Re: [PATCH 1/3] support: Add support_wait_for_thread_exit

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella:

> On 17/08/2021 10:50, Florian Weimer via Libc-alpha wrote:
>> ---
>>  support/support.h                      |  4 ++
>>  support/support_wait_for_thread_exit.c | 67 ++++++++++++++++++++++++++
>
> Missing the Makefile entry for support_wait_for_thread_exit

Huh, right.  I must have put that into one of the other patches.  Will
reshuffle.

>> +void
>> +support_wait_for_thread_exit (void)
>> +{
>> +  DIR *proc_self_task = opendir ("/proc/self/task");
>> +  if (proc_self_task == NULL)
>> +    /* Use a large timeout because we cannot verify that the thread
>> +       has exited.  */
>> +    usleep (5 * 1000 * 1000);
>> +  else
>> +    {
>> +      int count;
>> +
>> +    wait_again:
>> +      count = 0;
>> +      rewinddir (proc_self_task);
>> +      while (true)
>> +        {
>> +          errno = 0;
>> +          struct dirent *e = readdir (proc_self_task);
>> +          if (e == NULL && errno != 0)
>> +            FAIL_EXIT1 ("readdir: %m");
>> +          if (e == NULL)
>> +            {
>> +              /* Only the main thread remains.  Testing may continue.  */
>> +              TEST_VERIFY (count >= 1);
>> +              closedir (proc_self_task);
>> +              return;
>> +            }
>> +
>> +          if (strcmp (e->d_name, ".") == 0 || strcmp (e->d_name, "..") == 0)
>> +            continue;
>> +
>> +          ++count;
>> +          if (count > 1)
>> +            {
>> +              /* Small timeout to give the thread a chance to exit.  */
>> +              usleep (50 * 1000);
>> +              goto wait_again;
>> +            }
>> +        }
>> +    }
>> +}
>> 
>
> I am not very found of interfaces that poll kernel resources using timeouts,
> but I don't think we have a better Linux interface to handle this.

I think we could use pidfd_open and poll once we have a wrapper for
pidfd_open.  Not sure if it's possible to get a pollable descriptor via
/proc.

Thanks,
Florian