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 13:41:54 -0800

Dennis Kaarsemaker <[email-masked]> writes:

> On wo, 2015-12-30 at 13:20 -0800, Junio C Hamano wrote:
>> 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?
>
> lookup_commit actually returns NULL (via object_as_type) for objects
> that are not commits, so I don't think the above is true.

I think you did not read what you are responding to.  I was talking
about the error case where the object _is_ a commit (hence lookup
returns it), but parse_commit_buffer() does not like its contents.

> The code below also loses the diagnostic message about the object
> not being a commit.

Giving such a diagnostic message is a BUG.

A ref can legitimately point at any type of object (only refs under
refs/heads/, aka "branches", must point at commits), so you MUST NOT
complain about seeing a non-commit in a reflog in general.