rebase -x: sanity check command

If the user gives an empty argument to --exec then git creates a todo
list that it cannot parse. The rebase starts to run before erroring out
with

  error: missing arguments for exec
  error: invalid line 2: exec
  You can fix this with 'git rebase --edit-todo' and then run 'git rebase --continue'.
  Or you can abort the rebase with 'git rebase --abort'.

Instead check for empty commands before starting the rebase.

Also check that the command does not contain any newlines as the
todo-list format is unable to cope with multiline commands. Note that
this changes the behavior, before this change one could do

git rebase --exec='echo one
exec echo two'

and it would insert two exec lines in the todo list, now it will error
out.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -793,6 +793,19 @@ static void set_reflog_action(struct rebase_options *options)
 	strbuf_release(&buf);
 }
+static int check_exec_cmd(const char *cmd)
+{
+	if (strchr(cmd, '\n'))
+		return error(_("exec commands cannot contain newlines"));
+
+	/* Does the command consist purely of whitespace? */
+	if (!cmd[strspn(cmd, " \t\r\f\v")])
+		return error(_("empty exec command"));
+
+	return 0;
+}
+
+
 int cmd_rebase(int argc, const char **argv, const char *prefix)
 {
 	struct rebase_options options = {
@@ -1130,6 +1143,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		}
 	}
+	for (i = 0; i < exec.nr; i++)
+		if (check_exec_cmd(exec.items[i].string))
+			exit(1);
+
 	if (!(options.flags & REBASE_NO_QUIET))
 		argv_array_push(&options.git_am_opts, "-q");

t/t3404-rebase-interactive.sh

@@ -147,6 +147,25 @@ test_expect_success 'rebase -i with the exec command checks tree cleanness' '
 	git rebase --continue
 '
+test_expect_success 'rebase -x with empty command fails' '
+	test_when_finished "git rebase --abort ||:" &&
+	test_must_fail git rebase -x "" @ 2>actual &&
+	test_write_lines "error: empty exec command" >expected &&
+	test_i18ncmp expected actual &&
+	test_must_fail git rebase -x " " @ 2>actual &&
+	test_i18ncmp expected actual
+'
+
+LF='
+'
+test_expect_success 'rebase -x with newline in command fails' '
+	test_when_finished "git rebase --abort ||:" &&
+	test_must_fail git rebase -x "a${LF}b" @ 2>actual &&
+	test_write_lines "error: exec commands cannot contain newlines" \
+			 >expected &&
+	test_i18ncmp expected actual
+'
+
 test_expect_success 'rebase -i with exec of inexistent command' '
 	git checkout master &&
 	test_when_finished "git rebase --abort" &&