gpg-interface: allow sign_buffer() to use default signing key

The `sign_commit_to_strbuf()` helper in "commit.c" provides fallback
logic to get the default configured signing key when a key is not
provided and handles generating the commit signature accordingly. This
signing operation is not really specific to commits as any arbitrary
buffer can be signed. Also, in a subsequent commit, this same logic is
reused by git-fast-import(1) when signing commits with invalid
signatures.

Remove the `sign_commit_to_strbuf()` helper from "commit.c" and extend
`sign_buffer()` in "gpg-interface.c" to support using the default key as
a fallback when the `SIGN_BUFFER_USE_DEFAULT_KEY` flag is provided. Call
sites are updated accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jltobler

builtin/tag.c

@@ -167,7 +167,7 @@ static int do_sign(struct strbuf *buffer, struct object_id **compat_oid,
 	char *keyid = get_signing_key();
 	int ret = -1;
 
-	if (sign_buffer(buffer, &sig, keyid))
+	if (sign_buffer(buffer, &sig, keyid, 0))
 		goto out;
 
 	if (compat) {
@@ -176,7 +176,7 @@ static int do_sign(struct strbuf *buffer, struct object_id **compat_oid,
 		if (convert_object_file(the_repository ,&compat_buf, algo, compat,
 					buffer->buf, buffer->len, OBJ_TAG, 1))
 			goto out;
-		if (sign_buffer(&compat_buf, &compat_sig, keyid))
+		if (sign_buffer(&compat_buf, &compat_sig, keyid, 0))
 			goto out;
 		add_header_signature(&compat_buf, &sig, algo);
 		strbuf_addbuf(&compat_buf, &compat_sig);

commit.c

@@ -1170,18 +1170,6 @@ int add_header_signature(struct strbuf *buf, struct strbuf *sig, const struct gi
 	return 0;
 }
 
-static int sign_commit_to_strbuf(struct strbuf *sig, struct strbuf *buf, const char *keyid)
-{
-	char *keyid_to_free = NULL;
-	int ret = 0;
-	if (!keyid || !*keyid)
-		keyid = keyid_to_free = get_signing_key();
-	if (sign_buffer(buf, sig, keyid))
-		ret = -1;
-	free(keyid_to_free);
-	return ret;
-}
-
 int parse_signed_commit(const struct commit *commit,
 			struct strbuf *payload, struct strbuf *signature,
 			const struct git_hash_algo *algop)
@@ -1759,7 +1747,8 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 		oidcpy(&parent_buf[i++], &p->item->object.oid);
 
 	write_commit_tree(&buffer, msg, msg_len, tree, parent_buf, nparents, author, committer, extra);
-	if (sign_commit && sign_commit_to_strbuf(&sig, &buffer, sign_commit)) {
+	if (sign_commit && sign_buffer(&buffer, &sig, sign_commit,
+				       SIGN_BUFFER_USE_DEFAULT_KEY)) {
 		result = -1;
 		goto out;
 	}
@@ -1791,7 +1780,9 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 		free_commit_extra_headers(compat_extra);
 		free(mapped_parents);
 
-		if (sign_commit && sign_commit_to_strbuf(&compat_sig, &compat_buffer, sign_commit)) {
+		if (sign_commit && sign_buffer(&compat_buffer, &compat_sig,
+					       sign_commit,
+					       SIGN_BUFFER_USE_DEFAULT_KEY)) {
 			result = -1;
 			goto out;
 		}

gpg-interface.c

@@ -974,11 +974,20 @@ const char *gpg_trust_level_to_str(enum signature_trust_level level)
 	return sigcheck_gpg_trust_level[level].display_key;
 }
 
-int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)
+int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
+		const char *signing_key, enum sign_buffer_flags flags)
 {
+	char *keyid_to_free = NULL;
+	int ret = 0;
+
 	gpg_interface_lazy_init();
 
-	return use_format->sign_buffer(buffer, signature, signing_key);
+	if ((flags & SIGN_BUFFER_USE_DEFAULT_KEY) && (!signing_key || !*signing_key))
+		signing_key = keyid_to_free = get_signing_key();
+
+	ret = use_format->sign_buffer(buffer, signature, signing_key);
+	free(keyid_to_free);
+	return ret;
 }
 
 /*

gpg-interface.h

@@ -74,15 +74,23 @@ int parse_signature(const char *buf, size_t size, struct strbuf *payload, struct
  */
 size_t parse_signed_buffer(const char *buf, size_t size);
 
+/* Flags for sign_buffer(). */
+enum sign_buffer_flags {
+	/*
+	 * Use the default configured signing key as returned by `get_signing_key()`
+	 * when the provided "signing_key" is NULL or empty.
+	 */
+	SIGN_BUFFER_USE_DEFAULT_KEY = (1 << 0),
+};
+
 /*
  * Create a detached signature for the contents of "buffer" and append
  * it after "signature"; "buffer" and "signature" can be the same
  * strbuf instance, which would cause the detached signature appended
  * at the end.  Returns 0 on success, non-zero on failure.
  */
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
-		const char *signing_key);
-
+		const char *signing_key, enum sign_buffer_flags flags);
 
 /*
  * Returns corresponding string in lowercase for a given member of

send-pack.c

@@ -391,7 +391,7 @@ static int generate_push_cert(struct strbuf *req_buf,
 	if (!update_seen)
 		goto free_return;
 
-	if (sign_buffer(&cert, &cert, signing_key))
+	if (sign_buffer(&cert, &cert, signing_key, 0))
 		die(_("failed to sign the push certificate"));
 
 	packet_buf_write(req_buf, "push-cert%c%s", 0, cap_string);