Subject: Re: [PATCH v3] reflog-walk: don't segfault on non-commit sha1's in the reflog

From: Junio C Hamano <[email-masked]>
Date: Wed, 30 Dec 2015 16:02:41 -0800

Dennis Kaarsemaker <[email-masked]> writes:

> This turned out to be doable in the same code segment: just keep on
> processing reflog entries until you hit a commit or run out of entries.
> That (and the updated foremerly-failing test) are the only changes
> between v2 and v3.
>
> I'll try to actually implement the proper solution, but that'll take a
> while. Until then, this at least stops a segfault :)

Yeah, that would be an ambitious project.

> diff --git a/reflog-walk.c b/reflog-walk.c
> index 85b8a54..0ebd1da 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;

This thing is not initialized...

>  	struct reflog_info *reflog;
>  
>  	info->last_commit_reflog = NULL;
> @@ -232,15 +233,20 @@ void fake_reflog_parent(struct reflog_walk_info *info, struct commit *commit)
>  		commit->parents = NULL;
>  		return;
>  	}
> -
> -	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) {
> +
> +	do {
> +		reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
> +		commit_reflog->recno--;
> +		logobj = parse_object(reflog->osha1);
> +	} while (commit_reflog->recno && (logobj && logobj->type != OBJ_COMMIT));

But this loop runs at least once, so logobj will always have some
sane value when the loop exits.

> +	if (!logobj || logobj->type != OBJ_COMMIT) {

And the only case where this should trigger is when we ran out of
recno.  Am I reading the updated code correctly?

With the updated code, the number of times we return from this
function is different from the number initially set to recno.  I had
to wonder if the caller cares and misbehaves, but the caller does
not know how long the reflog is before starting to call
get_revision() in a loop anyway, so it already has to deal with a
case where it did .recno=20 and get_revision() did not return that
many times.  So this probably is safe.

> +		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;
> diff --git a/t/t1410-reflog.sh b/t/t1410-reflog.sh
> index b79049f..f97513c 100755
> --- a/t/t1410-reflog.sh
> +++ b/t/t1410-reflog.sh
> @@ -325,4 +325,17 @@ test_expect_success 'parsing reverse reflogs at BUFSIZ boundaries' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success 'no segfaults for reflog containing non-commit sha1s' '
> +	git update-ref --create-reflog -m "Creating ref" \
> +		refs/tests/tree-in-reflog HEAD &&
> +	git update-ref -m "Forcing tree" refs/tests/tree-in-reflog HEAD^{tree} &&
> +	git update-ref -m "Restoring to commit" refs/tests/tree-in-reflog HEAD &&
> +	git reflog refs/tests/tree-in-reflog
> +'
> +
> +test_expect_success 'reflog containing non-commit sha1s displays properly' '

In general, "properly" is a poor word to use in test description (or
a commit log message or a bug report, for that matter), as the whole
point of a test is to precisely define what is "proper".

And the code change declares that a proper thing to do is to omit
non-commit entries without segfaulting, so something like

    s/displays properly/omits them/

perhaps?

> +	git reflog refs/tests/tree-in-reflog >actual &&
> +	test_line_count = 2 actual
> +'
> +
>  test_done
> -- 
> 2.7.0-rc1-207-ga35084c