Subject: [PATCH v27] Make End-Of-Recovery error less scary

From: Kyotaro Horiguchi <[email-masked]>
Date: Tue, 7 Mar 2023 14:55:58 +0900

When recovery in any type ends, we see a bit scary error message like
"invalid record length" that suggests something serious is
happening. Actually if recovery meets a record with length = 0, that
usually means it finished applying all available WAL records.

Make this message less scary as "reached end of WAL". Instead, raise
the error level for other kind of WAL failure to WARNING.  To make
sure that the detection is correct, this patch checks if all trailing
bytes in the same page are zeroed in that case.
---
 src/backend/access/transam/xlogreader.c   | 134 ++++++++++++++++++----
 src/backend/access/transam/xlogrecovery.c |  94 +++++++++++----
 src/backend/replication/walreceiver.c     |   7 +-
 src/bin/pg_waldump/pg_waldump.c           |  13 ++-
 src/include/access/xlogreader.h           |   1 +
 src/test/recovery/t/035_recovery.pl       | 130 +++++++++++++++++++++
 6 files changed, 327 insertions(+), 52 deletions(-)
 create mode 100644 src/test/recovery/t/035_recovery.pl

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index e0baa86bd3..ce65f99c60 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -48,6 +48,8 @@ static int	ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr,
 							 int reqLen);
 static void XLogReaderInvalReadState(XLogReaderState *state);
 static XLogPageReadResult XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking);
+static bool ValidXLogRecordLength(XLogReaderState *state, XLogRecPtr RecPtr,
+								  XLogRecord *record);
 static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
 								  XLogRecPtr PrevRecPtr, XLogRecord *record, bool randAccess);
 static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record,
@@ -149,6 +151,7 @@ XLogReaderAllocate(int wal_segment_size, const char *waldir,
 		pfree(state);
 		return NULL;
 	}
+	state->EndOfWAL = false;
 	state->errormsg_buf[0] = '\0';
 
 	/*
@@ -525,6 +528,7 @@ XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking)
 	/* reset error state */
 	state->errormsg_buf[0] = '\0';
 	decoded = NULL;
+	state->EndOfWAL = false;
 
 	state->abortedRecPtr = InvalidXLogRecPtr;
 	state->missingContrecPtr = InvalidXLogRecPtr;
@@ -608,16 +612,12 @@ restart:
 	Assert(pageHeaderSize <= readOff);
 
 	/*
-	 * Read the record length.
+	 * Verify the record header.
 	 *
 	 * NB: Even though we use an XLogRecord pointer here, the whole record
-	 * header might not fit on this page. xl_tot_len is the first field of the
-	 * struct, so it must be on this page (the records are MAXALIGNed), but we
-	 * cannot access any other fields until we've verified that we got the
-	 * whole header.
+	 * header might not fit on this page.
 	 */
 	record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
-	total_len = record->xl_tot_len;
 
 	/*
 	 * If the whole record header is on this page, validate it immediately.
@@ -636,19 +636,19 @@ restart:
 	}
 	else
 	{
-		/* There may be no next page if it's too small. */
-		if (total_len < SizeOfXLogRecord)
-		{
-			report_invalid_record(state,
-								  "invalid record length at %X/%X: expected at least %u, got %u",
-								  LSN_FORMAT_ARGS(RecPtr),
-								  (uint32) SizeOfXLogRecord, total_len);
+		/*
+		 * xl_tot_len is the first field of the struct, so it must be on this
+		 * page (the records are MAXALIGNed), but we cannot access any other
+		 * fields until we've verified that we got the whole header.
+		 */
+		if (!ValidXLogRecordLength(state, RecPtr, record))
 			goto err;
-		}
-		/* We'll validate the header once we have the next page. */
+
 		gotheader = false;
 	}
 
+	total_len = record->xl_tot_len;
+
 	/*
 	 * Try to find space to decode this record, if we can do so without
 	 * calling palloc.  If we can't, we'll try again below after we've
@@ -1091,25 +1091,81 @@ XLogReaderInvalReadState(XLogReaderState *state)
 	state->readLen = 0;
 }
 
