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

From: René Scharfe <l.s.r@web.de>
Date: Jul 3, 2021, 12:56 UTC
Message-ID: <5cc24582-ed96-453a-fbad-d3fb83c8f949@web.de>
In-Reply-To: <87v95r8w86.fsf@evledraar.gmail.com>

Am 03.07.21 um 13:35 schrieb Ævar Arnfjörð Bjarmason:

Show 50 quoted lines

>
> 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).

Agreed.

> 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.

I don't think there's any confusion in our internal code about the macros' handling of allocation errors.

The following semantic patch finds a leery xmalloc() caller in compat/mmap.c, though:

@@
expression PTR, SIZE, SIZE2;
@@
(
  PTR = xmalloc(SIZE);
|
  PTR = xcalloc(SIZE, SIZE2);
|
  PTR = xrealloc(SIZE);
|
  ALLOC_ARRAY(PTR, SIZE);
|
  CALLOC_ARRAY(PTR, SIZE);
|
  REALLOC_ARRAY(PTR, SIZE);
)
  if (
- PTR == NULL
+ 0
  ) {...}

René