verify_packfile: check pack validity before accessing data

The verify_packfile() does not explicitly open the packfile;
instead, it starts with a sha1 checksum over the whole pack,
and relies on use_pack() to open the packfile as a side
effect.

If the pack cannot be opened for whatever reason (either
because its header information is corrupted, or perhaps
because a simultaneous repack deleted it), then use_pack()
will die(), as it has no way to return an error. This is not
ideal, as verify_packfile() otherwise tries to gently return
an error (this lets programs like git-fsck go on to check
other packs).

Instead, let's check is_pack_valid() up front, and return an
error if it fails. This will open the pack as a side effect,
and then use_pack() will later rely on our cached
descriptor, and avoid calling die().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-check.c

@@ -57,11 +57,8 @@ static int verify_packfile(struct packed_git *p,
 	int err = 0;
 	struct idx_entry *entries;
 
-	/* Note that the pack header checks are actually performed by
-	 * use_pack when it first opens the pack file.  If anything
-	 * goes wrong during those checks then the call will die out
-	 * immediately.
-	 */
+	if (!is_pack_valid(p))
+		return error("packfile %s cannot be accessed", p->pack_name);
 
 	git_SHA1_Init(&ctx);
 	do {