commit: refactor verify_commit_buffer()

In a following commit, we are going to check commit signatures, but we
won't have a commit yet, only a commit buffer, and we are going to
discard this commit buffer if the signature is invalid. So it would be
wasteful to create a commit that we might discard, just to be able to
check a commit signature.

It would be simpler instead to be able to check commit signatures
using only a commit buffer instead of a commit.

To be able to do that, let's extract some code from the
check_commit_signature() function into a new verify_commit_buffer()
function, and then let's make check_commit_signature() call
verify_commit_buffer().

Note that this doesn't fundamentally change how
check_commit_signature() works. It used to call parse_signed_commit()
which calls repo_get_commit_buffer(), parse_buffer_signed_by_header()
and repo_unuse_commit_buffer(). Now these 3 functions are called
directly by verify_commit_buffer().

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: chriscool

commit.c

@@ -1315,15 +1315,17 @@ static void handle_signed_tag(const struct commit *parent, struct commit_extra_h
 	free(buf);
 }
 
-int check_commit_signature(const struct commit *commit, struct signature_check *sigc)
+int verify_commit_buffer(const char *buffer, size_t size,
+			 struct signature_check *sigc)
 {
 	struct strbuf payload = STRBUF_INIT;
 	struct strbuf signature = STRBUF_INIT;
 	int ret = 1;
 
 	sigc->result = 'N';
 
-	if (parse_signed_commit(commit, &payload, &signature, the_hash_algo) <= 0)
+	if (parse_buffer_signed_by_header(buffer, size, &payload,
+					  &signature, the_hash_algo) <= 0)
 		goto out;
 
 	sigc->payload_type = SIGNATURE_PAYLOAD_COMMIT;
@@ -1337,6 +1339,17 @@ int check_commit_signature(const struct commit *commit, struct signature_check *
 	return ret;
 }
 
+int check_commit_signature(const struct commit *commit, struct signature_check *sigc)
+{
+	unsigned long size;
+	const char *buffer = repo_get_commit_buffer(the_repository, commit, &size);
+	int ret = verify_commit_buffer(buffer, size, sigc);
+
+	repo_unuse_commit_buffer(the_repository, commit, buffer);
+
+	return ret;
+}
+
 void verify_merge_signature(struct commit *commit, int verbosity,
 			    int check_trust)
 {

commit.h

@@ -333,6 +333,13 @@ int remove_signature(struct strbuf *buf);
  */
 int check_commit_signature(const struct commit *commit, struct signature_check *sigc);
 
+/*
+ * Same as check_commit_signature() but accepts a commit buffer and
+ * its size, instead of a `struct commit *`.
+ */
+int verify_commit_buffer(const char *buffer, size_t size,
+			 struct signature_check *sigc);
+
 /* record author-date for each commit object */
 struct author_date_slab;
 void record_author_date(struct author_date_slab *author_date,