PANIC on fsync() failure.

On some operating systems, it doesn't make sense to retry fsync(),
because dirty data cached by the kernel may have been dropped on
write-back failure.  In that case the only remaining copy of the
data is in the WAL.  A subsequent fsync() could appear to succeed,
but not have flushed the data.  That means that a future checkpoint
could apparently complete successfully but have lost data.

Therefore, violently prevent any future checkpoint attempts by
panicking on the first fsync() failure.  Note that we already
did the same for WAL data; this change extends that behavior to
non-temporary data files.

Provide a GUC data_sync_retry to control this new behavior, for
users of operating systems that don't eject dirty data, and possibly
forensic/testing uses.  If it is set to on and the write-back error
was transient, a later checkpoint might genuinely succeed (on a
system that does not throw away buffers on failure); if the error is
permanent, later checkpoints will continue to fail.  The GUC defaults
to off, meaning that we panic.

Back-patch to all supported releases.

There is still a narrow window for error-loss on some operating
systems: if the file is closed and later reopened and a write-back
error occurs in the intervening time, but the inode has the bad
luck to be evicted due to memory pressure before we reopen, we could
miss the error.  A later patch will address that with a scheme
for keeping files with dirty data open at all times, but we judge
that to be too complicated to back-patch.

Author: Craig Ringer, with some adjustments by Thomas Munro
Reported-by: Craig Ringer
Reviewed-by: Robert Haas, Thomas Munro, Andres Freund
Discussion: https://postgr.es/m/20180427222842.in2e4mibx45zdth5%40alap3.anarazel.de

11 files changed, 67 insertions, 18 deletions

diff --git a/src/backend/access/heap/rewriteheap.c b/src/backend/access/heap/rewriteheap.c
index 09b21c927cb..62c7683f148 100644
--- a/src/backend/access/heap/rewriteheap.c
+++ b/src/backend/access/heap/rewriteheap.c
@@ -977,7 +977,7 @@ logical_end_heap_rewrite(RewriteState state)
 	while ((src = (RewriteMappingFile *) hash_seq_search(&seq_status)) != NULL)
 	{
 		if (FileSync(src->vfd, WAIT_EVENT_LOGICAL_REWRITE_SYNC) != 0)
-			ereport(ERROR,
+			ereport(data_sync_elevel(ERROR),
 					(errcode_for_file_access(),
 					 errmsg("could not fsync file \"%s\": %m", src->path)));
 		FileClose(src->vfd);
@@ -1200,7 +1200,7 @@ heap_xlog_logical_rewrite(XLogReaderState *r)
 	 */
 	pgstat_report_wait_start(WAIT_EVENT_LOGICAL_REWRITE_MAPPING_SYNC);
 	if (pg_fsync(fd) != 0)
-		ereport(ERROR,
+		ereport(data_sync_elevel(ERROR),
 				(errcode_for_file_access(),
 				 errmsg("could not fsync file \"%s\": %m", path)));
 	pgstat_report_wait_end();
@@ -1299,7 +1299,7 @@ CheckPointLogicalRewriteHeap(void)
 			 */
 			pgstat_report_wait_start(WAIT_EVENT_LOGICAL_REWRITE_CHECKPOINT_SYNC);
 			if (pg_fsync(fd) != 0)
-				ereport(ERROR,
+				ereport(data_sync_elevel(ERROR),
 						(errcode_for_file_access(),
 						 errmsg("could not fsync file \"%s\": %m", path)));
 			pgstat_report_wait_end();
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index e0d26b75cf4..56ca77c4074 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -929,7 +929,7 @@ SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid)
 							   path, offset)));
 			break;
 		case SLRU_FSYNC_FAILED:
-			ereport(ERROR,
+			ereport(data_sync_elevel(ERROR),
 					(errcode_for_file_access(),
 					 errmsg("could not access status of transaction %u", xid),
 					 errdetail("Could not fsync file \"%s\": %m.",
diff --git a/src/backend/access/transam/timeline.c b/src/backend/access/transam/timeline.c
index 63db8a981dc..9975548f4b7 100644
--- a/src/backend/access/transam/timeline.c
+++ b/src/backend/access/transam/timeline.c
@@ -407,7 +407,7 @@ writeTimeLineHistory(TimeLineID newTLI, TimeLineID parentTLI,
 
