commit: convert mergetag before computing the signature of a commit

It so happens that commit mergetag lines embed a tag object.  So to
compute the compatible signature of a commit object that has mergetag
lines the compatible embedded tag must be computed first.

Implement this by duplicating and converting the commit extra headers
into the compatible version of the commit extra headers, that need
to be passed to commit_tree_extended.

To handle merge tags only the compatible extra headers need to be
computed.

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

Author: ebiederm

commit.c

@@ -1355,6 +1355,39 @@ void append_merge_tag_headers(struct commit_list *parents,
 	}
 }
 
+static int convert_commit_extra_headers(struct commit_extra_header *orig,
+					struct commit_extra_header **result)
+{
+	const struct git_hash_algo *compat = the_repository->compat_hash_algo;
+	const struct git_hash_algo *algo = the_repository->hash_algo;
+	struct commit_extra_header *extra = NULL, **tail = &extra;
+	struct strbuf out = STRBUF_INIT;
+	while (orig) {
+		struct commit_extra_header *new;
+		CALLOC_ARRAY(new, 1);
+		if (!strcmp(orig->key, "mergetag")) {
+			if (convert_object_file(&out, algo, compat,
+						orig->value, orig->len,
+						OBJ_TAG, 1)) {
+				free(new);
+				free_commit_extra_headers(extra);
+				return -1;
+			}
+			new->key = xstrdup("mergetag");
+			new->value = strbuf_detach(&out, &new->len);
+		} else {
+			new->key = xstrdup(orig->key);
+			new->len = orig->len;
+			new->value = xmemdupz(orig->value, orig->len);
+		}
+		*tail = new;
+		tail = &new->next;
+		orig = orig->next;
+	}
+	*result = extra;
+	return 0;
+}
+
 static void add_extra_header(struct strbuf *buffer,
 			     struct commit_extra_header *extra)
 {
@@ -1679,6 +1712,7 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 		goto out;
 	}
 	if (r->compat_hash_algo) {
+		struct commit_extra_header *compat_extra = NULL;
 		struct object_id mapped_tree;
 		struct object_id *mapped_parents;
 
@@ -1695,8 +1729,14 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 				free(mapped_parents);
 				goto out;
 			}
+		if (convert_commit_extra_headers(extra, &compat_extra)) {
+			result = -1;
+			free(mapped_parents);
+			goto out;
+		}
 		write_commit_tree(&compat_buffer, msg, msg_len, &mapped_tree,
-				  mapped_parents, nparents, author, committer, extra);
+				  mapped_parents, nparents, author, committer, compat_extra);
+		free_commit_extra_headers(compat_extra);
 		free(mapped_parents);
 
 		if (sign_commit && sign_commit_to_strbuf(&compat_sig, &compat_buffer, sign_commit)) {