Subject: Re: [PATCH 04/22] reftable/basics: handle allocation failures in `reftable_calloc()`

From: Junio C Hamano <[email-masked]>
Date: Sat, 21 Sep 2024 12:37:26 -0700

Patrick Steinhardt <[email-masked]> writes:

> Handle allocation failures in `reftable_calloc()`.
>
> Signed-off-by: Patrick Steinhardt <[email-masked]>
> ---
>  reftable/basics.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/reftable/basics.c b/reftable/basics.c
> index 4adc98cf5de..b404900b5d9 100644
> --- a/reftable/basics.c
> +++ b/reftable/basics.c
> @@ -39,6 +39,8 @@ void *reftable_calloc(size_t nelem, size_t elsize)
>  {
>  	size_t sz = st_mult(nelem, elsize);
>  	void *p = reftable_malloc(sz);
> +	if (!p)
> +		return NULL;
>  	memset(p, 0, sz);
>  	return p;
>  }

Since this series is not about eradicating all avenues in reftable
library code that can lead to die(), but only about dealing with
allocation errors from the underlying malloc/realloc routines, I
think the code posted is perfectly fine as-is as a part of this
series, but since I noticed something, let me comment before I
forget.

When st_mult() detects overflow, you'd still die(), wouldn't you?

We'd probably want a variant of st_mult() that lets us notice a
condition that would yield too large a result, and code the above
like so,

	size_t sz;
	void *p;

	if (st_mult_gently(nelem, elsize, &sz) ||
            !((p = reftable_malloc(sz))))
		return NULL;
	memset(p, 0, sz);
	return p;

or use the underlying helper ourselves, and say

	size_t sz;
	void *p;

	if (unsigned_mult_overflows(nelem, elsize)) ||
            !((sz = nelem * elsize, p = reftable_malloc(sz))))
		return NULL;
	memset(p, 0, sz);
	return p;

which lets us without an extra helper but after writing it myself, I
find it a bit too wordy.

In a sense, it is on the borderline to handle st_mult() overflow in
this function for a topic whose theme is about allocation failures.

>From the point of view of callers of reftable_calloc(), whether the
arguments they are feeding the function is too large to be
multiplied or whether the request is too big for the underlying
allocator to handle, the end result should be the same: they
requested too large an allocation.

So I wouldn't complain that it is out of scope, if use of st_mult()
that computes the allocation size is fixed as part of this series.
But as I already said, I am also OK if we leave it to a separate
series to tackle other potential callers of die().

Thanks.