Re: [PATCH v2 1/2] tree-diff: fix leak when not HAVE_ALLOCA_H

On Thu, Sep 16, 2021 at 01:55:22AM -0700, Carlo Marcelo Arenas Belón wrote:

b8ba412bf7 (tree-diff: avoid alloca for large allocations, 2016-06-07)
adds a way to route some bigger allocations out of the stack and free
them through the addition of two conveniently named macros, but leaves
the calls to free the xalloca part, which could be also in the heap,
if the system doesn't HAVE_ALLOCA_H (ex: macOS and other BSD).

Add the missing free call, xalloca_free(), which is a noop if we
allocated memory in the stack frame, but a real free() if we
allocated in the heap instead, and while at it, change the expression
to match in both macros for ease of readability.

Thanks, this is definitely my bug introduced by b8ba412bf7 and this is
the right fix.

I continue to find the whole xalloca() thing pretty gross, and doubly so
now that there are _two_ layers of "maybe alloca(), and maybe malloc()"
logic (one in xalloca(), and one in this FAST_ARRAY stuff).

We should definitely take this fix to address the immediate problem, but
I wonder if this size logic should be pushed into xalloca to make this
kind of problem harder. Of course this is the only caller, so it might
not matter much either way.

(I'd also be really happy to see it go away entirely, as alloca() is a
foot-gun in the first place. But I think it did make things slightly
faster. It might be worth re-measuring).

-Peff