Fix multiple portability issues in pg_upgrade's rewriteVisibilityMap().

This is new code in 9.6, and evidently we missed out testing it as
thoroughly as it should have been.  Bugs fixed here:

1. Use binary not text mode to open the files on Windows.  Before, if
the visibility map chanced to contain two bytes that looked like \r\n,
Windows' read() would convert that to \n, which both corrupts the map
data and causes the file to look shorter than it should.  Unless you
were *very* unlucky and had an exact multiple of 8K such occurrences
in each VM file, this would cause pg_upgrade to report a failure,
though with a rather obscure error message.

2. The code for copying rebuilt bytes into the output was simply wrong.
It chanced to work okay on little-endian machines but would emit the
bytes in the wrong order on big-endian, leading to silent corruption
of the visibility map data.

3. The code was careless about alignment of the working buffers.  Given
all three of an alignment-picky architecture, a compiler that chooses
to put the new_vmbuf[] local variable at an odd starting address, and
a checksum-enabled database, pg_upgrade would dump core.

Point one was reported by Thomas Kellerer, the other two detected by
code-reading.

Point two is much the nastiest of these issues from an impact standpoint,
though fortunately it affects only a minority of users.  The Windows issue
will definitely bite people, but it seems quite unlikely that there would
be undetected corruption from that.

In addition, I failed to resist the temptation to do some minor cosmetic
adjustments, mostly improving the comments.

It would be a good idea to try to improve the error reporting here, but
that seems like material for a separate patch.

Discussion: <nsjrbh$8li$1@blaine.gmane.org>

src/bin/pg_upgrade/file.c

@@ -18,8 +18,6 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
-#define BITS_PER_HEAPBLOCK_OLD 1
-
 
 #ifndef WIN32
 static int	copy_file(const char *fromfile, const char *tofile);
@@ -84,10 +82,11 @@ copy_file(const char *srcfile, const char *dstfile)
 		return -1;
 	}
 
-	if ((src_fd = open(srcfile, O_RDONLY, 0)) < 0)
+	if ((src_fd = open(srcfile, O_RDONLY | PG_BINARY, 0)) < 0)
 		return -1;
 
-	if ((dest_fd = open(dstfile, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR)) < 0)
+	if ((dest_fd = open(dstfile, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
+						S_IRUSR | S_IWUSR)) < 0)
 	{
 		save_errno = errno;
 
@@ -153,31 +152,30 @@ copy_file(const char *srcfile, const char *dstfile)
  * version, we could refuse to copy visibility maps from the old cluster
  * to the new cluster; the next VACUUM would recreate them, but at the
  * price of scanning the entire table.  So, instead, we rewrite the old
- * visibility maps in the new format.  That way, the all-visible bit
- * remains set for the pages for which it was set previously.  The
- * all-frozen bit is never set by this conversion; we leave that to
- * VACUUM.
+ * visibility maps in the new format.  That way, the all-visible bits
+ * remain set for the pages for which they were set previously.  The
+ * all-frozen bits are never set by this conversion; we leave that to VACUUM.
  */
 const char *
 rewriteVisibilityMap(const char *fromfile, const char *tofile)
 {
-	int			src_fd = 0;
-	int			dst_fd = 0;
-	char		buffer[BLCKSZ];
-	ssize_t		bytesRead;
+	int			src_fd;
+	int			dst_fd;
+	char	   *buffer;
+	char	   *new_vmbuf;
 	ssize_t		totalBytesRead = 0;
 	ssize_t		src_filesize;
 	int			rewriteVmBytesPerPage;
 	BlockNumber new_blkno = 0;
 	struct stat statbuf;
 
-	/* Compute we need how many old page bytes to rewrite a new page */
+	/* Compute number of old-format bytes per new page */
 	rewriteVmBytesPerPage = (BLCKSZ - SizeOfPageHeaderData) / 2;
 
 	if ((fromfile == NULL) || (tofile == NULL))
 		return "Invalid old file or new file";
 
-	if ((src_fd = open(fromfile, O_RDONLY, 0)) < 0)
+	if ((src_fd = open(fromfile, O_RDONLY | PG_BINARY, 0)) < 0)
 		return getErrorText();
 
 	if (fstat(src_fd, &statbuf) != 0)
@@ -186,7 +184,8 @@ rewriteVisibilityMap(const char *fromfile, const char *tofile)
 		return getErrorText();
 	}
 
-	if ((dst_fd = open(tofile, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR)) < 0)
+	if ((dst_fd = open(tofile, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
+					   S_IRUSR | S_IWUSR)) < 0)
 	{
 		close(src_fd);
 		return getErrorText();
@@ -195,14 +194,22 @@ rewriteVisibilityMap(const char *fromfile, const char *tofile)
 	/* Save old file size */
 	src_filesize = statbuf.st_size;
 
