Restore pg_pread and friends.

Commits cf112c12 and a0dc8271 were a little too hasty in getting rid of
the pg_ prefixes where we use pread(), pwrite() and vectored variants.

We dropped support for ancient Unixes where we needed to use lseek() to
implement replacements for those, but it turns out that Windows also
changes the current position even when you pass in an offset to
ReadFile() and WriteFile() if the file handle is synchronous, despite
its documentation saying otherwise.

Switching to asynchronous file handles would fix that, but have other
complications.  For now let's just put back the pg_ prefix and add some
comments to highlight the non-standard side-effect, which we can now
describe as Windows-only.

Reported-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/20220923202439.GA1156054%40nathanxps13

9 files changed, 14 insertions, 14 deletions

diff --git a/src/backend/access/heap/rewriteheap.c b/src/backend/access/heap/rewriteheap.c
index 2f08fbe8d31..b01b39b008c 100644
--- a/src/backend/access/heap/rewriteheap.c
+++ b/src/backend/access/heap/rewriteheap.c
@@ -1150,7 +1150,7 @@ heap_xlog_logical_rewrite(XLogReaderState *r)
 	/* write out tail end of mapping file (again) */
 	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_LOGICAL_REWRITE_MAPPING_WRITE);
-	if (pwrite(fd, data, len, xlrec->offset) != len)
+	if (pg_pwrite(fd, data, len, xlrec->offset) != len)
 	{
 		/* if write didn't set errno, assume problem is no disk space */
 		if (errno == 0)
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index c9a7b979495..b65cb49d7ff 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -718,7 +718,7 @@ SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno)
 
 	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_SLRU_READ);
-	if (pread(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
+	if (pg_pread(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
 	{
 		pgstat_report_wait_end();
 		slru_errcause = SLRU_READ_FAILED;
@@ -873,7 +873,7 @@ SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruWriteAll fdata)
 
 	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_SLRU_WRITE);
-	if (pwrite(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
+	if (pg_pwrite(fd, shared->page_buffer[slotno], BLCKSZ, offset) != BLCKSZ)
 	{
 		pgstat_report_wait_end();
 		/* if write didn't set errno, assume problem is no disk space */
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 00992a11b9e..8e15256db84 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2196,7 +2196,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 					INSTR_TIME_SET_CURRENT(start);
 
 				pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
-				written = pwrite(openLogFile, from, nleft, startoffset);
+				written = pg_pwrite(openLogFile, from, nleft, startoffset);
 				pgstat_report_wait_end();
 
 				/*
@@ -3018,7 +3018,7 @@ XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
 		 * enough.
 		 */
 		errno = 0;
-		if (pwrite(fd, zbuffer.data, 1, wal_segment_size - 1) != 1)
+		if (pg_pwrite(fd, zbuffer.data, 1, wal_segment_size - 1) != 1)
 		{
 			/* if write didn't set errno, assume no disk space */
 			save_errno = errno ? errno : ENOSPC;
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 4d6c34e0fc5..5a8fe81f826 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1544,7 +1544,7 @@ WALRead(XLogReaderState *state,
 
 		/* Reset errno first; eases reporting non-errno-affecting errors */
 		errno = 0;
-		readbytes = pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
+		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index b41e6826643..cb07694aea6 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3271,7 +3271,7 @@ retry:
 	readOff = targetPageOff;
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-	r = pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
+	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
 	if (r != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];
diff --git a/src/backend/backup/basebackup.c b/src/backend/backup/basebackup.c
index 411cac9be3f..e252ad74211 100644
--- a/src/backend/backup/basebackup.c
+++ b/src/backend/backup/basebackup.c
@@ -1828,7 +1828,7 @@ basebackup_read_file(int fd, char *buf, size_t nbytes, off_t offset,
 	int			rc;
 
 	pgstat_report_wait_start(WAIT_EVENT_BASEBACKUP_READ);
-	rc = pread(fd, buf, nbytes, offset);
+	rc = pg_pread(fd, buf, nbytes, offset);
 	pgstat_report_wait_end();
 
 	if (rc < 0)
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index f6ef0ace2c4..3767466ef31 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -915,7 +915,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		/* OK to write the logs */
 		errno = 0;
 
-		byteswritten = pwrite(recvFile, buf, segbytes, (off_t) startoff);
+		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
 		if (byteswritten <= 0)
 		{
 			char		xlogfname[MAXFNAMELEN];
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 073dab2be59..e4d954578c8 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -2053,7 +2053,7 @@ FileRead(File file, char *buffer, int amount, off_t offset,
 
 retry:
 	pgstat_report_wait_start(wait_event_info);
-	returnCode = pread(vfdP->fd, buffer, amount, offset);
+	returnCode = pg_pread(vfdP->fd, buffer, amount, offset);
 	pgstat_report_wait_end();
 
 	if (returnCode < 0)
@@ -2135,7 +2135,7 @@ FileWrite(File file, char *buffer, int amount, off_t offset,
 retry:
 	errno = 0;
 	pgstat_report_wait_start(wait_event_info);
-	returnCode = pwrite(VfdCache[file].fd, buffer, amount, offset);
+	returnCode = pg_pwrite(VfdCache[file].fd, buffer, amount, offset);
 	pgstat_report_wait_end();
 
 	/* if write didn't set errno, assume problem is no disk space */
@@ -3740,7 +3740,7 @@ data_sync_elevel(int elevel)
 }
 
 /*
- * A convenience wrapper for pwritev() that retries on partial write.  If an
+ * A convenience wrapper for pg_pwritev() that retries on partial write.  If an
  * error is returned, it is unspecified how much has been written.
  */
 ssize_t
@@ -3760,7 +3760,7 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 	for (;;)
 	{
 		/* Write as much as we can. */
-		part = pwritev(fd, iov, iovcnt, offset);
+		part = pg_pwritev(fd, iov, iovcnt, offset);
 		if (part < 0)
 			return -1;
 
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 683f616b1a8..4d341d3f7f1 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -1527,7 +1527,7 @@ AddToDataDirLockFile(int target_line, const char *str)
 	len = strlen(destbuffer);
 	errno = 0;
 	pgstat_report_wait_start(WAIT_EVENT_LOCK_FILE_ADDTODATADIR_WRITE);
-	if (pwrite(fd, destbuffer, len, 0) != len)
+	if (pg_pwrite(fd, destbuffer, len, 0) != len)
 	{
 		pgstat_report_wait_end();
 		/* if write didn't set errno, assume problem is no disk space */