+/*
+ * Validate record length of an XLOG record header.
+ *
+ * This is substantially a part of ValidXLogRecordHeader.  But XLogReadRecord
+ * needs this separate from the function in case of a partial record header.
+ *
+ * Returns true if the xl_tot_len header field has a seemingly valid value,
+ * which means the caller can proceed reading to the following part of the
+ * record.
+ */
+static bool
+ValidXLogRecordLength(XLogReaderState *state, XLogRecPtr RecPtr,
+					  XLogRecord *record)
+{
+	if (record->xl_tot_len == 0)
+	{
+		char	   *p;
+		char	   *pe;
+
+		/*
+		 * We are almost sure reaching the end of WAL, make sure that the
+		 * whole page after the record is filled with zeroes.
+		 */
+		p = (char *) record;
+		pe = p + XLOG_BLCKSZ - (RecPtr & (XLOG_BLCKSZ - 1));
+
+		while (p < pe && *p == 0)
+			p++;
+
+		if (p == pe)
+		{
+			/*
+			 * The page after the record is completely zeroed. That suggests
+			 * we don't have a record after this point. We don't bother
+			 * checking the pages after since they are not zeroed in the case
+			 * of recycled segments.
+			 */
+			report_invalid_record(state, "empty record at %X/%X",
+								  LSN_FORMAT_ARGS(RecPtr));
+
+			/* notify end-of-wal to callers */
+			state->EndOfWAL = true;
+			return false;
+		}
+	}
+
+	if (record->xl_tot_len < SizeOfXLogRecord)
+	{
+		report_invalid_record(state,
+							  "invalid record length at %X/%X: expected at least %u, got %u",
+							  LSN_FORMAT_ARGS(RecPtr),
+							  (uint32) SizeOfXLogRecord, record->xl_tot_len);
+		return false;
+	}
+
+	return true;
+}
+
 /*
  * Validate an XLOG record header.
  *
  * This is just a convenience subroutine to avoid duplicated code in
  * XLogReadRecord.  It's not intended for use from anywhere else.
+ *
+ * Returns true if the header fields have the valid values and the caller can
+ * proceed reading to the following part of the record.
  */
 static bool
 ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
 					  XLogRecPtr PrevRecPtr, XLogRecord *record,
 					  bool randAccess)
 {
-	if (record->xl_tot_len < SizeOfXLogRecord)
-	{
-		report_invalid_record(state,
-							  "invalid record length at %X/%X: expected at least %u, got %u",
-							  LSN_FORMAT_ARGS(RecPtr),
-							  (uint32) SizeOfXLogRecord, record->xl_tot_len);
+	if (!ValidXLogRecordLength(state, RecPtr, record))
 		return false;
-	}
+
 	if (!RmgrIdIsValid(record->xl_rmid))
 	{
 		report_invalid_record(state,
@@ -1207,6 +1263,32 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	XLByteToSeg(recptr, segno, state->segcxt.ws_segsize);
 	offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
 
+	StaticAssertStmt(XLOG_PAGE_MAGIC != 0, "XLOG_PAGE_MAGIC is zero");
+
+	if (hdr->xlp_magic == 0)
+	{
+		/* Regard an empty page as End-Of-WAL */
+		int			i;
+
+		for (i = 0; i < XLOG_BLCKSZ && phdr[i] == 0; i++);
+		if (i == XLOG_BLCKSZ)
+		{
+			char		fname[MAXFNAMELEN];
+
+			XLogFileName(fname, state->seg.ws_tli, segno,
+						 state->segcxt.ws_segsize);
+
+			report_invalid_record(state,
+								  "empty page in log segment %s, offset %u",
+								  fname,
+								  offset);
+			state->EndOfWAL = true;
+			return false;
+		}
+
+		/* The same condition will be caught as invalid magic number */
+	}
+
 	if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
 	{
 		char		fname[MAXFNAMELEN];
@@ -1296,6 +1378,14 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 							  fname,
 							  LSN_FORMAT_ARGS(recptr),
 							  offset);
+
+		/*
+		 * If the page address is less than expected we assume it is an unused
+		 * page in a recycled segment.
+		 */
+		if (hdr->xlp_pageaddr < recptr)
+			state->EndOfWAL = true;
+
 		return false;
 	}
 
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index c61566666a..9db27b765f 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -1672,7 +1672,7 @@ PerformWalRecovery(void)
 		/* just have to read next record after CheckPoint */
 		Assert(xlogreader->ReadRecPtr == CheckPointLoc);
 		replayTLI = CheckPointTLI;
-		record = ReadRecord(xlogprefetcher, LOG, false, replayTLI);
+		record = ReadRecord(xlogprefetcher, WARNING, false, replayTLI);
 	}
 
 	if (record != NULL)
@@ -1781,7 +1781,7 @@ PerformWalRecovery(void)
 			}
 
 			/* Else, try to fetch the next WAL record */
-			record = ReadRecord(xlogprefetcher, LOG, false, replayTLI);
+			record = ReadRecord(xlogprefetcher, WARNING, false, replayTLI);
 		} while (record != NULL);
 
