rebase -i: check for missing commits in the rebase--helper

In particular on Windows, where shell scripts are even more expensive
than on MacOSX or Linux, it makes sense to move a loop that forks
Git at least once for every line in the todo list into a builtin.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2519,3 +2519,125 @@ int transform_todo_ids(int shorten_ids)
 	todo_list_release(&todo_list);
 	return 0;
 }
+
+enum check_level {
+	CHECK_IGNORE = 0, CHECK_WARN, CHECK_ERROR
+};
+
+static enum check_level get_missing_commit_check_level(void)
+{
+	const char *value;
+
+	if (git_config_get_value("rebase.missingcommitscheck", &value) ||
+			!strcasecmp("ignore", value))
+		return CHECK_IGNORE;
+	if (!strcasecmp("warn", value))
+		return CHECK_WARN;
+	if (!strcasecmp("error", value))
+		return CHECK_ERROR;
+	warning(_("unrecognized setting %s for option"
+		  "rebase.missingCommitsCheck. Ignoring."), value);
+	return CHECK_IGNORE;
+}
+
+/*
+ * Check if the user dropped some commits by mistake
+ * Behaviour determined by rebase.missingCommitsCheck.
+ * Check if there is an unrecognized command or a
+ * bad SHA-1 in a command.
+ */
+int check_todo_list(void)
+{
+	enum check_level check_level = get_missing_commit_check_level();
+	struct strbuf todo_file = STRBUF_INIT;
+	struct todo_list todo_list = TODO_LIST_INIT;
+	struct strbuf missing = STRBUF_INIT;
+	int advise_to_edit_todo = 0, res = 0, fd, i;
+
+	strbuf_addstr(&todo_file, rebase_path_todo());
+	fd = open(todo_file.buf, O_RDONLY);
+	if (fd < 0) {
+		res = error_errno(_("could not open '%s'"), todo_file.buf);
+		goto leave_check;
+	}
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		res = error(_("could not read '%s'."), todo_file.buf);
+		goto leave_check;
+	}
+	close(fd);
+	advise_to_edit_todo = res =
+		parse_insn_buffer(todo_list.buf.buf, &todo_list);
+
+	if (res || check_level == CHECK_IGNORE)
+		goto leave_check;
+
+	/* Mark the commits in git-rebase-todo as seen */
+	for (i = 0; i < todo_list.nr; i++) {
+		struct commit *commit = todo_list.items[i].commit;
+		if (commit)
+			commit->util = (void *)1;
+	}
+
+	todo_list_release(&todo_list);
+	strbuf_addstr(&todo_file, ".backup");
+	fd = open(todo_file.buf, O_RDONLY);
+	if (fd < 0) {
+		res = error_errno(_("could not open '%s'"), todo_file.buf);
+		goto leave_check;
+	}
+	if (strbuf_read(&todo_list.buf, fd, 0) < 0) {
+		close(fd);
+		res = error(_("could not read '%s'."), todo_file.buf);
+		goto leave_check;
+	}
+	close(fd);
+	strbuf_release(&todo_file);
+	res = !!parse_insn_buffer(todo_list.buf.buf, &todo_list);
+
+	/* Find commits in git-rebase-todo.backup yet unseen */
+	for (i = todo_list.nr - 1; i >= 0; i--) {
+		struct todo_item *item = todo_list.items + i;
+		struct commit *commit = item->commit;
+		if (commit && !commit->util) {
+			strbuf_addf(&missing, " - %s %.*s\n",
+				    short_commit_name(commit),
+				    item->arg_len, item->arg);
+			commit->util = (void *)1;
+		}
+	}
+
+	/* Warn about missing commits */
+	if (!missing.len)
+		goto leave_check;
+
+	if (check_level == CHECK_ERROR)
+		advise_to_edit_todo = res = 1;
+
+	fprintf(stderr,
+		_("Warning: some commits may have been dropped accidentally.\n"
+		"Dropped commits (newer to older):\n"));
+
+	/* Make the list user-friendly and display */
+	fputs(missing.buf, stderr);
+	strbuf_release(&missing);
+
+	fprintf(stderr, _("To avoid this message, use \"drop\" to "
+		"explicitly remove a commit.\n\n"
+		"Use 'git config rebase.missingCommitsCheck' to change "
+		"the level of warnings.\n"
+		"The possible behaviours are: ignore, warn, error.\n\n"));
+
+leave_check:
+	strbuf_release(&todo_file);
+	todo_list_release(&todo_list);
+
+	if (advise_to_edit_todo)
+		fprintf(stderr,
+			_("You can fix this with 'git rebase --edit-todo' "
+			  "and then run 'git rebase --continue'.\n"
+			  "Or you can abort the rebase with 'git rebase"
+			  " --abort'.\n"));
+
+	return res;
+}