revert: Don't create invalid replay_opts in parse_args

The "--ff" command-line option cannot be used with some other
command-line options.  However, parse_args still parses these
incompatible options into a replay_opts structure for use by the rest
of the program.  Although pick_commits, the current gatekeeper to the
cherry-pick machinery, checks the validity of the replay_opts
structure before before starting its operation, there will be multiple
entry points to the cherry-pick machinery in future.  To futureproof
the code and catch these errors in one place, make sure that an
invalid replay_opts structure is not created by parse_args in the
first place.  We still check the replay_opts structure for validity in
pick_commits, but this is an assert() now to emphasize that it's the
caller's responsibility to get it right.

Inspired-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Jonathan Nieder <jrnieder@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -86,9 +86,26 @@ static int option_parse_x(const struct option *opt,
 	return 0;
 }
 
+static void verify_opt_compatible(const char *me, const char *base_opt, ...)
+{
+	const char *this_opt;
+	va_list ap;
+
+	va_start(ap, base_opt);
+	while ((this_opt = va_arg(ap, const char *))) {
+		if (va_arg(ap, int))
+			break;
+	}
+	va_end(ap);
+
+	if (this_opt)
+		die(_("%s: %s cannot be used with %s"), me, this_opt, base_opt);
+}
+
 static void parse_args(int argc, const char **argv, struct replay_opts *opts)
 {
 	const char * const * usage_str = revert_or_cherry_pick_usage(opts);
+	const char *me = action_name(opts);
 	int noop;
 	struct option options[] = {
 		OPT_BOOLEAN('n', "no-commit", &opts->no_commit, "don't automatically commit"),
@@ -122,6 +139,13 @@ static void parse_args(int argc, const char **argv, struct replay_opts *opts)
 	if (opts->commit_argc < 2)
 		usage_with_options(usage_str, options);
 
+	if (opts->allow_ff)
+		verify_opt_compatible(me, "--ff",
+				"--signoff", opts->signoff,
+				"--no-commit", opts->no_commit,
+				"-x", opts->record_origin,
+				"--edit", opts->edit,
+				NULL);
 	opts->commit_argv = argv;
 }
 
@@ -569,17 +593,9 @@ static int pick_commits(struct replay_opts *opts)
 	struct commit *commit;
 
 	setenv(GIT_REFLOG_ACTION, action_name(opts), 0);
-	if (opts->allow_ff) {
-		if (opts->signoff)
-			die(_("cherry-pick --ff cannot be used with --signoff"));
-		if (opts->no_commit)
-			die(_("cherry-pick --ff cannot be used with --no-commit"));
-		if (opts->record_origin)
-			die(_("cherry-pick --ff cannot be used with -x"));
-		if (opts->edit)
-			die(_("cherry-pick --ff cannot be used with --edit"));
-	}
-
+	if (opts->allow_ff)
+		assert(!(opts->signoff || opts->no_commit ||
+				opts->record_origin || opts->edit));
 	read_and_refresh_cache(opts);
 
 	prepare_revs(&revs, opts);