Fix unnecessary padding in incremental backups

Commit 10e3226 added padding to incremental backups to ensure the
block data is properly aligned. The code in sendFile() however failed to
consider that the header may be a multiple of BLCKSZ and thus already
aligned, adding a full BLCKSZ of unnecessary padding.

Not only does this make the incremental file a bit larger, but the other
places calculating the amount of padding did realize it's not needed and
did not include it in the formula. This resulted in pg_basebackup
getting confused while parsing the data stream, trying to access files
with invalid filenames (e.g. with binary data etc.) and failing.

Author: tvondra

src/backend/backup/basebackup.c

@@ -1638,11 +1638,12 @@ sendFile(bbsink *sink, const char *readfilename, const char *tarfilename,
 
 		/*
 		 * Add padding to align header to a multiple of BLCKSZ, but only if
-		 * the incremental file has some blocks. If there are no blocks we
-		 * don't want make the file unnecessarily large, as that might make
-		 * some filesystem optimizations impossible.
+		 * the incremental file has some blocks, and the alignment is actually
+		 * needed (i.e. header is not already a multiple of BLCKSZ). If there
+		 * are no blocks we don't want to make the file unnecessarily large,
+		 * as that might make some filesystem optimizations impossible.
 		 */
-		if (num_incremental_blocks > 0)
+		if ((num_incremental_blocks > 0) && (header_bytes_done % BLCKSZ != 0))
 		{
 			paddinglen = (BLCKSZ - (header_bytes_done % BLCKSZ));