Return ssize_t in fd.c I/O functions.

In the past, FileRead() and FileWrite() used types based on the Unix
read() and write() functions from before C and POSIX standardization,
though not exactly (we had int for amount instead of unsigned).  In
commit 2d4f1ba we changed to the appropriate standard C types, just
like the modern POSIX functions they wrap, but again not exactly: the
return type stayed as int.  In theory, a ssize_t value could be returned
by the underlying call that is too large for an int.

That wasn't really a live bug, because we don't expect PostgreSQL code
to perform reads or writes of gigabytes, and OSes probably apply
internal caps smaller than that anyway.  This change is done on the
principle that the return might as well follow the standard interfaces
consistently.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/1672202.1703441340%40sss.pgh.pa.us

Author: macdice

src/backend/storage/file/fd.c

@@ -2132,11 +2132,11 @@ FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
 	pgstat_report_wait_end();
 }
 
-int
+ssize_t
 FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
 		  uint32 wait_event_info)
 {
-	int			returnCode;
+	ssize_t		returnCode;
 	Vfd		   *vfdP;
 
 	Assert(FileIsValid(file));
@@ -2188,11 +2188,11 @@ FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
 	return returnCode;
 }
 
-int
+ssize_t
 FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
 		   uint32 wait_event_info)
 {
-	int			returnCode;
+	ssize_t		returnCode;
 	Vfd		   *vfdP;
 
 	Assert(FileIsValid(file));

src/include/storage/fd.h

@@ -107,8 +107,8 @@ extern File PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fil
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
 extern int	FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern int	FileReadV(File file, const struct iovec *ioc, int iovcnt, off_t offset, uint32 wait_event_info);
-extern int	FileWriteV(File file, const struct iovec *ioc, int iovcnt, off_t offset, uint32 wait_event_info);
+extern ssize_t FileReadV(File file, const struct iovec *ioc, int iovcnt, off_t offset, uint32 wait_event_info);
+extern ssize_t FileWriteV(File file, const struct iovec *ioc, int iovcnt, off_t offset, uint32 wait_event_info);
 extern int	FileSync(File file, uint32 wait_event_info);
 extern int	FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info);
 extern int	FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info);
@@ -192,7 +192,7 @@ extern int	durable_unlink(const char *fname, int elevel);
 extern void SyncDataDirectory(void);
 extern int	data_sync_elevel(int elevel);
 
-static inline int
+static inline ssize_t
 FileRead(File file, void *buffer, size_t amount, off_t offset,
 		 uint32 wait_event_info)
 {
@@ -204,7 +204,7 @@ FileRead(File file, void *buffer, size_t amount, off_t offset,
 	return FileReadV(file, &iov, 1, offset, wait_event_info);
 }
 
-static inline int
+static inline ssize_t
 FileWrite(File file, const void *buffer, size_t amount, off_t offset,
 		  uint32 wait_event_info)
 {