Subject: Re: [PATCH] reflog-walk: don't segfault on non-commit sha1's in the reflog

From: Junio C Hamano <[email-masked]>
Date: Wed, 30 Dec 2015 13:20:44 -0800

Dennis Kaarsemaker <[email-masked]> writes:

> diff --git a/reflog-walk.c b/reflog-walk.c
> index 85b8a54..b85c8e8 100644
> --- a/reflog-walk.c
> +++ b/reflog-walk.c
> @@ -236,8 +236,8 @@ void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
>  	reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
>  	info->last_commit_reflog = commit_reflog;
>  	commit_reflog->recno--;
> -	commit_info->commit = (struct commit *)parse_object(reflog->osha1);
> -	if (!commit_info->commit) {
> +	commit_info->commit = lookup_commit(reflog->osha1);
> +	if (!commit_info->commit || parse_commit(commit_info->commit)) {
>  		commit->parents = NULL;
>  		return;

This looks somewhat roundabout and illogical.  The original was bad
because it blindly assumed reflgo->osha1 refers to a commit without
making sure that assumption holds.  Calling lookup_commit() blindly
is not much better, even though you are helped that the function
happens not to barf if the given object is not a commit.

Also this changes semantics, no?  Trace the original flow and think
what happens, when we see a commit object that cannot be parsed in
parse_commit_buffer().  parse_object() calls parse_object_buffer()
which in turn calls parse_commit_buffer() and the entire callchain
returns NULL.  commit_info->commit will become NULL in such a case.

With your code, lookup_commit() will store a non NULL in
commit_info->commit, and parse_commit() calls parse_commit_buffer()
and that would fail, so you clear commit->parents to NULL but fail
to set commit_info->commit to NULL.

Why not keep the parse_object() as-is and make sure we error out
unless the result is a commit with a more explicit check, perhaps
like this, instead?

 reflog-walk.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/reflog-walk.c b/reflog-walk.c
index 85b8a54..861d7c4 100644
--- a/reflog-walk.c
+++ b/reflog-walk.c
@@ -221,6 +221,7 @@ void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
 	struct commit_info *commit_info =
 		get_commit_info(commit, &info->reflogs, 0);
 	struct commit_reflog *commit_reflog;
+	struct object *logobj;
 	struct reflog_info *reflog;
 
 	info->last_commit_reflog = NULL;
@@ -236,11 +237,13 @@ void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
 	reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
 	info->last_commit_reflog = commit_reflog;
 	commit_reflog->recno--;
-	commit_info->commit = (struct commit *)parse_object(reflog->osha1);
-	if (!commit_info->commit) {
+	logobj = parse_object(reflog->osha1);
+	if (!logobj || logobj->type != OBJ_COMMIT) {
+		commit_info->commit = NULL;
 		commit->parents = NULL;
 		return;
 	}
+	commit_info->commit = (struct commit *)logobj;
 
 	commit->parents = xcalloc(1, sizeof(struct commit_list));
 	commit->parents->item = commit_info->commit;

> +test_expect_success 'reflog containing non-commit sha1s' '
> +	git checkout -b broken-reflog &&
> +	echo "$(git rev-parse HEAD^{tree}) $(git rev-parse HEAD) abc <xyz> 0000000001 +0000" >> .git/logs/refs/heads/broken-reflog &&
> +	git reflog broken-reflog
> +'
> +

This will negatively affect the ongoing effort to abstract out the
on-disk implementation of the reflog.  In some future installation
of Git, the reflog may not even be in .git/logs/refs/whatever file.

Use a non-branch ref, so that you can store any valid object not
just commits, and use a Git command (e.g. "git update-ref" or "git
tag") instead of the raw filesystem access to update it, perhaps
like this?

	git tag --create-reflog test-logs HEAD^ &&
	git tag -f test-logs HEAD^{tree} &&
	git tag -f test-logs HEAD &&
	git reflog test-logs