rebase: introduce git_commit_create_cb

Introduce a new mechanism for `git_rebase_commit` for callers to
customize the experience.  Instead of assuming that we produce the
commit for them, provide a commit creation callback that allows callers
to produce the commit themselves and return the resulting commit id.

Author: ethomson

include/git2/commit.h

@@ -502,6 +502,43 @@ GIT_EXTERN(int) git_commit_create_with_signature(
  */
 GIT_EXTERN(int) git_commit_dup(git_commit **out, git_commit *source);
 
+/**
+ * Commit creation callback: used when a function is going to create
+ * commits (for example, in `git_rebase_commit`) to allow callers to
+ * override the commit creation behavior.  For example, users may
+ * wish to sign commits by providing this information to
+ * `git_commit_create_buffer`, signing that buffer, then calling
+ * `git_commit_create_with_signature`.  The resultant commit id
+ * should be set in the `out` object id parameter.
+ *
+ * @param out pointer that this callback will populate with the object
+ *            id of the commit that is created
+ * @param author the author name and time of the commit
+ * @param committer the committer name and time of the commit
+ * @param message_encoding the encoding of the given message, or NULL
+ *                         to assume UTF8
+ * @param message the commit message
+ * @param tree the tree to be committed
+ * @param parent_count the number of parents for this commit
+ * @param parents the commit parents
+ * @param payload the payload pointer in the rebase options
+ * @return 0 if this callback has created the commit and populated the out
+ *         parameter, GIT_PASSTHROUGH if the callback has not created a
+ *         commit and wants the calling function to create the commit as
+ *         if no callback had been specified, any other value to stop
+ *         and return a failure
+ */
+typedef int (*git_commit_create_cb)(
+	git_oid *out,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	const git_tree *tree,
+	size_t parent_count,
+	const git_commit *parents[],
+	void *payload);
+
 /**
  * Commit signing callback.
  *

include/git2/rebase.h

@@ -74,12 +74,27 @@ typedef struct {
 	 */
 	git_checkout_options checkout_options;
 
+	/**
+	 * Optional callback that allows users to override commit
+	 * creation in `git_rebase_commit`.  If specified, users can
+	 * create their own commit and provide the commit ID, which
+	 * may be useful for signing commits or otherwise customizing
+	 * the commit creation.
+	 *
+	 * If this callback returns `GIT_PASSTHROUGH`, then
+	 * `git_rebase_commit` will continue to create the commit.
+	 */
+	git_commit_create_cb commit_create_cb;
+
 	/**
 	 * If provided, this will be called with the commit content, allowing
 	 * a signature to be added to the rebase commit. Can be skipped with
 	 * GIT_PASSTHROUGH. If GIT_PASSTHROUGH is returned, a commit will be made
 	 * without a signature.
 	 * This field is only used when performing git_rebase_commit.
+	 *
+	 * This callback is not invoked if a `git_commit_create_cb` is
+	 * specified.
 	 */
 	git_commit_signing_cb signing_cb;
 

