Use pg_pwrite_zeros() in walmethods.c

This change impacts pg_receivewal and pg_basebackup, for the pre-padding
with zeros of all the new non-compressed WAL segments, so as the code is
more robust on partial writes.  This makes the code consistent with the
backend (XLogFileInitInternal) when wal_init_zeros is enabled for the
WAL segment initialization.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Andres Freund, Thomas Munro, Michael
Paquier
Discussion: https://postgr.es/m/CALj2ACUq7nAb7=bJNbK3yYmp-SZhJcXFR_pLk8un6XgDzDF3OA@mail.gmail.com

src/bin/pg_basebackup/walmethods.c

@@ -220,22 +220,21 @@ dir_open_for_write(WalWriteMethod *wwmethod, const char *pathname,
 	/* Do pre-padding on non-compressed files */
 	if (pad_to_size && wwmethod->compression_algorithm == PG_COMPRESSION_NONE)
 	{
-		PGAlignedXLogBlock zerobuf;
-		int			bytes;
+		ssize_t		rc;
 
-		memset(zerobuf.data, 0, XLOG_BLCKSZ);
-		for (bytes = 0; bytes < pad_to_size; bytes += XLOG_BLCKSZ)
-		{
-			errno = 0;
-			if (write(fd, zerobuf.data, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+		rc = pg_pwrite_zeros(fd, pad_to_size);
+
+		if (rc < 0)
 		{
-				/* If write didn't set errno, assume problem is no disk space */
-				wwmethod->lasterrno = errno ? errno : ENOSPC;
+			wwmethod->lasterrno = errno;
 			close(fd);
 			return NULL;
 		}
-		}
 
+		/*
+		 * pg_pwrite() (called via pg_pwrite_zeros()) may have moved the file
+		 * position, so reset it (see win32pwrite.c).
+		 */
 		if (lseek(fd, 0, SEEK_SET) != 0)
 		{
 			wwmethod->lasterrno = errno;