 	pgstat_report_wait_start(WAIT_EVENT_TIMELINE_HISTORY_SYNC);
 	if (pg_fsync(fd) != 0)
-		ereport(ERROR,
+		ereport(data_sync_elevel(ERROR),
 				(errcode_for_file_access(),
 				 errmsg("could not fsync file \"%s\": %m", tmppath)));
 	pgstat_report_wait_end();
@@ -487,7 +487,7 @@ writeTimeLineHistoryFile(TimeLineID tli, char *content, int size)
 
 	pgstat_report_wait_start(WAIT_EVENT_TIMELINE_HISTORY_FILE_SYNC);
 	if (pg_fsync(fd) != 0)
-		ereport(ERROR,
+		ereport(data_sync_elevel(ERROR),
 				(errcode_for_file_access(),
 				 errmsg("could not fsync file \"%s\": %m", tmppath)));
 	pgstat_report_wait_end();
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 45ca90ca6c2..a93017feecf 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3445,7 +3445,7 @@ XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno,
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_SYNC);
 	if (pg_fsync(fd) != 0)
-		ereport(ERROR,
+		ereport(data_sync_elevel(ERROR),
 				(errcode_for_file_access(),
 				 errmsg("could not fsync file \"%s\": %m", tmppath)));
 	pgstat_report_wait_end();
diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 8549cd506df..67e939883bc 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -1630,6 +1630,9 @@ SnapBuildSerialize(SnapBuild *builder, XLogRecPtr lsn)
 	 * fsync the file before renaming so that even if we crash after this we
 	 * have either a fully valid file or nothing.
 	 *
+	 * It's safe to just ERROR on fsync() here because we'll retry the whole
+	 * operation including the writes.
+	 *
 	 * TODO: Do the fsync() via checkpoints/restartpoints, doing it here has
 	 * some noticeable overhead since it's performed synchronously during
 	 * decoding?
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 3172092a914..0e50e4f4bac 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -138,6 +138,8 @@ int			max_files_per_process = 1000;
  */
 int			max_safe_fds = 32;	/* default if not changed */
 
+/* Whether it is safe to continue running after fsync() fails. */
+bool		data_sync_retry = false;
 
 /* Debugging.... */
 
@@ -433,11 +435,9 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 		 */
 		rc = sync_file_range(fd, offset, nbytes,
 							 SYNC_FILE_RANGE_WRITE);
-
-		/* don't error out, this is just a performance optimization */
 		if (rc != 0)
 		{
-			ereport(WARNING,
+			ereport(data_sync_elevel(WARNING),
 					(errcode_for_file_access(),
 					 errmsg("could not flush dirty data: %m")));
 		}
@@ -509,7 +509,7 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 			rc = msync(p, (size_t) nbytes, MS_ASYNC);
 			if (rc != 0)
 			{
-				ereport(WARNING,
+				ereport(data_sync_elevel(WARNING),
 						(errcode_for_file_access(),
 						 errmsg("could not flush dirty data: %m")));
 				/* NB: need to fall through to munmap()! */
@@ -565,7 +565,7 @@ pg_flush_data(int fd, off_t offset, off_t nbytes)
 void
 fsync_fname(const char *fname, bool isdir)
 {
-	fsync_fname_ext(fname, isdir, false, ERROR);
+	fsync_fname_ext(fname, isdir, false, data_sync_elevel(ERROR));
 }
 
 /*
@@ -1031,7 +1031,8 @@ LruDelete(File file)
 	 * to leak the FD than to mess up our internal state.
 	 */
 	if (close(vfdP->fd))
-		elog(LOG, "could not close file \"%s\": %m", vfdP->fileName);
+		elog(vfdP->fdstate & FD_TEMPORARY ? LOG : data_sync_elevel(LOG),
+			 "could not close file \"%s\": %m", vfdP->fileName);
 	vfdP->fd = VFD_CLOSED;
 	--nfile;
 
@@ -1510,7 +1511,14 @@ FileClose(File file)
 	{
 		/* close the file */
 		if (close(vfdP->fd))
-			elog(LOG, "could not close file \"%s\": %m", vfdP->fileName);
+		{
+			/*
+			 * We may need to panic on failure to close non-temporary files;
+			 * see LruDelete.
+			 */
+			elog(vfdP->fdstate & FD_TEMPORARY ? LOG : data_sync_elevel(LOG),
+				"could not close file \"%s\": %m", vfdP->fileName);
+		}
 
