pretty: move trailer formatting to trailer.c

The next commit will add many features to the %(trailer)
placeholder in pretty.c. We'll need to access some internal
functions of trailer.c for that, so our options are either:

  1. expose those functions publicly

or

  2. make an entry point into trailer.c to do the formatting

Doing (2) ends up exposing less surface area, though do note
that caveats in the docstring of the new function.

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

pretty.c

@@ -870,16 +870,6 @@ 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;
@@ -1273,7 +1263,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	}
 
 	if (starts_with(placeholder, "(trailers)")) {
-		format_trailers(sb, msg + c->subject_off);
+		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		format_trailers_from_commit(sb, msg + c->subject_off, &opts);
 		return strlen("(trailers)");
 	}
 

trailer.c

@@ -1090,3 +1090,21 @@ void trailer_info_release(struct trailer_info *info)
 		free(info->trailers[i]);
 	free(info->trailers);
 }
+
+static void format_trailer_info(struct strbuf *out,
+				const struct trailer_info *info,
+				const struct process_trailer_options *opts)
+{
+	strbuf_add(out, info->trailer_start,
+		   info->trailer_end - info->trailer_start);
+}
+
+void format_trailers_from_commit(struct strbuf *out, const char *msg,
+				 const struct process_trailer_options *opts)
+{
+	struct trailer_info info;
+
+	trailer_info_get(&info, msg);
+	format_trailer_info(out, &info, opts);
+	trailer_info_release(&info);
+}

trailer.h

@@ -40,4 +40,18 @@ void trailer_info_get(struct trailer_info *info, const char *str);
 
 void trailer_info_release(struct trailer_info *info);
 
+/*
+ * Format the trailers from the commit msg "msg" into the strbuf "out".
+ * Note two caveats about "opts":
+ *
+ *   - this is primarily a helper for pretty.c, and not
+ *     all of the flags are supported.
+ *
+ *   - this differs from process_trailers slightly in that we always format
+ *     only the trailer block itself, even if the "only_trailers" option is not
+ *     set.
+ */
+void format_trailers_from_commit(struct strbuf *out, const char *msg,
+				 const struct process_trailer_options *opts);
+
 #endif /* TRAILER_H */