sequencer: refactor append_todo_help() to write its message to a buffer

This refactors append_todo_help() to write its message to a buffer
instead of the todo-list.  This is needed for the rewrite of
complete_action(), which will come after the next commit.

As rebase--helper still needs the file manipulation part of
append_todo_help(), it is extracted to a temporary function,
append_todo_help_to_file().  This function will go away after the
rewrite of complete_action().

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase--helper.c

@@ -97,7 +97,7 @@ int cmd_rebase__helper(int argc, const char **argv, const char *prefix)
 	if (command == ADD_EXEC && argc == 2)
 		return !!sequencer_add_exec_commands(argv[1]);
 	if (command == APPEND_TODO_HELP && argc == 1)
-		return !!append_todo_help(0, keep_empty);
+		return !!append_todo_help_to_file(0, keep_empty);
 	if (command == EDIT_TODO && argc == 1)
 		return !!edit_todo_list(flags);
 	if (command == PREPARE_BRANCH && argc == 2)

rebase-interactive.c

@@ -4,11 +4,9 @@
 #include "sequencer.h"
 #include "strbuf.h"
 
-int append_todo_help(unsigned edit_todo, unsigned keep_empty)
+void append_todo_help(unsigned edit_todo, unsigned keep_empty,
+		      struct strbuf *buf)
 {
-	struct strbuf buf = STRBUF_INIT;
-	FILE *todo;
-	int ret;
 	const char *msg = _("\nCommands:\n"
 "p, pick <commit> = use commit\n"
 "r, reword <commit> = use commit, but edit the commit message\n"
@@ -26,11 +24,7 @@ int append_todo_help(unsigned edit_todo, unsigned keep_empty)
 "\n"
 "These lines can be re-ordered; they are executed from top to bottom.\n");
 
-	todo = fopen_or_warn(rebase_path_todo(), "a");
-	if (!todo)
-		return 1;
-
-	strbuf_add_commented_lines(&buf, msg, strlen(msg));
+	strbuf_add_commented_lines(buf, msg, strlen(msg));
 
 	if (get_missing_commit_check_level() == MISSING_COMMIT_CHECK_ERROR)
 		msg = _("\nDo not remove any line. Use 'drop' "
@@ -39,7 +33,7 @@ int append_todo_help(unsigned edit_todo, unsigned keep_empty)
 		msg = _("\nIf you remove a line here "
 			 "THAT COMMIT WILL BE LOST.\n");
 
-	strbuf_add_commented_lines(&buf, msg, strlen(msg));
+	strbuf_add_commented_lines(buf, msg, strlen(msg));
 
 	if (edit_todo)
 		msg = _("\nYou are editing the todo file "
@@ -50,12 +44,25 @@ int append_todo_help(unsigned edit_todo, unsigned keep_empty)
 		msg = _("\nHowever, if you remove everything, "
 			"the rebase will be aborted.\n\n");
 
-	strbuf_add_commented_lines(&buf, msg, strlen(msg));
+	strbuf_add_commented_lines(buf, msg, strlen(msg));
 
 	if (!keep_empty) {
 		msg = _("Note that empty commits are commented out");
-		strbuf_add_commented_lines(&buf, msg, strlen(msg));
+		strbuf_add_commented_lines(buf, msg, strlen(msg));
 	}
+}
+
+int append_todo_help_to_file(unsigned edit_todo, unsigned keep_empty)
+{
+	struct strbuf buf = STRBUF_INIT;
+	FILE *todo;
+	int ret;
+
+	todo = fopen_or_warn(rebase_path_todo(), "a");
+	if (!todo)
+		return -1;
+
+	append_todo_help(edit_todo, keep_empty, &buf);
 
 	ret = fputs(buf.buf, todo);
 	if (ret < 0)
@@ -84,7 +91,17 @@ int edit_todo_list(unsigned flags)
 	strbuf_release(&buf);
 
 	transform_todos(flags | TODO_LIST_SHORTEN_IDS);
-	append_todo_help(1, 0);
+
+	if (strbuf_read_file(&buf, todo_file, 0) < 0)
+		return error_errno(_("could not read '%s'."), todo_file);
+
+	append_todo_help(1, 0, &buf);
+	if (write_message(buf.buf, buf.len, todo_file, 0)) {
+		strbuf_release(&buf);
+		return -1;
+	}
+
+	strbuf_release(&buf);
 
 	if (launch_sequence_editor(todo_file, NULL, NULL))
 		return -1;

rebase-interactive.h

@@ -1,7 +1,9 @@
 #ifndef REBASE_INTERACTIVE_H
 #define REBASE_INTERACTIVE_H
 
-int append_todo_help(unsigned edit_todo, unsigned keep_empty);
+void append_todo_help(unsigned edit_todo, unsigned keep_empty,
+		      struct strbuf *buf);
+int append_todo_help_to_file(unsigned edit_todo, unsigned keep_empty);
 int edit_todo_list(unsigned flags);
 
 #endif