 		/*
@@ -1835,11 +1835,18 @@ PerformWalRecovery(void)
 
 		InRedo = false;
 	}
-	else
+	else if (xlogreader->EndOfWAL)
 	{
 		/* there are no WAL records following the checkpoint */
 		ereport(LOG,
-				(errmsg("redo is not required")));
+				errmsg("redo is not required"));
+	}
+	else
+	{
+		/* broken record found */
+		ereport(WARNING,
+				errmsg("redo is skipped"),
+				errhint("This suggests WAL file corruption. You might need to check the database."));
 	}
 
 	/*
@@ -3091,6 +3098,7 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 	for (;;)
 	{
 		char	   *errormsg;
+		XLogRecPtr	ErrRecPtr = InvalidXLogRecPtr;
 
 		record = XLogPrefetcherReadRecord(xlogprefetcher, &errormsg);
 		if (record == NULL)
@@ -3112,6 +3120,18 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 			{
 				abortedRecPtr = xlogreader->abortedRecPtr;
 				missingContrecPtr = xlogreader->missingContrecPtr;
+				ErrRecPtr = abortedRecPtr;
+			}
+			else
+			{
+				/*
+				 * EndRecPtr is the LSN we tried to read but failed. In the
+				 * case of decoding error, it is at the end of the failed
+				 * record but we don't have a means for now to know EndRecPtr
+				 * is pointing to which of the beginning or ending of the
+				 * failed record.
+				 */
+				ErrRecPtr = xlogreader->EndRecPtr;
 			}
 
 			if (readFile >= 0)
@@ -3122,13 +3142,15 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 
 			/*
 			 * We only end up here without a message when XLogPageRead()
-			 * failed - in that case we already logged something. In
-			 * StandbyMode that only happens if we have been triggered, so we
-			 * shouldn't loop anymore in that case.
+			 * failed- in that case we already logged something. In StandbyMode
+			 * that only happens if we have been triggered, so we shouldn't
+			 * loop anymore in that case. When EndOfWAL is true, we don't emit
+			 * the message immediately and instead will show it as a part of a
+			 * decent end-of-wal message later.
 			 */
-			if (errormsg)
-				ereport(emode_for_corrupt_record(emode, xlogreader->EndRecPtr),
-						(errmsg_internal("%s", errormsg) /* already translated */ ));
+			if (!xlogreader->EndOfWAL && errormsg)
+				ereport(emode_for_corrupt_record(emode, ErrRecPtr),
+						errmsg_internal("%s", errormsg) /* already translated */ );
 		}
 
 		/*
@@ -3159,11 +3181,14 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 			/* Great, got a record */
 			return record;
 		}
-		else
+
+		Assert(ErrRecPtr != InvalidXLogRecPtr);
+
+		/* No valid record available from this source */
+		lastSourceFailed = true;
+
+		if (!fetching_ckpt)
 		{
-			/* No valid record available from this source */
-			lastSourceFailed = true;
-
 			/*
 			 * If archive recovery was requested, but we were still doing
 			 * crash recovery, switch to archive recovery and retry using the
@@ -3176,11 +3201,16 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 			 * we'd have no idea how far we'd have to replay to reach
 			 * consistency.  So err on the safe side and give up.
 			 */
-			if (!InArchiveRecovery && ArchiveRecoveryRequested &&
-				!fetching_ckpt)
+			if (!InArchiveRecovery && ArchiveRecoveryRequested)
 			{
+				/*
+				 * We don't report this as LOG, since we don't stop recovery
+				 * here
+				 */
 				ereport(DEBUG1,
-						(errmsg_internal("reached end of WAL in pg_wal, entering archive recovery")));
+						errmsg_internal("reached end of WAL at %X/%X on timeline %u in pg_wal during crash recovery, entering archive recovery",
+										LSN_FORMAT_ARGS(ErrRecPtr),
+										replayTLI));
 				InArchiveRecovery = true;
 				if (StandbyModeRequested)
 					EnableStandbyMode();
@@ -3201,12 +3231,24 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 				continue;
 			}
 
