commit: allow parsing arbitrary buffers with headers

Currently only commits are signed with headers.  However, in the future,
we'll also sign tags with headers as well.  Let's refactor out a
function called parse_buffer_signed_by_header which does exactly that.
In addition, since we'll want to sign things other than commits this
way, let's call the function sign_with_header instead of do_sign_commit.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -995,7 +995,7 @@ static const char *gpg_sig_headers[] = {
 	"gpgsig-sha256",
 };
 
-static int do_sign_commit(struct strbuf *buf, const char *keyid)
+int sign_with_header(struct strbuf *buf, const char *keyid)
 {
 	struct strbuf sig = STRBUF_INIT;
 	int inspos, copypos;
@@ -1035,13 +1035,26 @@ static int do_sign_commit(struct strbuf *buf, const char *keyid)
 	return 0;
 }
 
+
+
 int parse_signed_commit(const struct commit *commit,
 			struct strbuf *payload, struct strbuf *signature,
 			const struct git_hash_algo *algop)
 {
-
 	unsigned long size;
 	const char *buffer = get_commit_buffer(commit, &size);
+	int ret = parse_buffer_signed_by_header(buffer, size, payload, signature, algop);
+
+	unuse_commit_buffer(commit, buffer);
+	return ret;
+}
+
+int parse_buffer_signed_by_header(const char *buffer,
+				  unsigned long size,
+				  struct strbuf *payload,
+				  struct strbuf *signature,
+				  const struct git_hash_algo *algop)
+{
 	int in_signature = 0, saw_signature = 0, other_signature = 0;
 	const char *line, *tail, *p;
 	const char *gpg_sig_header = gpg_sig_headers[hash_algo_by_ptr(algop)];
@@ -1078,7 +1091,6 @@ int parse_signed_commit(const struct commit *commit,
 		}
 		line = next;
 	}
-	unuse_commit_buffer(commit, buffer);
 	return saw_signature;
 }
 
@@ -1532,7 +1544,7 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 	if (encoding_is_utf8 && !verify_utf8(&buffer))
 		fprintf(stderr, _(commit_utf8_warn));
 
-	if (sign_commit && do_sign_commit(&buffer, sign_commit)) {
+	if (sign_commit && sign_with_header(&buffer, sign_commit)) {
 		result = -1;
 		goto out;
 	}

commit.h

@@ -360,4 +360,13 @@ int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void
 LAST_ARG_MUST_BE_NULL
 int run_commit_hook(int editor_is_used, const char *index_file, const char *name, ...);
 
+/* Sign a commit or tag buffer, storing the result in a header. */
+int sign_with_header(struct strbuf *buf, const char *keyid);
+/* Parse the signature out of a header. */
+int parse_buffer_signed_by_header(const char *buffer,
+				  unsigned long size,
+				  struct strbuf *payload,
+				  struct strbuf *signature,
+				  const struct git_hash_algo *algop);
+
 #endif /* COMMIT_H */