src/rebase.c

@@ -943,6 +943,52 @@ int git_rebase_inmemory_index(
 	return 0;
 }
 
+static int create_signed(
+	git_oid *out,
+	git_rebase *rebase,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	git_tree *tree,
+	size_t parent_count,
+	const git_commit **parents)
+{
+	git_buf commit_content = GIT_BUF_INIT,
+	        commit_signature = GIT_BUF_INIT,
+	        signature_field = GIT_BUF_INIT;
+	int error;
+
+	git_error_clear();
+
+	if ((error = git_commit_create_buffer(&commit_content,
+		rebase->repo, author, committer, message_encoding,
+		message, tree, parent_count, parents)) < 0)
+		goto done;
+
+	error = rebase->options.signing_cb(&commit_signature,
+		&signature_field, commit_content.ptr,
+		rebase->options.payload);
+
+	if (error) {
+		if (error != GIT_PASSTHROUGH)
+			git_error_set_after_callback_function(error, "signing_cb");
+
+		goto done;
+	}
+
+	error = git_commit_create_with_signature(out, rebase->repo,
+		commit_content.ptr,
+		commit_signature.size > 0 ? commit_signature.ptr : NULL,
+		signature_field.size > 0 ? signature_field.ptr : NULL);
+
+done:
+	git_buf_dispose(&commit_signature);
+	git_buf_dispose(&signature_field);
+	git_buf_dispose(&commit_content);
+	return error;
+}
+
 static int rebase_commit__create(
 	git_commit **out,
 	git_rebase *rebase,
@@ -957,10 +1003,6 @@ static int rebase_commit__create(
 	git_commit *current_commit = NULL, *commit = NULL;
 	git_tree *parent_tree = NULL, *tree = NULL;
 	git_oid tree_id, commit_id;
-	git_buf commit_content = GIT_BUF_INIT, commit_signature = GIT_BUF_INIT,
-		signature_field = GIT_BUF_INIT;
-	const char *signature_field_string = NULL,
-		*commit_signature_string = NULL;
 	int error;
 
 	operation = git_array_get(rebase->operations, rebase->current);
@@ -991,37 +1033,29 @@ static int rebase_commit__create(
 		message = git_commit_message(current_commit);
 	}
 
-	if ((error = git_commit_create_buffer(&commit_content, rebase->repo, author, committer,
-			message_encoding, message, tree, 1, (const git_commit **)&parent_commit)) < 0)
-		goto done;
-
-	if (rebase->options.signing_cb) {
-		git_error_clear();
-		error = git_error_set_after_callback_function(rebase->options.signing_cb(
-			&commit_signature, &signature_field, git_buf_cstr(&commit_content),
-			rebase->options.payload), "commit signing_cb failed");
-		if (error == GIT_PASSTHROUGH) {
-			git_buf_dispose(&commit_signature);
-			git_buf_dispose(&signature_field);
-			git_error_clear();
-			error = GIT_OK;
-		} else if (error < 0)
-			goto done;
-	}
-
-	if (git_buf_is_allocated(&commit_signature)) {
-		GIT_ASSERT(git_buf_contains_nul(&commit_signature));
-		commit_signature_string = git_buf_cstr(&commit_signature);
+	git_error_clear();
+	error = GIT_PASSTHROUGH;
+
+	if (rebase->options.commit_create_cb) {
+		error = rebase->options.commit_create_cb(&commit_id,
+			author, committer, message_encoding, message,
+			tree, 1, (const git_commit **)&parent_commit,
+			rebase->options.payload);
+
+		git_error_set_after_callback_function(error,
+			"commit_create_cb");
+	} else if (rebase->options.signing_cb) {
+		error = create_signed(&commit_id, rebase, author,
+			committer, message_encoding, message, tree,
+			1, (const git_commit **)&parent_commit);
 	}
 
-	if (git_buf_is_allocated(&signature_field)) {
-		GIT_ASSERT(git_buf_contains_nul(&signature_field));
-		signature_field_string = git_buf_cstr(&signature_field);
-	}
+	if (error == GIT_PASSTHROUGH)
+		error = git_commit_create(&commit_id, rebase->repo, NULL,
+			author, committer, message_encoding, message,
+			tree, 1, (const git_commit **)&parent_commit);
 
-	if ((error = git_commit_create_with_signature(&commit_id, rebase->repo,
-			git_buf_cstr(&commit_content), commit_signature_string,
-			signature_field_string)))
+	if (error)
 		goto done;
 
 	if ((error = git_commit_lookup(&commit, rebase->repo, &commit_id)) < 0)
@@ -1033,9 +1067,6 @@ static int rebase_commit__create(
 	if (error < 0)
 		git_commit_free(commit);
 
-	git_buf_dispose(&commit_signature);
-	git_buf_dispose(&signature_field);
-	git_buf_dispose(&commit_content);
 	git_commit_free(current_commit);
 	git_tree_free(parent_tree);
 	git_tree_free(tree);