fix accidental corrdump

[Maxim Dounin]

Hello!

On Thu, Sep 29, 2022 at 04:30:45PM +0800, zjd wrote:

> Actually, I'm not sure where is coredump; So I think  
> l->alloc=NULL after free(l-alloc) is reasonable, because  
> l->alloc's address can be reused in the pool.  Of course, 
> memzero  after get mem from the pool can be  solved  about this 
> question.But for example, ngx_array_push maybe reuse l->alloc's 
> address, and ngx_array_push is not memzero from the start  of 
> get memory.
> So I think  l->alloc=NULL after free(l-alloc) is necessary.  
> Thanks for discussing this with me

As previously explained, l->alloc is not used after free().  
Clearing unused memory without reasons is certainly not necessary, 
much like clearing allocated memory.  While it might be helpful to 
mitigate various bugs, a better approach would be to find and fix 
bugs.

To find and fix bugs a better approach is usually to set the 
unused memory to a pattern which is more likely to cause segfault 
if used, such as memset(0x5A).  In nginx, various mechanisms to 
facilitate memory debugging are available with NGX_DEBUG_MALLOC 
and NGX_DEBUG_PALLOC defines, see code for details.  Using system 
allocator options, Address Sanitizer, and tools like Valgrind also 
might be helpful.

-- 
Maxim Dounin
http://mdounin.ru/