update thread list, delete exited threads

update thread list, delete exited threads

On GNU/Linux, if the running kernel supports clone events, then
linux-thread-db.c defers thread listing to the target beneath:

static void
thread_db_update_thread_list (struct target_ops *ops)
{
...
  if (target_has_execution && !thread_db_use_events ())
    ops->beneath->to_update_thread_list (ops->beneath);
  else
    thread_db_update_thread_list_td_ta_thr_iter (ops);
...
}

However, when live debugging, the target beneath, linux-nat.c, does
not implement the to_update_thread_list method.  The result is that if
a thread is marked exited (because it can't be deleted right now,
e.g., it was the selected thread), then it won't ever be deleted,
until the process exits or is killed/detached.

A similar thing happens with the remote.c target.  Because its
target_update_thread_list implementation skips exited threads when it
walks the current thread list looking for threads that no longer exits
on the target side, using ALL_NON_EXITED_THREADS_SAFE, stale exited
threads are never deleted.

This is not a big deal -- I can't think of any way this might be user
visible, other than gdb's memory growing a tiny bit whenever a thread
gets stuck in exited state.  Still, might as well clean things up
properly.

All other targets use prune_threads, so are unaffected.

The fix adds a ALL_THREADS_SAFE macro, that like
ALL_NON_EXITED_THREADS_SAFE, walks the thread list and allows deleting
the iterated thread, and uses that in places that are walking the
thread list in order to delete threads.  Actually, after converting
linux-nat.c and remote.c to use this, we find the only other user of
ALL_NON_EXITED_THREADS_SAFE is also walking the list to delete
threads.  So we convert that too, and end up deleting
ALL_NON_EXITED_THREADS_SAFE.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/ChangeLog
2015-04-07  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (ALL_NON_EXITED_THREADS_SAFE): Rename to ...
	(ALL_THREADS_SAFE): ... this, and don't skip exited threads.
	(delete_exited_threads): New declaration.
	* infrun.c (follow_exec): Use ALL_THREADS_SAFE.
	* linux-nat.c (linux_nat_update_thread_list): New function.
	(linux_nat_add_target): Install it.
	* remote.c (remote_update_thread_list): Use ALL_THREADS_SAFE.
	* thread.c (prune_threads): Use ALL_THREADS_SAFE.
	(delete_exited_threads): New function.