Re: [PATCH 1/2] commit-graph: clean up leaked memory during write

["Martin Ågren" <martin.agren@gmail.com>]

On Tue, 2 Oct 2018 at 19:59, Stefan Beller <sbeller@google.com> wrote:
> > > +
> > > +       string_list_clear(&list, 0);
> > >  }
> >
> > Nit: The blank line adds some asymmetry, IMVHO.
>
> I think these blank lines are super common, as in:
>
>     {
>       declarations;
>
>       multiple;
>       lines(of);
>       code;
>
>       cleanup;
>       and_frees;
>     }
>
> (c.f. display_table in column.c, which I admit to have
> cherry-picked as an example).
>
> While in nit territory, I would rather move the string list init
> into the first block:
>
>   {
>     struct string_list list = STRING_LIST_INIT_DUP;
>
>     for_each_ref(add_ref_to_list, &list);
>     write_commit_graph(obj_dir, NULL, &list, append);
>
>     string_list_clear(&list, 0);
>   }

Now this looks very symmetrical. :-)

> > >  void write_commit_graph(const char *obj_dir,
> > > @@ -846,9 +848,11 @@ void write_commit_graph(const char *obj_dir,
> > >         compute_generation_numbers(&commits, report_progress);
> > >
> > >         graph_name = get_commit_graph_filename(obj_dir);
> > > -       if (safe_create_leading_directories(graph_name))
> > > +       if (safe_create_leading_directories(graph_name)) {
> > > +               UNLEAK(graph_name);
> > >                 die_errno(_("unable to create leading directories of %s"),
> > >                           graph_name);
> > > +       }
> >
> > Do you really need this hunk?
>
> graph_name is produced via xstrfmt in get_commit_graph_filename,
> so it needs to be free'd in any return/exit path.

Agreed. Although I am questioning that `die()` and its siblings count.

> > In my testing with LeakSanitizer and
> > valgrind, I don't need this hunk to be leak-free.
>
>
> > Generally speaking, it
> > seems impossible to UNLEAK when dying, since we don't know what we have
> > allocated higher up in the call-stack.
>
> I do not understand; I thought UNLEAK was specifically for the purpose of
> die() calls without imposing extra overhead; rereading 0e5bba53af
> (add UNLEAK annotation for reducing leak false positives, 2017-09-08)
> doesn't provide an example for prematurely die()ing, only for regular
> program exit.
>
> > [...] With this hunk, I am
> > puzzled and feel uneasy, both about having to UNLEAK before dying and
> > about having to UNLEAK outside of builtin/.
>
> I am not uneasy about an UNLEAK before dying, but about dying outside
> builtin/ in general

Yeah, not dying would be even better (out of scope for this patch).

> (but having a die call accompanied by UNLEAK seems
> to be the right thing). Can you explain the worries you have regarding the
> allocations on the call stack, as xstrfmt is allocating on the heap and we
> only UNLEAK the pointer to that?

I think we agree that leaking things "allocat[ed] on the call stack"
isn't much of a worry. The reason I mentioned the call stack is that
we've got any number of calls behind us on it, and we might have made
all sorts of allocations on the heap, and at this point, we have no
idea about what we should be UNLEAK-ing.

My worry is that one of these would seem to be true:

* UNLEAK is unsuitable for the job. Whenever we have a `die()` as we do
  here, we can UNLEAK the variables we know of, but we can't do anything
  about the allocations we have made higher up the call-chain. Our test
  suite obviously provokes lots of calls to `die()` -- imagine that each
  of those leaves a few leaked allocations behind. We'd have a semi-huge
  number of leaks being reported. While we could mark with UNLEAK to
  reduce that number, we wouldn't be able to bring the number of leaks
  down to anywhere near manageable where we'd be able to find the last
  few true positives.

* We add code with no purpose. In this case, we're not talking a lot of
  lines, but across the code base, if they bring no gain, they are bound
  to provide a negative net value given enough time.

Martin