Improve wording of log messages triggered by max_slot_wal_keep_size.

The one about "terminating process to release replication slot" told
you nothing about why that was happening.  The one about "invalidating
slot because its restart_lsn exceeds max_slot_wal_keep_size" told you
what was happening, but violated our message style guideline about
keeping the primary message short.  Add DETAIL/HINT lines to carry
the appropriate detail and make the two cases more uniform.

While here, fix bogus test logic in 019_replslot_limit.pl: if it timed
out without seeing the expected log message, no test failure would be
reported.  This is flat broken since commit 549ec201d removed the test
counts; even before that it was horribly bad style, since you'd only
get told that not all tests had been run.

Kyotaro Horiguchi, reviewed by Bertrand Drouvot; test fixes by me

Discussion: https://postgr.es/m/20211214.130456.2233153190058148084.horikyota.ntt@gmail.com

src/backend/replication/slot.c

@@ -1293,8 +1293,12 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlot *s, XLogRecPtr oldestLSN,
 			if (last_signaled_pid != active_pid)
 			{
 				ereport(LOG,
-						(errmsg("terminating process %d to release replication slot \"%s\"",
-								active_pid, NameStr(slotname))));
+						errmsg("terminating process %d to release replication slot \"%s\"",
+							   active_pid, NameStr(slotname)),
+						errdetail("The slot's restart_lsn %X/%X exceeds the limit by %llu bytes.",
+								  LSN_FORMAT_ARGS(restart_lsn),
+								  (unsigned long long) (oldestLSN - restart_lsn)),
+						errhint("You might need to increase max_slot_wal_keep_size."));
 
 				(void) kill(active_pid, SIGTERM);
 				last_signaled_pid = active_pid;
@@ -1331,9 +1335,12 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlot *s, XLogRecPtr oldestLSN,
 			ReplicationSlotRelease();
 
 			ereport(LOG,
-					(errmsg("invalidating slot \"%s\" because its restart_lsn %X/%X exceeds max_slot_wal_keep_size",
-							NameStr(slotname),
-							LSN_FORMAT_ARGS(restart_lsn))));
+					errmsg("invalidating obsolete replication slot \"%s\"",
+						   NameStr(slotname)),
+					errdetail("The slot's restart_lsn %X/%X exceeds the limit by %llu bytes.",
+							  LSN_FORMAT_ARGS(restart_lsn),
+							  (unsigned long long) (oldestLSN - restart_lsn)),
+					errhint("You might need to increase max_slot_wal_keep_size."));
 
 			/* done with this slot for now */
 			break;

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

@@ -185,8 +185,7 @@ my $invalidated = 0;
 for (my $i = 0; $i < 10000; $i++)
 {
 	if (find_in_log(
-			$node_primary,
-			"invalidating slot \"rep1\" because its restart_lsn [0-9A-F/]+ exceeds max_slot_wal_keep_size",
+			$node_primary, 'invalidating obsolete replication slot "rep1"',
 			$logstart))
 	{
 		$invalidated = 1;
@@ -379,6 +378,7 @@ $logstart = get_log_size($node_primary3);
 kill 'STOP', $senderpid, $receiverpid;
 advance_wal($node_primary3, 2);
 
+my $msg_logged   = 0;
 my $max_attempts = $PostgreSQL::Test::Utils::timeout_default;
 while ($max_attempts-- >= 0)
 {
@@ -387,11 +387,12 @@ while ($max_attempts-- >= 0)
 			"terminating process $senderpid to release replication slot \"rep3\"",
 			$logstart))
 	{
-		ok(1, "walsender termination logged");
+		$msg_logged = 1;
 		last;
 	}
 	sleep 1;
 }
+ok($msg_logged, "walsender termination logged");
 
 # Now let the walsender continue; slot should be killed now.
 # (Must not let walreceiver run yet; otherwise the standby could start another
@@ -402,18 +403,20 @@ $node_primary3->poll_query_until('postgres',
 	"lost")
   or die "timed out waiting for slot to be lost";
 
+$msg_logged   = 0;
 $max_attempts = $PostgreSQL::Test::Utils::timeout_default;
 while ($max_attempts-- >= 0)
 {
 	if (find_in_log(
-			$node_primary3,
-			'invalidating slot "rep3" because its restart_lsn', $logstart))
+			$node_primary3, 'invalidating obsolete replication slot "rep3"',
+			$logstart))
 	{
-		ok(1, "slot invalidation logged");
+		$msg_logged = 1;
 		last;
 	}
 	sleep 1;
 }
+ok($msg_logged, "slot invalidation logged");
 
 # Now let the walreceiver continue, so that the node can be stopped cleanly
 kill 'CONT', $receiverpid;