Subject: Re: Whether to keep using UNLEAK() in built-ins

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 18 Feb 2022 20:31:23 +0100

On Fri, Feb 18 2022, Junio C Hamano wrote:

> Ævar Arnfjörð Bjarmason <[email-masked]> writes:
>
>>>> Specifically, the cited commit turned an existing strbuf_release()
>>>> on &err into UNLEAK().  If that and the other strbuf (sb) were so
>>>> easily releasable, why didn't we do so back then already?
>>>
>>> I suspect that the answer to the above question is because these
>>> allocations are in the top-level cmd_commit() function, which is
>>> never called recursively or repeatedly as a subroutine.  The only
>>> significant thing that happens after we return from it is to exit.
>>>
>>> In such a code path, marking a variable as UNLEAK() is a better
>>> thing to do than calling strbuf_release().  Both will work as a way
>>> to squelch sanitizers from reporting a leak that does not matter,
>>> but calling strbuf_release() means we'd spend extra cycles to return
>>> pieces of memory to the pool, even though we know that the pool
>>> itself will be cleaned immediately later at exit.
>>>
>>> We already have UNLEAK to tell sanitizers not to distract us from
>>> spotting and plugging real leaks by reporting these apparent leaks
>>> that do not matter.  It is of somewhat dubious value to do a "we
>>> care too much about pleasing sanitizer and spend extra cycles at
>>> runtime while real users are doing real work" change.
>
>> Per https://lore.kernel.org/git/[email-masked]/ the
>> real goal I have in mind here is to use the built-ins as a stand-in for
>> testing whether the underlying APIs are leak-free.
>>
>> Because of that having to reason about UNLEAK() doing the right thing or
>> not is just unneeded distraction. Yes for a "struct strbuf" it won't
>> matter, but most of what we UNLEAK() is more complex stuff like "struct
>> rev_info". We won't really make headway making revision.c not leak
>> memory without using "git log" et al as the test subjects for whether
>> that API leaks.
>
> I have to say that you have a wrong goal and wrong priority.  The
> number of UNLEAK in cmd_foo() functions is not even a poor
> approximation of our progress.

To clarify I'm not saying it's an approximation of how close we are to
getting rid of memory leaks. I think I'll know better than anyone what
the current state of shoveling shit uphill that is.

I was suggesting that these's a trivial number of these, and mostly we
strbuf_release() already, and that on modern libc's free() is pretty
much free anyway, so worrying about using UNLEAK() v.s. just freeing
before exit is some combination of a premature optimization and
needlessly retaining a special-case for the built-ins.

> Imagine that a subsystem that are repeatedly set-up and used during
> a single program invocation was leaky.  Let's say a program calls
> diff_setup() to prepare the diff_options structure, feeds two things
> to compare, and calls diff_flush() to show the comparison result,
> but let's assume this sequence leaks some resources.
>
> Now cmd_diff() may be such a program that does a setup, feeds two
> trees, calls diff_flush() and exits.  If we didn't do anything to
> it, diff_options may "leak".  Marking it with UNLEAK may be a good
> measure, if we want to keep the leak checker from reporting a leak
> that does not matter in practice so that we can concentrate on
> plugging real leaks that matter.
>
> But consider cmd_log(), running something like "git log -p".  It
> iterates over commits, does the <setup, compare two trees, flush>
> repeatedly for each commit it encounters during the walk on the same
> diff_options structure.  Now, the leak in the code path does matter.
> If diff_flush() is left leaky, it will show up in the leak checker's
> output, and that is reporting real leaks that matter.  The thing is,
> the <setup, compare, flush> sequence whose leak matters is not in
> cmd_log(); it is much deeper in the revision walking machinery.  We
> do not want to paper over such leaks with UNLEAK.
>
> See the difference?  The number of UNLEAK OUTside built-ins does
> matter.  We do not want to have UNLEAK there to hide leaks in
> possible callees that may be called arbitrary number of times in
> arbitrary order.  Compared to that, UNLEAK in cmd_foo() to mark an
> on-stack variable that was used only once without even a recursion
> is purely to squelch the leak checker from reporting leaks that does
> not matter.  For simple things like strbuf on stack of the top-level
> cmd_foo() functions, we could even leave strbuf_release() not called
> and leave the resource reclamation to _exit(2).  That would cause
> the leak checkers to report them and distract us, and UNLEAK is a
> better way to squelch the distraction without wasting extra cycles
> at runtime to actually free them.
>
> So, don't look at "built-ins as a stand-in".  It is a wrong
> direction to go in to let the "leak checker" tail wag the dog.

Yes, I see the difference and I fully accept the point you're making.

I just don't find it to be a useful distinction in practice, because for
e.g. "git log" we do have certain uses of the API which are only
performed by the top-level part of a "git log --whatever".

So if we take the view that e.g. the "struct rev_info rev" should be
UNLEAK()'d because "we're about to exit anyway" then as soon as we *do*
use that part of the API we'll run into the previously unfixed and
hidden leak.

Which is why as noted in the linked-to-above
https://lore.kernel.org/git/[email-masked]/ I think
it's useful to fix leaks even in built-ins, because they're useful
canaries for those APIs.

Now, in this case does it really matter? No, it doesn't currently. But I
think the direction we should be heading in is turning more of these
cmd_whatever() into properly functioning libraries, as e.g. John Cai's
just-sent series to do that for "reflog delete" does:
https://lore.kernel.org/git/[email-masked]/

Once we do that the UNLEAK() here will absolutely need to be changed to
a strbuf_release() or equivalent, since if you make two commits with
that new library we'll have leaked memory.

So I think it makes sense to just fix leaks everywhere if it's easy
without worrying about the distinction, particularly since I haven't
seen that "wasting cycles" concern matter in practice.

If it did I'd think adding a "GIT_DESTRUCT_LEVEL" as I suggested in the
linked E-Mail would make more sense, since e.g. you could also use that
if you knew you were walking 100 commits & exiting, which can be useful
to further reduce free() churn, and is how a similar global in the Perl
API (PERL_DESTRUCT_LEVEL) works.