Re: [PATCH 10/17] get_revision_internal(): make check less mysterious

[Michael Haggerty <mhagger@alum.mit.edu>]

On 05/21/2013 07:38 PM, Junio C Hamano wrote:
> Michael Haggerty <mhagger@alum.mit.edu> writes:
> 
>> The condition under which gc_boundary() is called was previously
>>
>>     if (alloc <= nr)
>>
>> .  But by construction, nr can never exceed alloc, so the check looks
>> unnecessarily mysterious.  In fact, the purpose of the check is to try
>> to avoid a realloc() call by shrinking the array if possible if it is
>> at its allocation limit when a new element is about to be added.  So
>> change the check to
>>
>>     if (nr == alloc)
>>
>> and add a comment to explain what's going on.
>>
>> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
>> ---
>> Please check that I have properly described the purpose of this check.
>>
>> The way the code is written, it looks like a bad pattern of growth and
>> shrinkage of the array (namely, just under the resize limit) could
>> cause gc_boundary() to be called over and over again with (most of)
>> the same data.  I hope that the author had some reason to believe that
>> such a pattern is unlikely.
> 
> That is about comparing with "alloc", not having high and low
> watermarks, right?
> 
> I do not see "alloc <= nr" is mysterious at all; it is merely being
> defensive.

If nr would ever exceed alloc, then the code would be broken and would
probably have already performed an illegal memory access.  Pretending to
support nr > alloc here is not defensive but misleading, because by that
time the ship is going down anyway.

On a more practical level, when I saw this code I thought to myself
"that's strange, I'd better look into it because it suggests that I
don't understand the meaning of nr and alloc".  I think that the
suggested change will help prevent the next reader from repeating the
same pointless investigation.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/