pretty: add %(trailers) format for displaying trailers of a commit message

Recent patches have expanded on the trailers.c code and we have the
builtin commant git-interpret-trailers which can be used to add or
modify trailer lines. However, there is no easy way to simply display
the trailers of a commit message.

Add support for %(trailers) format modifier which will use the
trailer_info_get() calls to read trailers in an identical way as git
interpret-trailers does. Use a long format option instead of a short
name so that future work can more easily unify ref-filter and pretty
formats.

Add documentation and tests for the same.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -199,6 +199,8 @@ endif::git-rev-list[]
   than given and there are spaces on its left, use those spaces
 - '%><(<N>)', '%><|(<N>)': similar to '% <(<N>)', '%<|(<N>)'
   respectively, but padding both sides (i.e. the text is centered)
+-%(trailers): display the trailers of the body as interpreted by
+  linkgit:git-interpret-trailers[1]
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will

pretty.c

@@ -10,6 +10,7 @@
 #include "color.h"
 #include "reflog-walk.h"
 #include "gpg-interface.h"
+#include "trailer.h"
 
 static char *user_format;
 static struct cmt_fmt_map {
@@ -889,6 +890,16 @@ const char *format_subject(struct strbuf *sb, const char *msg,
 	return msg;
 }
 
+static void format_trailers(struct strbuf *sb, const char *msg)
+{
+	struct trailer_info info;
+
+	trailer_info_get(&info, msg);
+	strbuf_add(sb, info.trailer_start,
+		   info.trailer_end - info.trailer_start);
+	trailer_info_release(&info);
+}
+
 static void parse_commit_message(struct format_commit_context *c)
 {
 	const char *msg = c->message + c->message_off;
@@ -1292,6 +1303,12 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		strbuf_addstr(sb, msg + c->body_off);
 		return 1;
 	}
+
+	if (starts_with(placeholder, "(trailers)")) {
+		format_trailers(sb, msg + c->subject_off);
+		return strlen("(trailers)");
+	}
+
 	return 0;	/* unknown placeholder */
 }
 

t/t4205-log-pretty-formats.sh

@@ -535,4 +535,30 @@ test_expect_success 'clean log decoration' '
 	test_cmp expected actual1
 '
 
+cat >trailers <<EOF
+Signed-off-by: A U Thor <author@example.com>
+Acked-by: A U Thor <author@example.com>
+[ v2 updated patch description ]
+Signed-off-by: A U Thor <author@example.com>
+EOF
+
+test_expect_success 'pretty format %(trailers) shows trailers' '
+	echo "Some contents" >trailerfile &&
+	git add trailerfile &&
+	git commit -F - <<-EOF &&
+	trailers: this commit message has trailers
+
+	This commit is a test commit with trailers at the end. We parse this
+	message and display the trailers using %bT
+
+	$(cat trailers)
+	EOF
+	git log --no-walk --pretty="%(trailers)" >actual &&
+	cat >expect <<-EOF &&
+	$(cat trailers)
+
+	EOF
+	test_cmp expect actual
+'
+
 test_done