object-file-convert: convert tag objects when writing

When writing a tag object in a repository with both SHA-1 and SHA-256,
we'll need to convert our commit objects so that we can write the hash
values for both into the repository.  To do so, let's add a function to
convert tag objects.

Note that signatures for tag objects in the current algorithm trail the
message, and those for the alternate algorithm are in headers.
Therefore, we parse the tag object for both a trailing signature and a
header and then, when writing the other format, swap the two around.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Eric W. Biederman <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

object-file-convert.c

@@ -7,6 +7,8 @@
 #include "hash.h"
 #include "object.h"
 #include "loose.h"
+#include "commit.h"
+#include "gpg-interface.h"
 #include "object-file-convert.h"
 
 int repo_oid_to_algop(struct repository *repo, const struct object_id *src,
@@ -83,6 +85,52 @@ static int convert_tree_object(struct strbuf *out,
 	return 0;
 }
 
+static int convert_tag_object(struct strbuf *out,
+			      const struct git_hash_algo *from,
+			      const struct git_hash_algo *to,
+			      const char *buffer, size_t size)
+{
+	struct strbuf payload = STRBUF_INIT, temp = STRBUF_INIT, oursig = STRBUF_INIT, othersig = STRBUF_INIT;
+	size_t payload_size;
+	struct object_id oid, mapped_oid;
+	const char *p;
+
+	/* Add some slop for longer signature header in the new algorithm. */
+	strbuf_grow(out, size + 7);
+
+	/* Is there a signature for our algorithm? */
+	payload_size = parse_signed_buffer(buffer, size);
+	strbuf_add(&payload, buffer, payload_size);
+	if (payload_size != size) {
+		/* Yes, there is. */
+		strbuf_add(&oursig, buffer + payload_size, size - payload_size);
+	}
+	/* Now, is there a signature for the other algorithm? */
+	if (parse_buffer_signed_by_header(payload.buf, payload.len, &temp, &othersig, to)) {
+		/* Yes, there is. */
+		strbuf_swap(&payload, &temp);
+		strbuf_release(&temp);
+	}
+
+	/*
+	 * Our payload is now in payload and we may have up to two signatrures
+	 * in oursig and othersig.
+	 */
+	if (strncmp(payload.buf, "object ", 7) || payload.buf[from->hexsz + 7] != '\n')
+		return error("bogus tag object");
+	if (parse_oid_hex_algop(payload.buf + 7, &oid, &p, from) < 0)
+		return error("bad tag object ID");
+	if (repo_oid_to_algop(the_repository, &oid, to, &mapped_oid))
+		return error("unable to map tree %s in tag object",
+			     oid_to_hex(&oid));
+	strbuf_addf(out, "object %s", oid_to_hex(&mapped_oid));
+	strbuf_add(out, p, payload.len - (p - payload.buf));
+	strbuf_addbuf(out, &othersig);
+	if (oursig.len)
+		add_header_signature(out, &oursig, from);
+	return 0;
+}
+
 int convert_object_file(struct strbuf *outbuf,
 			const struct git_hash_algo *from,
 			const struct git_hash_algo *to,
@@ -100,8 +148,10 @@ int convert_object_file(struct strbuf *outbuf,
 	case OBJ_TREE:
 		ret = convert_tree_object(outbuf, from, to, buf, len);
 		break;
-	case OBJ_COMMIT:
 	case OBJ_TAG:
+		ret = convert_tag_object(outbuf, from, to, buf, len);
+		break;
+	case OBJ_COMMIT:
 	default:
 		/* Not implemented yet, so fail. */
 		ret = -1;