 		--nfile;
 		vfdP->fd = VFD_CLOSED;
@@ -2949,6 +2957,9 @@ looks_like_temp_rel_name(const char *name)
  * harmless cases such as read-only files in the data directory, and that's
  * not good either.
  *
+ * Note that if we previously crashed due to a PANIC on fsync(), we'll be
+ * rewriting all changes again during recovery.
+ *
  * Note we assume we're chdir'd into PGDATA to begin with.
  */
 void
@@ -3235,3 +3246,26 @@ fsync_parent_path(const char *fname, int elevel)
 
 	return 0;
 }
+
+/*
+ * Return the passed-in error level, or PANIC if data_sync_retry is off.
+ *
+ * Failure to fsync any data file is cause for immediate panic, unless
+ * data_sync_retry is enabled.  Data may have been written to the operating
+ * system and removed from our buffer pool already, and if we are running on
+ * an operating system that forgets dirty data on write-back failure, there
+ * may be only one copy of the data remaining: in the WAL.  A later attempt to
+ * fsync again might falsely report success.  Therefore we must not allow any
+ * further checkpoints to be attempted.  data_sync_retry can in theory be
+ * enabled on systems known not to drop dirty buffered data on write-back
+ * failure (with the likely outcome that checkpoints will continue to fail
+ * until the underlying problem is fixed).
+ *
+ * Any code that reports a failure from fsync() or related functions should
+ * filter the error level with this function.
+ */
+int
+data_sync_elevel(int elevel)
+{
+	return data_sync_retry ? elevel : PANIC;
+}
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 64455a41550..d08e837c931 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -1040,7 +1040,7 @@ mdimmedsync(SMgrRelation reln, ForkNumber forknum)
 		MdfdVec    *v = &reln->md_seg_fds[forknum][segno - 1];
 
 		if (FileSync(v->mdfd_vfd, WAIT_EVENT_DATA_FILE_IMMEDIATE_SYNC) < 0)
-			ereport(ERROR,
+			ereport(data_sync_elevel(ERROR),
 					(errcode_for_file_access(),
 					 errmsg("could not fsync file \"%s\": %m",
 							FilePathName(v->mdfd_vfd))));
@@ -1285,7 +1285,7 @@ mdsync(void)
 							bms_join(new_requests, requests);
 
 						errno = save_errno;
-						ereport(ERROR,
+						ereport(data_sync_elevel(ERROR),
 								(errcode_for_file_access(),
 								 errmsg("could not fsync file \"%s\": %m",
 										path)));
@@ -1459,7 +1459,7 @@ register_dirty_segment(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
 				(errmsg("could not forward fsync request because request queue is full")));
 
 		if (FileSync(seg->mdfd_vfd, WAIT_EVENT_DATA_FILE_SYNC) < 0)
-			ereport(ERROR,
+			ereport(data_sync_elevel(ERROR),
 					(errcode_for_file_access(),
 					 errmsg("could not fsync file \"%s\": %m",
 							FilePathName(seg->mdfd_vfd))));
diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index f5394dc43d4..8d4215cb981 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -798,7 +798,7 @@ write_relmap_file(bool shared, RelMapFile *newmap,
 	 */
 	pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_SYNC);
 	if (pg_fsync(fd) != 0)
-		ereport(ERROR,
+		ereport(data_sync_elevel(ERROR),
 				(errcode_for_file_access(),
 				 errmsg("could not fsync relation mapping file \"%s\": %m",
 						mapfilename)));
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 40da074d7b2..4298a0dde23 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1667,6 +1667,15 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"data_sync_retry", PGC_POSTMASTER, ERROR_HANDLING_OPTIONS,
+			gettext_noop("Whether to continue running after a failure to sync data files."),
+		},
+		&data_sync_retry,
+		false,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 22923350581..3e68afd2750 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -635,6 +635,7 @@
 
 #exit_on_error = off			# terminate session on any error?
 #restart_after_crash = on		# reinitialize after backend crash?
+#data_sync_retry = off			# retry or panic on failure to fsync data?
 
 
 #------------------------------------------------------------------------------
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index ffb3f4e10f1..3043af31b01 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -53,6 +53,7 @@ typedef int File;
 
 /* GUC parameter */
 extern PGDLLIMPORT int max_files_per_process;
+extern PGDLLIMPORT bool data_sync_retry;
 
 /*
  * This is private to fd.c, but exported for save/restore_backend_variables()
@@ -124,6 +125,7 @@ extern int	durable_rename(const char *oldfile, const char *newfile, int loglevel
 extern int	durable_unlink(const char *fname, int loglevel);
 extern int	durable_link_or_rename(const char *oldfile, const char *newfile, int loglevel);
 extern void SyncDataDirectory(void);
+extern int data_sync_elevel(int elevel);
 
 /* Filename components for OpenTemporaryFile */
 #define PG_TEMP_FILES_DIR "pgsql_tmp"