Re: [PATCH] khash: clarify that allocations never fail

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Sat, Jul 03 2021, Jeff King wrote:

> On Sat, Jul 03, 2021 at 12:05:46PM +0200, René Scharfe wrote:
>
>> We use our standard allocation functions and macros (xcalloc,
>> ALLOC_ARRAY, REALLOC_ARRAY) in our version of khash.h.  They terminate
>> the program on error, so code that's using them doesn't have to handle
>> allocation failures.  Make this behavior explicit by replacing the code
>> that handles allocation errors in kh_resize_ and kh_put_ with BUG calls.
>
> Seems like a good idea.
>
> We're very sloppy about checking the "ret" field from kh_put_* for
> errors (it's a tri-state for "already existed", "newly added", or
> "error"). I think that's not a problem because as you show here, we
> can't actually hit the error case. This makes that much more obvious.
>
> Two nits if we wanted to go further:
>
>> diff --git a/khash.h b/khash.h
>> index 21c2095216..84ff7230b6 100644
>> --- a/khash.h
>> +++ b/khash.h
>> @@ -126,7 +126,7 @@ static const double __ac_HASH_UPPER = 0.77;
>>  			if (h->size >= (khint_t)(new_n_buckets * __ac_HASH_UPPER + 0.5)) j = 0;	/* requested size is too small */ \
>>  			else { /* hash table size to be changed (shrink or expand); rehash */ \
>>  				ALLOC_ARRAY(new_flags, __ac_fsize(new_n_buckets)); \
>> -				if (!new_flags) return -1;								\
>> +				if (!new_flags) BUG("ALLOC_ARRAY failed");				\
>
> I converted this in b32fa95fd8 (convert trivial cases to ALLOC_ARRAY,
> 2016-02-22), but left the now-obsolete error-check.
>
> But a few lines below...
>
>>  				memset(new_flags, 0xaa, __ac_fsize(new_n_buckets) * sizeof(khint32_t)); \
>>  				if (h->n_buckets < new_n_buckets) {	/* expand */		\
>>  					REALLOC_ARRAY(h->keys, new_n_buckets); \
>
> These REALLOC_ARRAY() calls are in the same boat. You dropped the error
> check in 2756ca4347 (use REALLOC_ARRAY for changing the allocation size
> of arrays, 2014-09-16).
>
> Should we make the two match? I'd probably do so by making the former
> match the latter, and just drop the conditional and BUG entirely.

Yes, I don't see why we should be guarding theis anymore than we do
xmalloc() or other x*() functions in various places (which is what it
resolves to).

If anything we might consider renaming it via coccinelle to
XALLOC_ARRAY(), XREALLOC_ARRAY() etc. to make it clear that they handle
any errors themselves.