @@ $node_standby3->start;
 $node_primary3->wait_for_catchup($node_standby3);
 my $senderpid = $node_primary3->safe_psql('postgres',
 	"SELECT pid FROM pg_stat_activity WHERE backend_type = 'walsender'");
-like($senderpid, qr/^[0-9]+$/, "have walsender pid $senderpid");
+
+# We've seen occasional cases where multiple walsender pids are active. An
+# immediate shutdown may hide evidence of a locking bug. So if multiple
+# walsenders are observed, shut down in fast mode, and collect some more
+# information.
+if (not like($senderpid, qr/^[0-9]+$/, "have walsender pid $senderpid"))
+{
+	my ($stdout, $stderr);
+	$node_primary3->psql('postgres',
+						 "\\a\\t\nSELECT * FROM pg_stat_activity",
+						 stdout => \$stdout, stderr => \$stderr);
+	diag $stdout, $stderr;
+	$node_primary3->stop('fast');
+	$node_standby3->stop('fast');
+	die "could not determine walsender pid, can't continue";
+}
+
 my $receiverpid = $node_standby3->safe_psql('postgres',
 	"SELECT pid FROM pg_stat_activity WHERE backend_type = 'walreceiver'");
 like($receiverpid, qr/^[0-9]+$/, "have walreceiver pid $receiverpid");