RFC: pthread pid accessor (BZ# 27880)

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

It seems that this is trickier than it seems, some issues we might consider 
first:

  1. What should we do with detached threads? As for pthread_kill, issuing 
     a pthread_gettid_np might use an invalid handler (since the pthread_t 
     identifier might be reused).  This only solution I have is to define 
     it as undefined behavior, this is not great but to proper support it
     would incur to keep tracking or all possible pthread_t identifiers 
     (we already keep the glibc provided stacks, dl_stack_cache, so it 
      would be a matter to include the user provided one in the list as 
     special entries).

  2. I think that once we provide this API, developers will start to use to
     query if a thread is alive and I am not sure if this is really the 
     proper API for this. This is the same issue as 1.

  3. How do we handle the concurrent access between pthread_join and 
    pthread_gettid_np? Once a pthread_join is issued, the pthread_t identifier
    might be reused and accessing it should be invalid. pthread_join first
    synchronizes using 'joinid' to avoid concurrent pthread_join and then
    wait the kernel signal on 'tid' that the thread has finished.  
    The straightforward 'pthread_gettid_np' implementation would do a atomic 
    load on tid, however it might read a transient value between pthread_join
    'joinid' setup and the futex wait.  I am not sure how to handle it correctly.

Also, MacOSX signature is:

  int pthread_gettid_np (pthread_t thread, uint64_t *thread_id)

And it returns the current thread identification if THREAD is NULL, returns
ESRCH for invalid handle (the 1. and 2. issue below), and also consults 
the kernel if the identifier can no be obtained.

I think for possible glibc symbols we should use a pid_t instead, the NULL
special arguments is also tricky because if by POSIX pthread_t might be
NULL (this is an implementation detail), for ESRCH we can our 
INVALID_NOT_TERMINATED_TD_P (which not the best solution, but it the 
current practice).

Thoughts?