Re: [PATCH] xrealloc: do not reuse pointer freed by zero-length realloc()

From: Derrick Stolee <stolee@gmail.com>
To: Jeff King <peff@peff.net>
Cc: git@vger.kernel.org
Subject: Re: [PATCH] xrealloc: do not reuse pointer freed by zero-length realloc()
Date: Tue, 1 Sep 2020 10:24:18 -0400	[thread overview]
Message-ID: <a176961b-645f-4dcb-3439-596f814891f4@gmail.com> (raw)
In-Reply-To: <20200901135105.GA3284077@coredump.intra.peff.net>

On 9/1/2020 9:51 AM, Jeff King wrote:
> On Tue, Sep 01, 2020 at 09:04:36AM -0400, Derrick Stolee wrote:
>> Adding an `if (!size) {free(ptr); return NULL;}` block was what I
>> expected. Was that chosen just so we can rely more on the system
>> realloc(), or is there a performance implication that I'm not
>> seeing?
> 
> I went back and forth on whether to do that or not. This case should
> basically never happen, so I like both the performance and readability
> of only triggering it when realloc() returns NULL. But it would get rid
> of the hand-waving above, and I doubt the performance is measurable.
> 
> If we do handle it up-front, then I think we'd actually want:
> 
>   if (!size) {
>           free(ptr);
> 	  return xmalloc(0);
>   }
> 
> (i.e., to never return NULL for consistency with xmalloc() and
> xcalloc()).

Good point. In that sense, your change makes a lot more
sense for staying consistent without strangeness like xmalloc(0).

>>> @@ -120,7 +120,7 @@ void *xrealloc(void *ptr, size_t size)
>>>  	memory_limit_check(size, 0);
>>>  	ret = realloc(ptr, size);
>>>  	if (!ret && !size)
>>> -		ret = realloc(ptr, 1);
>>> +		ret = realloc(ret, 1);
>>
>> I appreciate all the additional context for such a small change.
> 
> Somebody's got to complete with you for ratio of commit message to diff
> lines. :)

Pretty sure I have a long way to match, but it is important to
have goals.

-Stolee