RTEMS | posix/aio: Implemented aio_suspend() (!275)

[Alessandro Nardin (@AlessandroNardin)]

Alessandro Nardin commented on a discussion on cpukit/posix/src/aio_suspend.c: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/275#note_116976

 > +      r_chain = rtems_aio_search_fd( idle_req_chain, list[i]->aio_fildes, 0 );
 > +    }
 > +
 > +    /** If not found continue */
 > +    if ( r_chain == NULL ) {
 > +      continue;
 > +    }
 > +
 > +    /* Search request in fd_chain */
 > +    request = rtems_aio_search_in_chain( list[i], &r_chain->perfd );
 > +      
 > +    if ( request != NULL ) {
 > +      if ( request->suspendcbp == NULL ) {
 > +        request->suspendcbp = suspendcbp;
 > +      }
 > +      suspendcbp->requests_left++;

Yes, this is an error. That part of the code handles the case where a request is involved in more than one suspend call. I have changed it and now i think it works, although maybe there are better solutions.
Now the code is as follows:
```
if ( request->suspendcbp == NULL ) {
  request->suspendcbp = suspendcbp;
  suspendcbp->requests_left++;

} else {
  pthread_mutex_lock( request->suspendcbp->mutex );
  if ( request->suspendcbp->notified == AIO_SIGNALED ) {
    pthread_mutex_unlock( request->suspendcbp->mutex );

    rtems_aio_update_suspendcbp( request->suspendcbp );
    request->suspendcbp = suspendcbp;
    suspendcbp->requests_left++;

  } else{
    thread_mutex_unlock( request->suspendcbp->mutex );
  }
}
```
If `request->suspendcbp == NULL`, I can simply set `request->suspendcbp` to the new suspendcb.
If `request->suspendcbp != NULL`, it means that the request has been involved in another suspend request. There are now two cases:
1. If the other suspendcb has yet to generate the signal, I don't need to do anything since the completion of the request will generate the signal.
2. However, if the suspendcbp has already generated a signal, I update it and then set `request->suspendcbp` to the new suspendcb.

-- 
View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/275#note_116976
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