Provide vectored variants of FileRead() and FileWrite().

FileReadV() and FileWriteV() adapt pg_preadv() and pg_pwritev() for
fd.c's virtual file descriptors.  The simple FileRead() and FileWrite()
functions are now implemented in terms of the vectored functions, to
avoid code duplication, and they are converted back to the corresponding
simple system calls further down (commit 15c9ac36).  Later work will
make more interesting multi-iovec calls.

The traditional behavior of reporting a "fake" ENOSPC error is
simplified.  It's now always set for non-failing writes, for the benefit
of callers that expect to log a meaningful "%m" if they determine that
the write was short.  (Perhaps we should consider getting rid of that
expectation one day.)

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

1 files changed, 25 insertions, 18 deletions

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index a185fb3d08c..fbffdd72f0b 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -2110,18 +2110,18 @@ FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
 }
 
 int
-FileRead(File file, void *buffer, size_t amount, off_t offset,
-		 uint32 wait_event_info)
+FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+		  uint32 wait_event_info)
 {
 	int			returnCode;
 	Vfd		   *vfdP;
 
 	Assert(FileIsValid(file));
 
-	DO_DB(elog(LOG, "FileRead: %d (%s) " INT64_FORMAT " %zu %p",
+	DO_DB(elog(LOG, "FileReadV: %d (%s) " INT64_FORMAT " %d",
 			   file, VfdCache[file].fileName,
 			   (int64) offset,
-			   amount, buffer));
+			   iovcnt));
 
 	returnCode = FileAccess(file);
 	if (returnCode < 0)
@@ -2131,7 +2131,7 @@ FileRead(File file, void *buffer, size_t amount, off_t offset,
 
 retry:
 	pgstat_report_wait_start(wait_event_info);
-	returnCode = pg_pread(vfdP->fd, buffer, amount, offset);
+	returnCode = pg_preadv(vfdP->fd, iov, iovcnt, offset);
 	pgstat_report_wait_end();
 
 	if (returnCode < 0)
@@ -2166,18 +2166,18 @@ retry:
 }
 
 int
-FileWrite(File file, const void *buffer, size_t amount, off_t offset,
-		  uint32 wait_event_info)
+FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+		   uint32 wait_event_info)
 {
 	int			returnCode;
 	Vfd		   *vfdP;
 
 	Assert(FileIsValid(file));
 
-	DO_DB(elog(LOG, "FileWrite: %d (%s) " INT64_FORMAT " %zu %p",
+	DO_DB(elog(LOG, "FileWriteV: %d (%s) " INT64_FORMAT " %d",
 			   file, VfdCache[file].fileName,
 			   (int64) offset,
-			   amount, buffer));
+			   iovcnt));
 
 	returnCode = FileAccess(file);
 	if (returnCode < 0)
@@ -2195,7 +2195,10 @@ FileWrite(File file, const void *buffer, size_t amount, off_t offset,
 	 */
 	if (temp_file_limit >= 0 && (vfdP->fdstate & FD_TEMP_FILE_LIMIT))
 	{
-		off_t		past_write = offset + amount;
+		off_t		past_write = offset;
+
+		for (int i = 0; i < iovcnt; ++i)
+			past_write += iov[i].iov_len;
 
 		if (past_write > vfdP->fileSize)
 		{
@@ -2211,23 +2214,27 @@ FileWrite(File file, const void *buffer, size_t amount, off_t offset,
 	}
 
 retry:
-	errno = 0;
 	pgstat_report_wait_start(wait_event_info);
-	returnCode = pg_pwrite(VfdCache[file].fd, buffer, amount, offset);
+	returnCode = pg_pwritev(vfdP->fd, iov, iovcnt, offset);
 	pgstat_report_wait_end();
 
-	/* if write didn't set errno, assume problem is no disk space */
-	if (returnCode != amount && errno == 0)
-		errno = ENOSPC;
-
 	if (returnCode >= 0)
 	{
 		/*
+		 * Some callers expect short writes to set errno, and traditionally we
+		 * have assumed that they imply disk space shortage.  We don't want to
+		 * waste CPU cycles adding up the total size here, so we'll just set
+		 * it for all successful writes in case such a caller determines that
+		 * the write was short and ereports "%m".
+		 */
+		errno = ENOSPC;
+
+		/*
 		 * Maintain fileSize and temporary_files_size if it's a temp file.
 		 */
 		if (vfdP->fdstate & FD_TEMP_FILE_LIMIT)
 		{
-			off_t		past_write = offset + amount;
+			off_t		past_write = offset + returnCode;
 
 			if (past_write > vfdP->fileSize)
 			{
@@ -2239,7 +2246,7 @@ retry:
 	else
 	{
 		/*
-		 * See comments in FileRead()
+		 * See comments in FileReadV()
 		 */
 #ifdef WIN32
 		DWORD		error = GetLastError();