The following patch fixes a problem when a user application creates a thread 
shortly after another thread has completed.  For nptl, thread ids are addresses. 
  If a thread completes/dies, the tid is available for reuse by a new thread.

On RH9 and RHEL3, nptl threads do not have exit events associated with them.  I 
have already discussed this with Daniel J. who feels that the kernels are not 
doing the right thing, but regardless, the current and previous RH nptl kernels 
are behaving this way and gdb needs to handle it.  As such, when a new thread is 
created, if it is reusing the tid of a previous thread that gdb hasn't figured 
out isn't around any more, gdb ignores the create event and the new thread is 
not added.  Ignoring the event is done because it is possible for gdb to find 
out about the thread before it's creation event is reported and so the create 
event can be redundant information.

The following fix removes the problem by also checking if the original lwp given 
to the old thread in the list matches the lwp for the new thread.  IIRC, there 
is at least one platform that allows threads to change their lwps dynamically. 
I do not believe that platform uses the thread-db layer, but to be safe, I did 
not use a direct compare between the original lwp and the new lwp.  Instead, I 
added a target vector routine that by default never returns an unequal 
comparison.  For linux, the comparison routine points to a new routine in 
lin-lwp.c which does the expected comparison.  The comparison is based on C 
compare routines whereby 0 means equal and non-zero means unequal.

Now, if a thread id for a new thread is found to already be on the list, the 
target comparison is made between the original lwp stored for the old thread and 
the lwp for the new thread.  If the comparison is unequal, the old thread is 
deleted and then the new thread is added.  Otherwise, the new thread event is 
ignored as before.

Ok to commit?

-- Jeff J.

2004-03-18  Jeff Johnston  <jjohnstn@redhat.com>

         * thread-db.c (struct private_thread_info): Add orig_lwp field.
         (attach_thread): Save original lwp for thread after attaching.
         (check_event): When a create event occurs and the thread is already
         in the thread list, check if the old thread has gone away and the
         thread id is being recycled.
         * target.h (struct target_ops): Add to_thread_compare function.
         * target.c (update_current_target): Add to_thread_compare support.
         (debug_to_thread_compare): New function.
         (setup_target_debug): Add debug_to_thread_compare.
         * lin-lwp.c (lin_lwp_thread_compare): New function.
         (init_lin_lwp_ops): Add lin_lwp_thread_compare.