-			/* In standby mode, loop back to retry. Otherwise, give up. */
-			if (StandbyMode && !CheckForStandbyTrigger())
-				continue;
-			else
-				return NULL;
+			/*
+			 * recovery ended.
+			 *
+			 * Emit a decent message if we met end-of-WAL. Otherwise we should
+			 * have already emitted an error message.
+			 */
+			if (xlogreader->EndOfWAL)
+				ereport(LOG,
+						errmsg("reached end of WAL at %X/%X on timeline %u",
+							   LSN_FORMAT_ARGS(ErrRecPtr), replayTLI),
+						(errormsg ? errdetail_internal("%s", errormsg) : 0));
 		}
+
+		/* In standby mode, loop back to retry. Otherwise, give up. */
+		if (StandbyMode && !CheckForStandbyTrigger())
+			continue;
+		else
+			return NULL;
 	}
 }
 
@@ -3303,11 +3345,16 @@ retry:
 			case XLREAD_WOULDBLOCK:
 				return XLREAD_WOULDBLOCK;
 			case XLREAD_FAIL:
+				Assert(!StandbyMode || CheckForStandbyTrigger());
+
 				if (readFile >= 0)
 					close(readFile);
 				readFile = -1;
 				readLen = 0;
 				readSource = XLOG_FROM_ANY;
+
+				/* promotion exit is not end-of-WAL */
+				xlogreader->EndOfWAL = !StandbyMode;
 				return XLREAD_FAIL;
 			case XLREAD_SUCCESS:
 				break;
@@ -3975,7 +4022,8 @@ emode_for_corrupt_record(int emode, XLogRecPtr RecPtr)
 {
 	static XLogRecPtr lastComplaint = 0;
 
-	if (readSource == XLOG_FROM_PG_WAL && emode == LOG)
+	/* use currentSource as readSource is reset at failure */
+	if (currentSource == XLOG_FROM_PG_WAL && emode <= WARNING)
 	{
 		if (RecPtr == lastComplaint)
 			emode = DEBUG1;
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 2398167f49..2271304814 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -496,10 +496,9 @@ WalReceiverMain(void)
 						else if (len < 0)
 						{
 							ereport(LOG,
-									(errmsg("replication terminated by primary server"),
-									 errdetail("End of WAL reached on timeline %u at %X/%X.",
-											   startpointTLI,
-											   LSN_FORMAT_ARGS(LogstreamResult.Write))));
+									errmsg("replication terminated by primary server at %X/%X on timeline %u.",
+										   LSN_FORMAT_ARGS(LogstreamResult.Write),
+										   startpointTLI));
 							endofwal = true;
 							break;
 						}
diff --git a/src/bin/pg_waldump/pg_waldump.c b/src/bin/pg_waldump/pg_waldump.c
index a3535bdfa9..11a69027d0 100644
--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@@ -1309,9 +1309,16 @@ main(int argc, char **argv)
 		exit(0);
 
 	if (errormsg)
-		pg_fatal("error in WAL record at %X/%X: %s",
-				 LSN_FORMAT_ARGS(xlogreader_state->ReadRecPtr),
-				 errormsg);
+	{
+		if (xlogreader_state->EndOfWAL)
+			pg_log_info("end of WAL at %X/%X: %s",
+						LSN_FORMAT_ARGS(xlogreader_state->EndRecPtr),
+						errormsg);
+		else
+			pg_fatal("error in WAL record at %X/%X: %s",
+					 LSN_FORMAT_ARGS(xlogreader_state->EndRecPtr),
+					 errormsg);
+	}
 
 	XLogReaderFree(xlogreader_state);
 
diff --git a/src/include/access/xlogreader.h b/src/include/access/xlogreader.h
index 0813722715..2be8ea7c37 100644
--- a/src/include/access/xlogreader.h
+++ b/src/include/access/xlogreader.h
@@ -205,6 +205,7 @@ struct XLogReaderState
 	 */
 	XLogRecPtr	ReadRecPtr;		/* start of last record read */
 	XLogRecPtr	EndRecPtr;		/* end+1 of last record read */
+	bool		EndOfWAL;		/* was the last attempt EOW? */
 
