shortlog: match commit trailers with --group

If a project uses commit trailers, this patch lets you use
shortlog to see who is performing each action. For example,
running:

  git shortlog -ns --group=trailer:reviewed-by

in git.git shows who has reviewed. You can even use a custom
format to see things like who has helped whom:

  git shortlog --format="...helped %an (%ad)" \
               --group=trailer:helped-by

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

Documentation/git-shortlog.txt

@@ -53,6 +53,19 @@ OPTIONS
 +
  - `author`, commits are grouped by author
  - `committer`, commits are grouped by committer (the same as `-c`)
+ - `trailer:<field>`, the `<field>` is interpreted as a case-insensitive
+   commit message trailer (see linkgit:git-interpret-trailers[1]). For
+   example, if your project uses `Reviewed-by` trailers, you might want
+   to see who has been reviewing with
+   `git shortlog -ns --group=trailer:reviewed-by`.
++
+Note that commits that do not include the trailer will not be counted.
+Likewise, commits with multiple trailers (e.g., multiple signoffs) may
+be counted more than once.
++
+The contents of each trailer value are taken literally and completely.
+No mailmap is applied, and the `-e` option has no effect (if the trailer
+contains a username and email, they are both always shown).
 
 -c::
 --committer::

builtin/shortlog.c

@@ -9,6 +9,7 @@
 #include "mailmap.h"
 #include "shortlog.h"
 #include "parse-options.h"
+#include "trailer.h"
 
 static char const * const shortlog_usage[] = {
 	N_("git shortlog [<options>] [<revision-range>] [[--] <path>...]"),
@@ -136,6 +137,8 @@ static void read_from_stdin(struct shortlog *log)
 	case SHORTLOG_GROUP_COMMITTER:
 		match = committer_match;
 		break;
+	case SHORTLOG_GROUP_TRAILER:
+		die(_("using --group=trailer with stdin is not supported"));
 	default:
 		BUG("unhandled shortlog group");
 	}
@@ -163,6 +166,37 @@ static void read_from_stdin(struct shortlog *log)
 	strbuf_release(&oneline);
 }
 
+static void insert_records_from_trailers(struct shortlog *log,
+					 struct commit *commit,
+					 struct pretty_print_context *ctx,
+					 const char *oneline)
+{
+	struct trailer_iterator iter;
+	const char *commit_buffer, *body;
+
+	/*
+	 * Using format_commit_message("%B") would be simpler here, but
+	 * this saves us copying the message.
+	 */
+	commit_buffer = logmsg_reencode(commit, NULL, ctx->output_encoding);
+	body = strstr(commit_buffer, "\n\n");
+	if (!body)
+		return;
+
+	trailer_iterator_init(&iter, body);
+	while (trailer_iterator_advance(&iter)) {
+		const char *value = iter.val.buf;
+
+		if (strcasecmp(iter.key.buf, log->trailer))
+			continue;
+
+		insert_one_record(log, value, oneline);
+	}
+	trailer_iterator_release(&iter);
+
+	unuse_commit_buffer(commit, commit_buffer);
+}
+
 void shortlog_add_commit(struct shortlog *log, struct commit *commit)
 {
 	struct strbuf ident = STRBUF_INIT;
@@ -197,6 +231,9 @@ void shortlog_add_commit(struct shortlog *log, struct commit *commit)
 				      &ident, &ctx);
 		insert_one_record(log, ident.buf, oneline_str);
 		break;
+	case SHORTLOG_GROUP_TRAILER:
+		insert_records_from_trailers(log, commit, &ctx, oneline_str);
+		break;
 	}
 
 	strbuf_release(&ident);
@@ -263,12 +300,17 @@ static int parse_wrap_args(const struct option *opt, const char *arg, int unset)
 static int parse_group_option(const struct option *opt, const char *arg, int unset)
 {
 	struct shortlog *log = opt->value;
+	const char *field;
 
 	if (unset || !strcasecmp(arg, "author"))
 		log->group = SHORTLOG_GROUP_AUTHOR;
 	else if (!strcasecmp(arg, "committer"))
 		log->group = SHORTLOG_GROUP_COMMITTER;
-	else
+	else if (skip_prefix(arg, "trailer:", &field)) {
+		log->group = SHORTLOG_GROUP_TRAILER;
+		free(log->trailer);
+		log->trailer = xstrdup(field);
+	} else
 		return error(_("unknown group type: %s"), arg);
 
 	return 0;

shortlog.h

@@ -19,7 +19,9 @@ struct shortlog {
 	enum {
 		SHORTLOG_GROUP_AUTHOR = 0,
 		SHORTLOG_GROUP_COMMITTER,
+		SHORTLOG_GROUP_TRAILER,
 	} group;
+	char *trailer;
 
 	char *common_repo_prefix;
 	int email;

t/t4201-shortlog.sh

@@ -220,4 +220,18 @@ test_expect_success '--group=committer is the same as --committer' '
 	test_cmp expect actual
 '
 
+test_expect_success 'shortlog --group=trailer:signed-off-by' '
+	git commit --allow-empty -m foo -s &&
+	GIT_COMMITTER_NAME="SOB One" \
+	GIT_COMMITTER_EMAIL=sob@example.com \
+		git commit --allow-empty -m foo -s &&
+	git commit --allow-empty --amend --no-edit -s &&
+	cat >expect <<-\EOF &&
+	     2	C O Mitter <committer@example.com>
+	     1	SOB One <sob@example.com>
+	EOF
+	git shortlog -ns --group=trailer:signed-off-by HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_done