gdb: use all_threads() in for_each_thread()

In for_each_thread(), we use all_threads_safe(), which allows deleting the
thread.  This it not needed in the cases where it is currently used and it
causes the iterator to get corrupted when threads are deleted in a nested
all_threads_safe() iteration.

One such scenario is as follows:
- remote debugging in non-stop mode
- one thread has hit a breakpoint
- one thread exited but GDB does not know about it, yet
- resume with 'continue -a'
- the continue command uses for_each_thread() to proceed threads individually
- the stopped thread starts an inline step-over (displaced stepping disabled)
- update_thread_list() learns about the exit and deletes the thread
- the safe iterator's m_next still points to the deleted thread

Approved-By: Tom Tromey <tom@tromey.com>

gdb/gdbthread.h

@@ -796,7 +796,14 @@ void thread_change_ptid (process_stratum_target *targ,
 using for_each_thread_callback_ftype
   = gdb::function_view<void (thread_info *)>;
 
-/* Call CALLBACK once for each known thread.  */
+/* Call CALLBACK once for each known thread.
+
+   CALLBACK must not delete the thread.  To delete threads, use:
+
+     for (thread_info &t : all_threads_safe ())
+       if (some_condition ())
+	 delete &t;
+*/
 
 extern void for_each_thread (for_each_thread_callback_ftype callback);
 

gdb/thread.c

@@ -605,7 +605,7 @@ find_thread_by_handle (gdb::array_view<const gdb_byte> handle,
 void
 for_each_thread (for_each_thread_callback_ftype callback)
 {
-  for (thread_info &tp : all_threads_safe ())
+  for (thread_info &tp : all_threads ())
     callback (&tp);
 }