fmt-merge-msg: Add contents of merged tag in the merge message

When a contributor asks the integrator to merge her history, a signed tag
can be a good vehicle to communicate the authenticity of the request while
conveying other information such as the purpose of the topic.

E.g. a signed tag "for-linus" can be created, and the integrator can run:

   $ git pull git://example.com/work.git/ for-linus

This would allow the integrator to run "git verify-tag FETCH_HEAD" to
validate the signed tag.

Update fmt-merge-msg so that it pre-fills the merge message template with
the body (but not signature) of the tag object to help the integrator write
a better merge message, in the same spirit as the existing merge.log summary
lines.

The message that comes from GPG signature validation is also included in
the merge message template to help the integrator verify it, but they are
prefixed with "#" to make them comments.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 92 insertions, 2 deletions

diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 3ff9564e75..7dae846c52 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -5,6 +5,7 @@
 #include "revision.h"
 #include "tag.h"
 #include "string-list.h"
+#include "gpg-interface.h"
 
 static const char * const fmt_merge_msg_usage[] = {
 	"git fmt-merge-msg [-m <message>] [--log[=<n>]|--no-log] [--file <file>]",
@@ -262,6 +263,70 @@ static void fmt_merge_msg_title(struct strbuf *out,
 		strbuf_addf(out, " into %s\n", current_branch);
 }
 
+static void fmt_tag_signature(struct strbuf *tagbuf,
+			      struct strbuf *sig,
+			      const char *buf,
+			      unsigned long len)
+{
+	const char *tag_body = strstr(buf, "\n\n");
+	if (tag_body) {
+		tag_body += 2;
+		strbuf_add(tagbuf, tag_body, buf + len - tag_body);
+	}
+	strbuf_complete_line(tagbuf);
+	strbuf_add_lines(tagbuf, "# ", sig->buf, sig->len);
+}
+
+static void fmt_merge_msg_sigs(struct strbuf *out)
+{
+	int i, tag_number = 0, first_tag = 0;
+	struct strbuf tagbuf = STRBUF_INIT;
+
+	for (i = 0; i < origins.nr; i++) {
+		unsigned char *sha1 = origins.items[i].util;
+		enum object_type type;
+		unsigned long size, len;
+		char *buf = read_sha1_file(sha1, &type, &size);
+		struct strbuf sig = STRBUF_INIT;
+
+		if (!buf || type != OBJ_TAG)
+			goto next;
+		len = parse_signature(buf, size);
+
+		if (size == len)
+			; /* merely annotated */
+		else if (verify_signed_buffer(buf, len, buf + len, size - len, &sig)) {
+			if (!sig.len)
+				strbuf_addstr(&sig, "gpg verification failed.\n");
+		}
+
+		if (!tag_number++) {
+			fmt_tag_signature(&tagbuf, &sig, buf, len);
+			first_tag = i;
+		} else {
+			if (tag_number == 2) {
+				struct strbuf tagline = STRBUF_INIT;
+				strbuf_addf(&tagline, "\n# %s\n",
+					    origins.items[first_tag].string);
+				strbuf_insert(&tagbuf, 0, tagline.buf,
+					      tagline.len);
+				strbuf_release(&tagline);
+			}
+			strbuf_addf(&tagbuf, "\n# %s\n",
+				    origins.items[i].string);
+			fmt_tag_signature(&tagbuf, &sig, buf, len);
+		}
+		strbuf_release(&sig);
+	next:
+		free(buf);
+	}
+	if (tagbuf.len) {
+		strbuf_addch(out, '\n');
+		strbuf_addbuf(out, &tagbuf);
+	}
+	strbuf_release(&tagbuf);
+}
+
 int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 		  struct fmt_merge_msg_opts *opts)
 {
@@ -293,6 +358,9 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 	if (opts->add_title && srcs.nr)
 		fmt_merge_msg_title(out, current_branch);
 
+	if (origins.nr)
+		fmt_merge_msg_sigs(out);
+
 	if (opts->shortlog_len) {
 		struct commit *head;
 		struct rev_info rev;
@@ -310,8 +378,8 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 			shortlog(origins.items[i].string, origins.items[i].util,
 				 head, &rev, opts->shortlog_len, out);
 	}
-	if (out->len && out->buf[out->len-1] != '\n')
-		strbuf_addch(out, '\n');
+
+	strbuf_complete_line(out);
 	return 0;
 }
 
diff --git a/strbuf.c b/strbuf.c
index 3ad2cc0016..a849705197 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -397,3 +397,17 @@ int strbuf_read_file(struct strbuf *sb, const char *path, size_t hint)
 
 	return len;
 }
+
+void strbuf_add_lines(struct strbuf *out, const char *prefix,
+		      const char *buf, size_t size)
+{
+	while (size) {
+		const char *next = memchr(buf, '\n', size);
+		next = next ? (next + 1) : (buf + size);
+		strbuf_addstr(out, prefix);
+		strbuf_add(out, buf, next - buf);
+		size -= next - buf;
+		buf = next;
+	}
+	strbuf_complete_line(out);
+}
diff --git a/strbuf.h b/strbuf.h
index 46a33f8c46..08fc13d386 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -100,6 +100,14 @@ extern void strbuf_addf(struct strbuf *sb, const char *fmt, ...);
 __attribute__((format (printf,2,0)))
 extern void strbuf_vaddf(struct strbuf *sb, const char *fmt, va_list ap);
 
+extern void strbuf_add_lines(struct strbuf *sb, const char *prefix, const char *buf, size_t size);
+
+static inline void strbuf_complete_line(struct strbuf *sb)
+{
+	if (sb->len && sb->buf[sb->len - 1] != '\n')
+		strbuf_addch(sb, '\n');
+}
+
 extern size_t strbuf_fread(struct strbuf *, size_t, FILE *);
 /* XXX: if read fails, any partial read is undone */
 extern ssize_t strbuf_read(struct strbuf *, int fd, size_t hint);