+	/*
+	 * Malloc the work buffers, rather than making them local arrays, to
+	 * ensure adequate alignment.
+	 */
+	buffer = (char *) pg_malloc(BLCKSZ);
+	new_vmbuf = (char *) pg_malloc(BLCKSZ);
+
 	/*
 	 * Turn each visibility map page into 2 pages one by one. Each new page
-	 * has the same page header as the old one.  If the last section of last
-	 * page is empty, we skip it, mostly to avoid turning one-page visibility
-	 * maps for small relations into two pages needlessly.
+	 * has the same page header as the old one.  If the last section of the
+	 * last page is empty, we skip it, mostly to avoid turning one-page
+	 * visibility maps for small relations into two pages needlessly.
 	 */
 	while (totalBytesRead < src_filesize)
 	{
+		ssize_t		bytesRead;
 		char	   *old_cur;
 		char	   *old_break;
 		char	   *old_blkend;
@@ -225,61 +232,59 @@ rewriteVisibilityMap(const char *fromfile, const char *tofile)
 		/*
 		 * These old_* variables point to old visibility map page. old_cur
 		 * points to current position on old page. old_blkend points to end of
-		 * old block. old_break points to old page break position for
-		 * rewriting a new page. After wrote a new page, old_break proceeds
-		 * rewriteVmBytesPerPage bytes.
+		 * old block.  old_break is the end+1 position on the old page for the
+		 * data that will be transferred to the current new page.
 		 */
 		old_cur = buffer + SizeOfPageHeaderData;
 		old_blkend = buffer + bytesRead;
 		old_break = old_cur + rewriteVmBytesPerPage;
 
-		while (old_blkend >= old_break)
+		while (old_break <= old_blkend)
 		{
-			char		new_vmbuf[BLCKSZ];
-			char	   *new_cur = new_vmbuf;
+			char	   *new_cur;
 			bool		empty = true;
 			bool		old_lastpart;
 
-			/* Copy page header in advance */
+			/* First, copy old page header to new page */
 			memcpy(new_vmbuf, &pageheader, SizeOfPageHeaderData);
 
-			/* Rewrite the last part of the old page? */
-			old_lastpart = old_lastblk && (old_blkend == old_break);
+			/* Rewriting the last part of the last old page? */
+			old_lastpart = old_lastblk && (old_break == old_blkend);
 
-			new_cur += SizeOfPageHeaderData;
+			new_cur = new_vmbuf + SizeOfPageHeaderData;
 
 			/* Process old page bytes one by one, and turn it into new page. */
-			while (old_break > old_cur)
+			while (old_cur < old_break)
 			{
+				uint8		byte = *(uint8 *) old_cur;
 				uint16		new_vmbits = 0;
 				int			i;
 
 				/* Generate new format bits while keeping old information */
 				for (i = 0; i < BITS_PER_BYTE; i++)
 				{
-					uint8		byte = *(uint8 *) old_cur;
-
-					if (byte & (1 << (BITS_PER_HEAPBLOCK_OLD * i)))
+					if (byte & (1 << i))
 					{
 						empty = false;
-						new_vmbits |= 1 << (BITS_PER_HEAPBLOCK * i);
+						new_vmbits |=
+							VISIBILITYMAP_ALL_VISIBLE << (BITS_PER_HEAPBLOCK * i);
 					}
 				}
 
-				/* Copy new visibility map bit to new format page */
-				memcpy(new_cur, &new_vmbits, BITS_PER_HEAPBLOCK);
+				/* Copy new visibility map bytes to new-format page */
+				new_cur[0] = (char) (new_vmbits & 0xFF);
+				new_cur[1] = (char) (new_vmbits >> 8);
 
-				old_cur += BITS_PER_HEAPBLOCK_OLD;
+				old_cur++;
 				new_cur += BITS_PER_HEAPBLOCK;
 			}
 
-			/* If the last part of the old page is empty, skip writing it */
+			/* If the last part of the last page is empty, skip writing it */
 			if (old_lastpart && empty)
 				break;
 
-			/* Set new checksum for a visibility map page (if enabled) */
-			if (old_cluster.controldata.data_checksum_version != 0 &&
-				new_cluster.controldata.data_checksum_version != 0)
+			/* Set new checksum for visibility map page, if enabled */
+			if (new_cluster.controldata.data_checksum_version != 0)
 				((PageHeader) new_vmbuf)->pd_checksum =
 					pg_checksum_page(new_vmbuf, new_blkno);
 
@@ -290,17 +295,19 @@ rewriteVisibilityMap(const char *fromfile, const char *tofile)
 				return getErrorText();
 			}
 
+			/* Advance for next new page */
 			old_break += rewriteVmBytesPerPage;
 			new_blkno++;
 		}
 	}
 
-	/* Close files */
+	/* Clean up */
+	pg_free(buffer);
+	pg_free(new_vmbuf);
 	close(dst_fd);
 	close(src_fd);
 
 	return NULL;
-
 }
 
 void