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

In theory it's easy for any reader who wants to parse
trailers to do so. But there are a lot of subtle corner
cases around what counts as a trailer, when the trailer
block begins and ends, etc. Since interpret-trailers already
has our parsing logic, let's let callers ask it to just
output the trailers.

They still have to parse the "key: value" lines, but at
least they can ignore all of the other corner cases.

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

Documentation/git-interpret-trailers.txt

@@ -80,6 +80,9 @@ OPTIONS
 	trailer to the input messages. See the description of this
 	command.
 
+--only-trailers::
+	Output only the trailers, not any other parts of the input.
+
 CONFIGURATION VARIABLES
 -----------------------
 

builtin/interpret-trailers.c

@@ -24,6 +24,7 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		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_STRING_LIST(0, "trailer", &trailers, N_("trailer"),
 				N_("trailer(s) to add")),
 		OPT_END()

t/t7513-interpret-trailers.sh

@@ -1275,4 +1275,43 @@ test_expect_success 'with cut line' '
 	test_cmp expected actual
 '
 
+test_expect_success 'only trailers' '
+	git config trailer.sign.command "echo config-value" &&
+	cat >expected <<-\EOF &&
+		existing: existing-value
+		sign: config-value
+		added: added-value
+	EOF
+	git interpret-trailers \
+		--trailer added:added-value \
+		--only-trailers >actual <<-\EOF &&
+		my subject
+
+		my body
+
+		existing: existing-value
+	EOF
+	test_cmp expected actual
+'
+
+test_expect_success 'only-trailers omits non-trailer in middle of block' '
+	git config trailer.sign.command "echo config-value" &&
+	cat >expected <<-\EOF &&
+		Signed-off-by: nobody <nobody@nowhere>
+		Signed-off-by: somebody <somebody@somewhere>
+		sign: config-value
+	EOF
+	git interpret-trailers --only-trailers >actual <<-\EOF &&
+		subject
+
+		it is important that the trailers below are signed-off-by
+		so that they meet the "25% trailers Git knows about" heuristic
+
+		Signed-off-by: nobody <nobody@nowhere>
+		this is not a trailer
+		Signed-off-by: somebody <somebody@somewhere>
+	EOF
+	test_cmp expected actual
+'
+
 test_done

trailer.c

@@ -163,13 +163,15 @@ static void print_tok_val(FILE *outfile, const char *tok, const char *val)
 		fprintf(outfile, "%s%c %s\n", tok, separators[0], val);
 }
 
-static void print_all(FILE *outfile, struct list_head *head, int trim_empty)
+static void print_all(FILE *outfile, struct list_head *head,
+		      const struct process_trailer_options *opts)
 {
 	struct list_head *pos;
 	struct trailer_item *item;
 	list_for_each(pos, head) {
 		item = list_entry(pos, struct trailer_item, list);
-		if (!trim_empty || strlen(item->value) > 0)
+		if ((!opts->trim_empty || strlen(item->value) > 0) &&
+		    (!opts->only_trailers || item->token))
 			print_tok_val(outfile, item->token, item->value);
 	}
 }
@@ -886,7 +888,8 @@ static int ends_with_blank_line(const char *buf, size_t len)
 
 static int process_input_file(FILE *outfile,
 			      const char *str,
-			      struct list_head *head)
+			      struct list_head *head,
+			      const struct process_trailer_options *opts)
 {
 	struct trailer_info info;
 	struct strbuf tok = STRBUF_INIT;
@@ -896,9 +899,10 @@ static int process_input_file(FILE *outfile,
 	trailer_info_get(&info, str);
 
 	/* Print lines before the trailers as is */
-	fwrite(str, 1, info.trailer_start - str, outfile);
+	if (!opts->only_trailers)
+		fwrite(str, 1, info.trailer_start - str, outfile);
 
-	if (!info.blank_line_before_trailer)
+	if (!opts->only_trailers && !info.blank_line_before_trailer)
 		fprintf(outfile, "\n");
 
 	for (i = 0; i < info.trailer_nr; i++) {
@@ -913,7 +917,7 @@ static int process_input_file(FILE *outfile,
 			add_trailer_item(head,
 					 strbuf_detach(&tok, NULL),
 					 strbuf_detach(&val, NULL));
-		} else {
+		} else if (!opts->only_trailers) {
 			strbuf_addstr(&val, trailer);
 			strbuf_strip_suffix(&val, "\n");
 			add_trailer_item(head,
@@ -985,18 +989,19 @@ void process_trailers(const char *file,
 		outfile = create_in_place_tempfile(file);
 
 	/* Print the lines before the trailers */
-	trailer_end = process_input_file(outfile, sb.buf, &head);
+	trailer_end = process_input_file(outfile, sb.buf, &head, opts);
 
 	process_command_line_args(&arg_head, trailers);
 
 	process_trailers_lists(&head, &arg_head);
 
-	print_all(outfile, &head, opts->trim_empty);
+	print_all(outfile, &head, opts);
 
 	free_all(&head);
 
 	/* Print the lines after the trailers as is */
-	fwrite(sb.buf + trailer_end, 1, sb.len - trailer_end, outfile);
+	if (!opts->only_trailers)
+		fwrite(sb.buf + trailer_end, 1, sb.len - trailer_end, outfile);
 
 	if (opts->in_place)
 		if (rename_tempfile(&trailers_tempfile, file))

trailer.h

@@ -25,6 +25,7 @@ struct trailer_info {
 struct process_trailer_options {
 	int in_place;
 	int trim_empty;
+	int only_trailers;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}