Introduce pg_pwrite_zeros() in fileutils.c

This routine is designed to write zeros to a file using vectored I/O,
for a size given by its caller, being useful when it comes to
initializing a file with a final size already known.

XLogFileInitInternal() in xlog.c is changed to use this new routine when
initializing WAL segments with zeros (wal_init_zero enabled).  Note that
the aligned buffers used for the vectored I/O writes have a size of
XLOG_BLCKSZ, and not BLCKSZ anymore, as pg_pwrite_zeros() relies on
PGAlignedBlock while xlog.c originally used PGAlignedXLogBlock.

This routine will be used in a follow-up patch to do the pre-padding of
WAL segments for pg_receivewal and pg_basebackup when these are not
compressed.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Andres Freund, Thomas Munro, Michael
Paquier
Discussion: https://www.postgresql.org/message-id/CALj2ACUq7nAb7%3DbJNbK3yYmp-SZhJcXFR_pLk8un6XgDzDF3OA%40mail.gmail.com

src/common/file_utils.c

@@ -527,3 +527,76 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 
 	return sum;
 }
+
+/*
+ * pg_pwrite_zeros
+ *
+ * Writes zeros to file worth "size" bytes, using vectored I/O.
+ *
+ * Returns the total amount of data written.  On failure, a negative value
+ * is returned with errno set.
+ */
+ssize_t
+pg_pwrite_zeros(int fd, size_t size)
+{
+	PGAlignedBlock zbuffer;		/* worth BLCKSZ */
+	size_t		zbuffer_sz;
+	struct iovec iov[PG_IOV_MAX];
+	int			blocks;
+	size_t		remaining_size = 0;
+	int			i;
+	ssize_t		written;
+	ssize_t		total_written = 0;
+
+	zbuffer_sz = sizeof(zbuffer.data);
+
+	/* Zero-fill the buffer. */
+	memset(zbuffer.data, 0, zbuffer_sz);
+
+	/* Prepare to write out a lot of copies of our zero buffer at once. */
+	for (i = 0; i < lengthof(iov); ++i)
+	{
+		iov[i].iov_base = zbuffer.data;
+		iov[i].iov_len = zbuffer_sz;
+	}
+
+	/* Loop, writing as many blocks as we can for each system call. */
+	blocks = size / zbuffer_sz;
+	remaining_size = size % zbuffer_sz;
+	for (i = 0; i < blocks;)
+	{
+		int			iovcnt = Min(blocks - i, lengthof(iov));
+		off_t		offset = i * zbuffer_sz;
+
+		written = pg_pwritev_with_retry(fd, iov, iovcnt, offset);
+
+		if (written < 0)
+			return written;
+
+		i += iovcnt;
+		total_written += written;
+	}
+
+	/* Now, write the remaining size, if any, of the file with zeros. */
+	if (remaining_size > 0)
+	{
+		/* We'll never write more than one block here */
+		int			iovcnt = 1;
+
+		/* Jump on to the end of previously written blocks */
+		off_t		offset = i * zbuffer_sz;
+
+		iov[0].iov_len = remaining_size;
+
+		written = pg_pwritev_with_retry(fd, iov, iovcnt, offset);
+
+		if (written < 0)
+			return written;
+
+		total_written += written;
+	}
+
+	Assert(total_written == size);
+
+	return total_written;
+}