Re: [PATCH v2 15/22] combine-diff: fix leaking lost lines

[Patrick Steinhardt <ps@pks.im>]

On Mon, Nov 04, 2024 at 04:43:38PM -0600, Justin Tobler wrote:
> On 24/10/21 11:28AM, Patrick Steinhardt wrote:
> > The `cnt` variable tracks the number of lines in a patch diff. It can
> > happen though that there are no newlines, in which case we'd still end
> > up allocating our array of `sline`s. In fact, we always allocate it with
> > `cnt + 2` entries. But when we loop through the array to clear it at the
> > end of this function we only loop until `lno < cnt`, and thus we may not
> > end up releasing whatever the two extra `sline`s contain.
> > 
> > Plug this memory leak.
> > 
> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> >  combine-diff.c           | 2 +-
> >  t/t4038-diff-combined.sh | 1 +
> >  2 files changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/combine-diff.c b/combine-diff.c
> > index f6b624dc288..3c6d9507fec 100644
> > --- a/combine-diff.c
> > +++ b/combine-diff.c
> > @@ -1220,7 +1220,7 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
> >  	}
> >  	free(result);
> >  
> > -	for (lno = 0; lno < cnt; lno++) {
> > +	for (lno = 0; lno < cnt + 2; lno++) {
> 
> From looking only at the code, its not very obvious to me where the "+2"
> comes from. Not really worth a reroll, but it might be nice to leave a
> note with some context.

The code is quite convoluted and hard to read, agreed. In any case, we
call `CALLOC_ARRAY(sline, st_add(cnt, 2))`, and as a comment further up
mentions:

    /* Even p_lno[cnt+1] is valid -- that is for the end line number for
     * deletion hunk at the end.
     */

This explains the +1. The second +1 seems to never be populated and acts
more like a sentinel value.

I don't really think it makes a ton of sense to explain why the sline
array is overallocated when freeing it, and we already do explain it.
But I'll touch up the commit message a bit and sneak in a fix of the
above comment to be properly formatted, which also gives the necessary
context when reading the diff, only.

Patrick