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>

1 files changed, 16 insertions, 0 deletions

diff --git a/csum-file.c b/csum-file.c
index 7510950..60f58f6 100644
--- a/csum-file.c
+++ b/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);
+}