gpg-interface: introduce sign_buffer_with_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 re-signing invalid commit signatures.

Move the `sign_commit_to_strbuf()` helper from "commit.c" to
"gpg-interface.c" and rename it to `sign_buffer_with_key()`. Also export
this function so it can be used by "commit.c" and
"builtin/fast-import.c" in the subsequent commit.

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

Author: jltobler

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 +1747,7 @@ 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_with_key(&buffer, &sig, sign_commit)) {
 		result = -1;
 		goto out;
 	}
@@ -1791,7 +1779,7 @@ 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_with_key(&compat_buffer, &compat_sig, sign_commit)) {
 			result = -1;
 			goto out;
 		}

gpg-interface.c

@@ -981,6 +981,19 @@ int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *sig
 	return use_format->sign_buffer(buffer, signature, signing_key);
 }
 
+int sign_buffer_with_key(struct strbuf *buffer, struct strbuf *signature,
+			 const char *signing_key)
+{
+	char *keyid_to_free = NULL;
+	int ret = 0;
+	if (!signing_key || !*signing_key)
+		signing_key = keyid_to_free = get_signing_key();
+	if (sign_buffer(buffer, signature, signing_key))
+		ret = -1;
+	free(keyid_to_free);
+	return ret;
+}
+
 /*
  * Strip CR from the line endings, in case we are on Windows.
  * NEEDSWORK: make it trim only CRs before LFs and rename

gpg-interface.h

@@ -83,6 +83,13 @@ size_t parse_signed_buffer(const char *buf, size_t size);
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
 		const char *signing_key);
 
+/*
+ * Similar to `sign_buffer()`, but uses the default configured signing key as
+ * returned by `get_signing_key()` when the provided "signing_key" is NULL or
+ * empty. Returns 0 on success, non-zero on failure.
+ */
+int sign_buffer_with_key(struct strbuf *buffer, struct strbuf *signature,
+			 const char *signing_key);
 
 /*
  * Returns corresponding string in lowercase for a given member of