Reset properly errno before calling write()

6cb3372 enforces errno to ENOSPC when less bytes than what is expected
have been written when it is unset, though it forgot to properly reset
errno before doing a system call to write(), causing errno to
potentially come from a previous system call.

Reported-by: Tom Lane
Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/31797.1533326676@sss.pgh.pa.us

src/backend/access/heap/rewriteheap.c

@@ -1166,6 +1166,7 @@ heap_xlog_logical_rewrite(XLogReaderState *r)
 	len = xlrec->num_mappings * sizeof(LogicalRewriteMappingData);
 
 	/* write out tail end of mapping file (again) */
+	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_LOGICAL_REWRITE_MAPPING_WRITE);
 	if (write(fd, data, len) != len)
 	{

src/backend/access/transam/twophase.c

@@ -1669,6 +1669,7 @@ RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
 				 errmsg("could not recreate file \"%s\": %m", path)));
 
 	/* Write content and CRC */
+	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_TWOPHASE_FILE_WRITE);
 	if (write(fd, content, len) != len)
 	{

src/backend/replication/logical/origin.c

@@ -576,6 +576,7 @@ CheckPointReplicationOrigin(void)
 						tmppath)));
 
 	/* write magic */
+	errno = 0;
 	if ((write(tmpfd, &magic, sizeof(magic))) != sizeof(magic))
 	{
 		int			save_errno = errno;
@@ -619,6 +620,7 @@ CheckPointReplicationOrigin(void)
 		/* make sure we only write out a commit that's persistent */
 		XLogFlush(local_lsn);
 
+		errno = 0;
 		if ((write(tmpfd, &disk_state, sizeof(disk_state))) !=
 			sizeof(disk_state))
 		{
@@ -641,6 +643,7 @@ CheckPointReplicationOrigin(void)
 
 	/* write out the CRC */
 	FIN_CRC32C(crc);
+	errno = 0;
 	if ((write(tmpfd, &crc, sizeof(crc))) != sizeof(crc))
 	{
 		int			save_errno = errno;

src/backend/replication/logical/reorderbuffer.c

@@ -2421,6 +2421,7 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 	ondisk->size = sz;
 
+	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_REORDER_BUFFER_WRITE);
 	if (write(fd, rb->outbuf, ondisk->size) != ondisk->size)
 	{

src/backend/replication/logical/snapbuild.c

@@ -1609,6 +1609,7 @@ SnapBuildSerialize(SnapBuild *builder, XLogRecPtr lsn)
 		ereport(ERROR,
 				(errmsg("could not open file \"%s\": %m", path)));
 
+	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_SNAPBUILD_WRITE);
 	if ((write(fd, ondisk, needed_length)) != needed_length)
 	{

src/backend/replication/slot.c

@@ -1275,6 +1275,7 @@ SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
 				SnapBuildOnDiskChecksummedSize);
 	FIN_CRC32C(cp.checksum);
 
+	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_REPLICATION_SLOT_WRITE);
 	if ((write(fd, &cp, sizeof(cp))) != sizeof(cp))
 	{

src/bin/pg_basebackup/walmethods.c

@@ -122,6 +122,7 @@ dir_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 		zerobuf = pg_malloc0(XLOG_BLCKSZ);
 		for (bytes = 0; bytes < pad_to_size; bytes += XLOG_BLCKSZ)
 		{
+			errno = 0;
 			if (write(fd, zerobuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
 			{
 				int			save_errno = errno;
@@ -445,6 +446,7 @@ tar_write_compressed_data(void *buf, size_t count, bool flush)
 		{
 			size_t		len = ZLIB_OUT_SIZE - tar_data->zp->avail_out;
 
+			errno = 0;
 			if (write(tar_data->fd, tar_data->zlibOut, len) != len)
 			{
 				/*
@@ -629,6 +631,7 @@ tar_open_for_write(const char *pathname, const char *temp_suffix, size_t pad_to_
 
 	if (!tar_data->compression)
 	{
+		errno = 0;
 		if (write(tar_data->fd, tar_data->currentfile->header, 512) != 512)
 		{
 			save_errno = errno;
@@ -829,6 +832,7 @@ tar_close(Walfile f, WalCloseMethod method)
 		return -1;
 	if (!tar_data->compression)
 	{
+		errno = 0;
 		if (write(tar_data->fd, tf->header, 512) != 512)
 		{
 			/* if write didn't set errno, assume problem is no disk space */
@@ -901,6 +905,7 @@ tar_finish(void)
 	MemSet(zerobuf, 0, sizeof(zerobuf));
 	if (!tar_data->compression)
 	{
+		errno = 0;
 		if (write(tar_data->fd, zerobuf, sizeof(zerobuf)) != sizeof(zerobuf))
 		{
 			/* if write didn't set errno, assume problem is no disk space */
@@ -933,6 +938,7 @@ tar_finish(void)
 			{
 				size_t		len = ZLIB_OUT_SIZE - tar_data->zp->avail_out;
 
+				errno = 0;
 				if (write(tar_data->fd, tar_data->zlibOut, len) != len)
 				{
 					/*