snapshot: support archive signatures

Read signatures from the notes refs refs/notes/signatures/$FORMAT where
FORMAT is one of our archive formats ("tar", "tar.gz", ...).  The note
is expected to simply contain the signature content to be returned when
the snapshot "${filename}.asc" is requested, so the signature for
cgit-1.1.tar.xz can be stored against the v1.1 tag with:

	git notes --ref=refs/notes/signatures/tar.xz add -C "$(
		gpg --output - --armor --detach-sign cgit-1.1.tar.xz |
		git hash-object -w --stdin
	)" v1.1

and then downloaded by simply appending ".asc" to the archive URL.

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>

cgit.h

@@ -374,6 +374,8 @@ extern void cgit_parse_url(const char *url);
 extern const char *cgit_repobasename(const char *reponame);
 
 extern int cgit_parse_snapshots_mask(const char *str);
+extern const struct object_id *cgit_snapshot_get_sig(const char *ref,
+						     const struct cgit_snapshot_format *f);
 
 extern int cgit_open_filter(struct cgit_filter *filter, ...);
 extern int cgit_close_filter(struct cgit_filter *filter);

ui-shared.c

@@ -1133,6 +1133,13 @@ void cgit_print_snapshot_links(const struct cgit_repo *repo, const char *ref,
 		strbuf_addstr(&filename, f->suffix);
 		cgit_snapshot_link(filename.buf, NULL, NULL, NULL, NULL,
 				   filename.buf);
+		if (cgit_snapshot_get_sig(ref, f)) {
+			strbuf_addstr(&filename, ".asc");
+			html(" (");
+			cgit_snapshot_link("sig", NULL, NULL, NULL, NULL,
+					   filename.buf);
+			html(")");
+		}
 		html(separator);
 	}
 	strbuf_release(&filename);

ui-snapshot.c

@@ -94,6 +94,31 @@ const struct cgit_snapshot_format cgit_snapshot_formats[] = {
 	{ NULL }
 };
 
+static struct notes_tree snapshot_sig_notes[ARRAY_SIZE(cgit_snapshot_formats)];
+
+const struct object_id *cgit_snapshot_get_sig(const char *ref,
+					      const struct cgit_snapshot_format *f)
+{
+	struct notes_tree *tree;
+	struct object_id oid;
+
+	if (get_oid(ref, &oid))
+		return NULL;
+
+	tree = &snapshot_sig_notes[f - &cgit_snapshot_formats[0]];
+	if (!tree->initialized) {
+		struct strbuf notes_ref = STRBUF_INIT;
+
+		strbuf_addf(&notes_ref, "refs/notes/signatures/%s",
+			    f->suffix + 1);
+
+		init_notes(tree, notes_ref.buf, combine_notes_ignore, 0);
+		strbuf_release(&notes_ref);
+	}
+
+	return get_note(tree, &oid);
+}
+
 static const struct cgit_snapshot_format *get_format(const char *filename)
 {
 	const struct cgit_snapshot_format *fmt;
@@ -129,6 +154,39 @@ static int make_snapshot(const struct cgit_snapshot_format *format,
 	return 0;
 }
 
+static int write_sig(const struct cgit_snapshot_format *format,
+		     const char *hex, const char *archive,
+		     const char *filename)
+{
+	const struct object_id *note = cgit_snapshot_get_sig(hex, format);
+	enum object_type type;
+	unsigned long size;
+	char *buf;
+
+	if (!note) {
+		cgit_print_error_page(404, "Not found",
+				"No signature for %s", archive);
+		return 0;
+	}
+
+	buf = read_sha1_file(note->hash, &type, &size);
+	if (!buf) {
+		cgit_print_error_page(404, "Not found", "Not found");
+		return 0;
+	}
+
+	html("X-Content-Type-Options: nosniff\n");
+	html("Content-Security-Policy: default-src 'none'\n");
+	ctx.page.etag = oid_to_hex(note);
+	ctx.page.mimetype = xstrdup("application/pgp-signature");
+	ctx.page.filename = xstrdup(filename);
+	cgit_print_http_headers();
+
+	html_raw(buf, size);
+	free(buf);
+	return 0;
+}
+
 /* Try to guess the requested revision from the requested snapshot name.
  * First the format extension is stripped, e.g. "cgit-0.7.2.tar.gz" become
  * "cgit-0.7.2". If this is a valid commit object name we've got a winner.
@@ -185,6 +243,8 @@ void cgit_print_snapshot(const char *head, const char *hex,
 			 const char *filename, int dwim)
 {
 	const struct cgit_snapshot_format* f;
+	const char *sig_filename = NULL;
+	char *adj_filename = NULL;
 	char *prefix = NULL;
 
 	if (!filename) {
@@ -193,6 +253,15 @@ void cgit_print_snapshot(const char *head, const char *hex,
 		return;
 	}
 
+	if (ends_with(filename, ".asc")) {
+		sig_filename = filename;
+
+		/* Strip ".asc" from filename for common format processing */
+		adj_filename = xstrdup(filename);
+		adj_filename[strlen(adj_filename) - 4] = '\0';
+		filename = adj_filename;
+	}
+
 	f = get_format(filename);
 	if (!f || !(ctx.repo->snapshots & f->bit)) {
 		cgit_print_error_page(400, "Bad request",
@@ -216,6 +285,11 @@ void cgit_print_snapshot(const char *head, const char *hex,
 	if (!prefix)
 		prefix = xstrdup(cgit_snapshot_prefix(ctx.repo));
 
+	if (sig_filename)
+		write_sig(f, hex, filename, sig_filename);
+	else
 		make_snapshot(f, hex, prefix, filename);
+
 	free(prefix);
+	free(adj_filename);
 }