Re: [PATCH][PING][PR gdb/19361] Fix invalid comparison functions

[Pedro Alves <palves@redhat.com>]

On 12/30/2015 09:25 PM, Pedro Alves wrote:
> On 12/30/2015 08:18 PM, Yuri Gribov wrote:
> 
>> Sorry, I should have been more wordy about the actual problem. With
>> current approach i.e.
>>
>>   if (pid1 == pgid1)
>>     return -1;
>>   else if (pid2 == pgid2)
>>     return 1;
>>
>> comparison of two group leaders is not going to be symmetric:
>>
>>   cmp(lead_1, lead_2) == cmp(lead_2, lead_1) == -1
> 
> Aaaaaaah, d'oh!  Thanks, it's obvious now, yes, we fail to consider
> the case of both elements being leaders.  I couldn't see that
> even after staring at the code for a while.  That hunk is OK as
> is then.  (Please clarify this in the commit log.)

Wait, no we don't...  If both are leaders when you get there, then
they must have different pgid's, and that case is handled before:

  /* Sort by PGID.  */
  if (pgid1 < pgid2)
    return -1;
  else if (pgid1 > pgid2)
    return 1;
  else

But let's assume not.  Let's assume we see two leaders when you get to the
code in question.  That means they have pgid1==pgid2.  Since by definition
being leader means pgid==pid, it must be that pid1 == pid2 as well.  That
is, this is really about comparing equivalent elements.  Which
brings us back again to:

      /* Easier to check for equivalent element first.  */
      if (pid1 == pid2)
        return 0;

Or am I confused again?

Thanks,
Pedro Alves