In basebackup.c, perform end-of-file test after checksum validation.

We read blocks of data from files that we're backing up in chunks,
some multiple of BLCKSZ for each read. If checksum verification fails,
we then try rereading just the one block for which validation failed.
If that block happened to be the first block of the chunk, and if
the file was concurrently truncated to remove that block, then we'd
reach a call to bbsink_archive_contents() with a buffer length of 0.
That causes an assertion failure.

As far as I can see, there are no particularly bad consequences if
this happens in a non-assert build, and it's pretty unlikely to happen
in the first place because it requires a series of somewhat unlikely
things to happen in very quick succession. However, assertion failures
are bad, so rearrange the code to avoid that possibility.

Patch by me, reviewed by Michael Paquier.

Discussion: http://postgr.es/m/CA+TgmoZ_fFAoU6mrHt9QBs+dcYhN6yXenGTTMRebZNhtwPwHyg@mail.gmail.com

src/backend/backup/basebackup.c

@@ -1568,14 +1568,6 @@ sendFile(bbsink *sink, const char *readfilename, const char *tarfilename,
 								   Min(sink->bbs_buffer_length, remaining),
 								   len, readfilename, true);
 
-		/*
-		 * If we hit end-of-file, a concurrent truncation must have occurred.
-		 * That's not an error condition, because WAL replay will fix things
-		 * up.
-		 */
-		if (cnt == 0)
-			break;
-
 		/*
 		 * The checksums are verified at block level, so we iterate over the
 		 * buffer in chunks of BLCKSZ, after making sure that
@@ -1678,6 +1670,15 @@ sendFile(bbsink *sink, const char *readfilename, const char *tarfilename,
 			}
 		}
 
+		/*
+		 * If we hit end-of-file, a concurrent truncation must have occurred.
+		 * That's not an error condition, because WAL replay will fix things
+		 * up.
+		 */
+		if (cnt == 0)
+			break;
+
+		/* Archive the data we just read. */
 		bbsink_archive_contents(sink, cnt);
 
 		/* Also feed it to the checksum machinery. */