interpret-trailers: add an option to show only existing trailers

It can be useful to invoke interpret-trailers for the
primary purpose of parsing existing trailers. But in that
case, we don't want to apply existing ifMissing or ifExists
rules from the config. Let's add a special mode where we
avoid applying those rules. Coupled with --only-trailers,
this gives us a reasonable parsing tool.

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

Documentation/git-interpret-trailers.txt

@@ -83,6 +83,11 @@ OPTIONS
 --only-trailers::
 	Output only the trailers, not any other parts of the input.
 
+--only-input::
+	Output only trailers that exist in the input; do not add any
+	from the command-line or by following configured `trailer.*`
+	rules.
+
 CONFIGURATION VARIABLES
 -----------------------
 

builtin/interpret-trailers.c

@@ -25,6 +25,7 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "in-place", &opts.in_place, N_("edit files in place")),
 		OPT_BOOL(0, "trim-empty", &opts.trim_empty, N_("trim empty trailers")),
 		OPT_BOOL(0, "only-trailers", &opts.only_trailers, N_("output only the trailers")),
+		OPT_BOOL(0, "only-input", &opts.only_input, N_("do not apply config rules")),
 		OPT_STRING_LIST(0, "trailer", &trailers, N_("trailer"),
 				N_("trailer(s) to add")),
 		OPT_END()
@@ -33,6 +34,12 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, options,
 			     git_interpret_trailers_usage, 0);
 
+	if (opts.only_input && trailers.nr)
+		usage_msg_opt(
+			_("--trailer with --only-input does not make sense"),
+			git_interpret_trailers_usage,
+			options);
+
 	if (argc) {
 		int i;
 		for (i = 0; i < argc; i++)

t/t7513-interpret-trailers.sh

@@ -1314,4 +1314,20 @@ test_expect_success 'only-trailers omits non-trailer in middle of block' '
 	test_cmp expected actual
 '
 
+test_expect_success 'only input' '
+	git config trailer.sign.command "echo config-value" &&
+	cat >expected <<-\EOF &&
+		existing: existing-value
+	EOF
+	git interpret-trailers \
+		--only-trailers --only-input >actual <<-\EOF &&
+		my subject
+
+		my body
+
+		existing: existing-value
+	EOF
+	test_cmp expected actual
+'
+
 test_done

trailer.c

@@ -976,7 +976,6 @@ void process_trailers(const char *file,
 		      struct string_list *trailers)
 {
 	LIST_HEAD(head);
-	LIST_HEAD(arg_head);
 	struct strbuf sb = STRBUF_INIT;
 	int trailer_end;
 	FILE *outfile = stdout;
@@ -991,9 +990,11 @@ void process_trailers(const char *file,
 	/* Print the lines before the trailers */
 	trailer_end = process_input_file(outfile, sb.buf, &head, opts);
 
-	process_command_line_args(&arg_head, trailers);
-
-	process_trailers_lists(&head, &arg_head);
+	if (!opts->only_input) {
+		LIST_HEAD(arg_head);
+		process_command_line_args(&arg_head, trailers);
+		process_trailers_lists(&head, &arg_head);
+	}
 
 	print_all(outfile, &head, opts);
 

trailer.h

@@ -26,6 +26,7 @@ struct process_trailer_options {
 	int in_place;
 	int trim_empty;
 	int only_trailers;
+	int only_input;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}