check return value of verify_ref_format()

Users of the ref-filter code must call verify_ref_format()
before formatting any refs, but most ignore its return
value. This means we may print an error on a syntactically
bogus pattern, but keep going anyway.

In most cases this results in a fatal error when we actually
try to format a ref. But if you have no refs to show at all,
then the behavior is confusing: git prints the error from
verify_ref_format(), then exits with code 0 without showing
any output.  Let's instead abort immediately if we know we
have a bogus format.

We'll output the usage information if we have it handy (just
like the existing call in cmd_for_each_ref() does), and
otherwise just die().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -409,7 +409,9 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 
 	if (!format)
 		format = to_free = build_format(filter, maxwidth, remote_prefix);
-	verify_ref_format(format);
+
+	if (verify_ref_format(format))
+		die(_("unable to parse format string"));
 
 	ref_array_sort(sorting, &array);
 

builtin/tag.c

@@ -53,7 +53,8 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting, con
 			format = "%(refname:lstrip=2)";
 	}
 
-	verify_ref_format(format);
+	if (verify_ref_format(format))
+		die(_("unable to parse format string"));
 	filter->with_commit_tag_algo = 1;
 	filter_refs(&array, filter, FILTER_REFS_TAGS);
 	ref_array_sort(sorting, &array);
@@ -501,8 +502,8 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	if (cmdmode == 'd')
 		return for_each_tag_name(argv, delete_tag, NULL);
 	if (cmdmode == 'v') {
-		if (format)
-			verify_ref_format(format);
+		if (format && verify_ref_format(format))
+			usage_with_options(git_tag_usage, options);
 		return for_each_tag_name(argv, verify_tag, format);
 	}
 

builtin/verify-tag.c

@@ -51,7 +51,9 @@ int cmd_verify_tag(int argc, const char **argv, const char *prefix)
 		flags |= GPG_VERIFY_VERBOSE;
 
 	if (fmt_pretty) {
-		verify_ref_format(fmt_pretty);
+		if (verify_ref_format(fmt_pretty))
+			usage_with_options(verify_tag_usage,
+					   verify_tag_options);
 		flags |= GPG_VERIFY_OMIT_STATUS;
 	}