Re: [PATCH 5/7] xdiff: use GALLOC_GROW(), not XDL_ALLOC_GROW()

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Ævar

On 11/07/2022 11:48, Ævar Arnfjörð Bjarmason wrote:
> 
> On Mon, Jul 11 2022, Phillip Wood wrote:
> 
>> Hi Ævar
>>
>> On 08/07/2022 15:20, Ævar Arnfjörð Bjarmason wrote:
>>> Replace the recently introduced XDL_ALLOC_GROW() with invocations of
>>> the GALLOC_GROW() from git-shared-util.h.
>>> As this change shows the macro + function indirection of
>>> XDL_ALLOC_GROW() is something we needed only because the two callsites
>>> we used it in wanted to use it as an expression, and we thus had to
>>> pass the "sizeof" down.
>>> Let's just check the value afterwards instead, which allows us to
>>> use
>>> the shared macro, we can also remove xdl_reallo(), this was its last
>>> user.
>>
>> I don't think this expression->statement change is an
>> improvement.
> 
> I think the use-as-statement is prettier too, but I think the uglyness
> of having to pass down the sizeof() & re-implementing the macro version
> of the alloc-or-die variant outweights that.

I think this is partly a choice between prioritizing ease of 
implementation or ease of use for callers.

>> This change also removes the overflow checks that are
>> present in XDL_ALLOC_GROW()[...]
> 
> We end up calling st_mult(), which does that overflow check. Do you mean
> that the POC shimmy layer I showed in another reply for libgit2 doesn't
> have an st_mult() that detects overflows?

I was referring to

#define alloc_nr(x) (((x)+16)*3/2)

in cache.h. XDL_ALLOC_GROW() detects overflows when growing the number 
of items as well as when calculating the number of bytes to allocate.

> That's true, but as noted downthread of that we can & could ship that as
> part of the shimmy layer, but that's unrelated to this change.
> 
> In your pre-image you use LONG_MAX instead of UINTMAX_MAX & I don't see
> (but maybe I haven't looked at it carefully enough) how it does the same
> dying on overflows. Doesn't it just fall back to LONG_MAX?

It does not die on overflow as we want to return errors rather than die 
in the xdiff code. It uses long to match the existing code.

> Part of this is that it's not clear to me from your commit(s) why you
> need to rewrite alloc_nr() and rewrite (or drop?) st_mult().

So that we don't die on overflow and so that the xdiff code is self 
contained.

I'm a bit disappointed that this patch seems to have been written 
without really taking the time to understand exactly what the code it is 
replacing is doing.

Best Wishes

Phillip

>> and fails to free the old allocation when
>> realloc() fails. It is not a like for like replacement.
> 
> Yes, we should have a free() there. Wel spotted. But again, doing that
> as part of the "gently" branch seems preferrable to have duplicate
> versions for expression (non-fatal) v.s. statement (fatal) variants.