 	/*
 	 * Set at the end of recovery: the start point of a partial record at the
diff --git a/src/test/recovery/t/035_recovery.pl b/src/test/recovery/t/035_recovery.pl
new file mode 100644
index 0000000000..7107df6509
--- /dev/null
+++ b/src/test/recovery/t/035_recovery.pl
@@ -0,0 +1,130 @@
+
+# Copyright (c) 2021-2023, PostgreSQL Global Development Group
+
+# Minimal test testing recovery process
+
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+use IPC::Run;
+
+my $reached_eow_pat = "reached end of WAL at ";
+my $node = PostgreSQL::Test::Cluster->new('primary');
+$node->init(allows_streaming => 1);
+$node->start;
+
+my ($stdout, $stderr) = ('', '');
+
+my $segsize = $node->safe_psql('postgres',
+	   qq[SELECT setting FROM pg_settings WHERE name = 'wal_segment_size';]);
+
+# make sure no records afterwards go to the next segment
+$node->safe_psql('postgres', qq[
+				 SELECT pg_switch_wal();
+				 CHECKPOINT;
+				 CREATE TABLE t();
+]);
+$node->stop('immediate');
+
+# identify REDO WAL file
+my $cmd = "pg_controldata -D " . $node->data_dir();
+$cmd = ['pg_controldata', '-D', $node->data_dir()];
+$stdout = '';
+$stderr = '';
+IPC::Run::run $cmd, '>', \$stdout, '2>', \$stderr;
+ok($stdout =~ /^Latest checkpoint's REDO WAL file:[ \t] *(.+)$/m,
+   "checkpoint file is identified");
+my $chkptfile = $1;
+
+# identify the last record
+my $walfile = $node->data_dir() . "/pg_wal/$chkptfile";
+$cmd = ['pg_waldump', $walfile];
+$stdout = '';
+$stderr = '';
+my $lastrec;
+IPC::Run::run $cmd, '>', \$stdout, '2>', \$stderr;
+foreach my $l (split(/\r?\n/, $stdout))
+{
+	$lastrec = $l;
+}
+ok(defined $lastrec, "last WAL record is extracted");
+ok($stderr =~ /end of WAL at ([0-9A-F\/]+): .* at \g1/,
+   "pg_waldump emits the correct ending message");
+
+# read the last record LSN excluding leading zeroes
+ok ($lastrec =~ /, lsn: 0\/0*([1-9A-F][0-9A-F]+),/,
+	"LSN of the last record identified");
+my $lastlsn = $1;
+
+# corrupt the last record
+my $offset = hex($lastlsn) % $segsize;
+open(my $segf, '+<', $walfile) or die "failed to open $walfile\n";
+seek($segf, $offset, 0);  # zero xl_tot_len, leaving following bytes alone.
+print $segf "\0\0\0\0";
+close($segf);
+
+# pg_waldump complains about the corrupted record
+$stdout = '';
+$stderr = '';
+IPC::Run::run $cmd, '>', \$stdout, '2>', \$stderr;
+ok($stderr =~ /error: error in WAL record at 0\/$lastlsn: .* at 0\/$lastlsn/,
+   "pg_waldump emits the correct error message");
+
+# also server complains for the same reason
+my $logstart = -s $node->logfile;
+$node->start;
+ok($node->wait_for_log(
+	   "WARNING:  invalid record length at 0/$lastlsn: expected at least 24, got 0",
+	   $logstart),
+   "header error is correctly logged at $lastlsn");
+
+# no end-of-wal message should be seen this time
+ok(!find_in_log($node, $reached_eow_pat, $logstart),
+   "false log message is not emitted");
+
+# Create streaming standby linking to primary
+my $backup_name = 'my_backup';
+$node->backup($backup_name);
+my $node_standby = PostgreSQL::Test::Cluster->new('standby');
+$node_standby->init_from_backup($node, $backup_name, has_streaming => 1);
+$node_standby->start;
+$node->safe_psql('postgres', 'CREATE TABLE t ()');
+my $primary_lsn = $node->lsn('write');
+$node->wait_for_catchup($node_standby, 'write', $primary_lsn);
+
+$node_standby->stop();
+$node->stop('immediate');
+
+# crash restart the primary
+$logstart = -s $node->logfile;
+$node->start();
+ok($node->wait_for_log($reached_eow_pat, $logstart),
+   'primary properly emits end-of-WAL message');
+
+# restart the standby
+$logstart = -s $node_standby->logfile;
+$node_standby->start();
+ok($node->wait_for_log($reached_eow_pat, $logstart),
+   'standby properly emits end-of-WAL message');
+
+$node_standby->stop();
+$node->stop();
+
+done_testing();
+
+#### helper routines
+# find $pat in logfile of $node after $off-th byte
+sub find_in_log
+{
+	my ($node, $pat, $off) = @_;
+
+	$off = 0 unless defined $off;
+	my $log = PostgreSQL::Test::Utils::slurp_file($node->logfile);
+	return 0 if (length($log) <= $off);
+
+	$log = substr($log, $off);
+
+	return $log =~ m/$pat/;
+}
-- 
2.39.3

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