[PATCH 2/2] Ensure selected thread after thread exit stop

[Pedro Alves <pedro@palves.net>]

While making step over thread exit work properly on AMDGPU, I noticed
that if there's a breakpoint on top of the exit syscall, and,
displaced stepping is off, then when GDB reports "Command aborted,
thread exited.", GDB also switches focus to a random thread, instead
of leaving the exited thread as selected:

 (gdb) thread
 [Current thread is 6, lane 0 (AMDGPU Lane 1:4:1:1/0 (0,0,0)[0,0,0])]
 (gdb) si
 [AMDGPU Wave 1:4:1:1 (0,0,0)/0 exited]
 Command aborted, thread exited.
 (gdb) thread
 [Current thread is 5 (Thread 0x7ffff626f640 (LWP 3248392))]
 (gdb)

The previous patch extended gdb.threads/step-over-thread-exit.exp to
exercise this on GNU/Linux (on the CPU side), and there, after that
"si", we always end up with the exiting thread as selected even
without this fix, but that's just a concidence.  This commit enforces
it, fixing the latent problem for GNU/Linux, and the actual problem on
AMDGPU.  I wrote a gdb.rocm/ testcase for this, but I can't be
upstreamed yet, until more pieces of the DWARF machinery are upstream
as well.

Change-Id: I6ff57a79514ac0142bba35c749fe83d53d9e4e51
---
 gdb/infrun.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index a1543ab2443..a74ec824a8e 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -5891,7 +5891,14 @@ handle_thread_exited (execution_control_state *ecs)
 
   if (abort_cmd)
     {
+      /* We're stopping for the thread exit event.  Switch to the
+	 event thread again, as finish_step_over may have switched
+	 threads.  */
+      switch_to_thread (ecs->event_thread);
+
+      /* Emit [Thread ... exited] notification.  */
       delete_thread (ecs->event_thread);
+
       ecs->event_thread = nullptr;
       return false;
     }
-- 
2.42.0