csum-file: introduce checksum_valid()

Introduce a new function which checks the validity of a file's trailing
checksum. This is similar to hashfd_check(), but different since it is
intended to be used by callers who aren't writing the same data (like
`git index-pack --verify`), but who instead want to validate the
integrity of data that they are reading.

Rewrite the first of two callers which could benefit from this new
function in pack-check.c. Subsequent callers will be added in the
following patches.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

csum-file.c

@@ -187,3 +187,19 @@ uint32_t crc32_end(struct hashfile *f)
 	f->do_crc = 0;
 	return f->crc32;
 }
+
+int hashfile_checksum_valid(const unsigned char *data, size_t total_len)
+{
+	unsigned char got[GIT_MAX_RAWSZ];
+	git_hash_ctx ctx;
+	size_t data_len = total_len - the_hash_algo->rawsz;
+
+	if (total_len < the_hash_algo->rawsz)
+		return 0; /* say "too short"? */
+
+	the_hash_algo->init_fn(&ctx);
+	the_hash_algo->update_fn(&ctx, data, data_len);
+	the_hash_algo->final_fn(got, &ctx);
+
+	return hasheq(got, data + data_len);
+}

csum-file.h

@@ -42,6 +42,9 @@ void hashflush(struct hashfile *f);
 void crc32_begin(struct hashfile *);
 uint32_t crc32_end(struct hashfile *);
 
+/* Verify checksum validity while reading. Returns non-zero on success. */
+int hashfile_checksum_valid(const unsigned char *data, size_t len);
+
 /*
  * Returns the total number of bytes fed to the hashfile so far (including ones
  * that have not been written out to the descriptor yet).

pack-check.c

@@ -164,22 +164,13 @@ static int verify_packfile(struct repository *r,
 
 int verify_pack_index(struct packed_git *p)
 {
-	size_t len;
-	const unsigned char *index_base;
-	git_hash_ctx ctx;
-	unsigned char hash[GIT_MAX_RAWSZ];
 	int err = 0;
 
 	if (open_pack_index(p))
 		return error("packfile %s index not opened", p->pack_name);
-	index_base = p->index_data;
-	len = p->index_size - the_hash_algo->rawsz;
 
 	/* Verify SHA1 sum of the index file */
-	the_hash_algo->init_fn(&ctx);
-	the_hash_algo->update_fn(&ctx, index_base, len);
-	the_hash_algo->final_fn(hash, &ctx);
-	if (!hasheq(hash, index_base + len))
+	if (!hashfile_checksum_valid(p->index_data, p->index_size))
 		err = error("Packfile index for %s hash mismatch",
 			    p->pack_name);
 	return err;