gdb/infrun: handle already-exited threads when attempting to stop

gdb/infrun: handle already-exited threads when attempting to stop

In stop_all_threads, GDB sends signals to other threads in an attempt
to stop them.  While in a typical scenario the expected wait status is
TARGET_WAITKIND_STOPPED, it is possible that the thread GDB attempted
to stop has already terminated.  If so, a waitstatus other than
TARGET_WAITKIND_STOPPED would be received.  Handle this case
appropriately.

If a wait status that denotes thread termination is ignored, GDB goes
into an infinite loop in stop_all_threads.
E.g.:

  $ gdb ./a.out
  (gdb) start
  ...
  (gdb) add-inferior -exec ./a.out
  ...
  (gdb) inferior 2
  ...
  (gdb) start
  ...
  (gdb) set schedule-multiple on
  (gdb) set debug infrun 2
  (gdb) continue
  Continuing.
  infrun: clear_proceed_status_thread (process 10449)
  infrun: clear_proceed_status_thread (process 10453)
  infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
  infrun: proceed: resuming process 10449
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 10449] at 0x55555555514e
  infrun: infrun_async(1)
  infrun: prepare_to_wait
  infrun: proceed: resuming process 10453
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 10453] at 0x55555555514e
  infrun: prepare_to_wait
  infrun: Found 2 inferiors, starting at #0
  infrun: target_wait (-1.0.0, status) =
  infrun:   10449.10449.0 [process 10449],
  infrun:   status->kind = exited, status = 0
  infrun: handle_inferior_event status->kind = exited, status = 0
  [Inferior 1 (process 10449) exited normally]
  infrun: stop_waiting
  infrun: stop_all_threads
  infrun: stop_all_threads, pass=0, iterations=0
  infrun:   process 10453 executing, need stop
  infrun: target_wait (-1.0.0, status) =
  infrun:   10453.10453.0 [process 10453],
  infrun:   status->kind = exited, status = 0
  infrun: stop_all_threads status->kind = exited, status = 0 process 10453
  infrun:   process 10453 executing, already stopping
  infrun: target_wait (-1.0.0, status) =
  infrun:   -1.0.0 [process -1],
  infrun:   status->kind = no-resumed
  infrun: infrun_async(0)
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  infrun: stop_all_threads status->kind = no-resumed process -1
  infrun:   process 10453 executing, already stopping
  ...

And this polling goes on forever.  This patch prevents the infinite
looping behavior.  For the same scenario above, we obtain the
following behavior:

  ...
  (gdb) continue
  Continuing.
  infrun: clear_proceed_status_thread (process 31229)
  infrun: clear_proceed_status_thread (process 31233)
  infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
  infrun: proceed: resuming process 31229
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 31229] at 0x55555555514e
  infrun: infrun_async(1)
  infrun: prepare_to_wait
  infrun: proceed: resuming process 31233
  infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 31233] at 0x55555555514e
  infrun: prepare_to_wait
  infrun: Found 2 inferiors, starting at #0
  infrun: target_wait (-1.0.0, status) =
  infrun:   31229.31229.0 [process 31229],
  infrun:   status->kind = exited, status = 0
  infrun: handle_inferior_event status->kind = exited, status = 0
  [Inferior 1 (process 31229) exited normally]
  infrun: stop_waiting
  infrun: stop_all_threads
  infrun: stop_all_threads, pass=0, iterations=0
  infrun:   process 31233 executing, need stop
  infrun: target_wait (-1.0.0, status) =
  infrun:   31233.31233.0 [process 31233],
  infrun:   status->kind = exited, status = 0
  infrun: stop_all_threads status->kind = exited, status = 0 process 31233
  infrun: saving status status->kind = exited, status = 0 for 31233.31233.0
  infrun:   process 31233 not executing
  infrun: stop_all_threads, pass=1, iterations=1
  infrun:   process 31233 not executing
  infrun: stop_all_threads done
  (gdb)

The exit event from Inferior 1 is received and shown to the user.
The exit event from Inferior 2 is not displayed, but kept pending.

  (gdb) info inferiors
    Num  Description       Connection           Executable
  * 1    <null>                                 a.out
    2    process 31233     1 (native)           a.out
  (gdb) inferior 2
  [Switching to inferior 2 [process 31233] (a.out)]
  [Switching to thread 2.1 (process 31233)]
  Couldn't get registers: No such process.
  (gdb) continue
  Continuing.
  infrun: clear_proceed_status_thread (process 31233)
  infrun: clear_proceed_status_thread: thread process 31233 has pending wait status status->kind = exited, status = 0 (currently_stepping=0).
  infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
  infrun: proceed: resuming process 31233
  infrun: resume: thread process 31233 has pending wait status status->kind = exited, status = 0 (currently_stepping=0).
  infrun: prepare_to_wait
  infrun: Using pending wait status status->kind = exited, status = 0 for process 31233.
  infrun: target_wait (-1.0.0, status) =
  infrun:   31233.31233.0 [process 31233],
  infrun:   status->kind = exited, status = 0
  infrun: handle_inferior_event status->kind = exited, status = 0
  [Inferior 2 (process 31233) exited normally]
  infrun: stop_waiting
  (gdb) info inferiors
    Num  Description       Connection           Executable
    1    <null>                                 a.out
  * 2    <null>                                 a.out
  (gdb)

When a process exits and we leave the process exit event pending, we
need to make sure that at least one thread is left listed in the
inferior's thread list.  This is necessary in order to make sure we
have a thread that we can later resume, so the process exit event can
be collected/reported.

When native debugging, the GNU/Linux back end already makes sure that
the last LWP isn't deleted.

When remote debugging against GNU/Linux GDBserver, the GNU/Linux
GDBserver backend also makes sure that the last thread isn't deleted
until the process exit event is reported to GDBserver core.

However, between the backend reporting the process exit event to
GDBserver core, and GDB consuming the event, GDB may update the thread
list and find no thread left in the process.  The process exit event
will be pending somewhere in GDBserver's stop reply queue, or
gdb/remote.c's queue, or whathever other event queue inbetween
GDBserver and infrun.c's handle_inferior_event.

This patch tweaks remote.c's target_update_thread_list implementation
to avoid deleting the last thread of an inferior.

In the past, this case of inferior-with-no-threads led to a special
case at the bottom of handle_no_resumed, where it reads:

  /* Note however that we may find no resumed thread because the whole
     process exited meanwhile (thus updating the thread list results
     in an empty thread list).  In this case we know we'll be getting
     a process exit event shortly.  */
  for (inferior *inf : all_non_exited_inferiors (ecs->target))

In current master, that code path is still reachable with the
gdb.threads/continue-pending-after-query.exp testcase, when tested
against GDBserver, with "maint set target-non-stop" forced "on".

With this patch, the scenario that loop was concerned about is still
properly handled, because the loop above it finds the process's last
thread with "executing" set to true, and thus the handle_no_resumed
function still returns true.

Since GNU/Linux native and remote are the only targets that support
non-stop mode, and with this patch, we always make sure the inferior
has at least one thread, this patch also removes that "inferior with
no threads" special case handling from handle_no_resumed.

Since remote.c now has a special case where we treat a thread that has
already exited as if it was still alive, we might need to tweak
remote.c's target_thread_alive implementation to return true for that
thread without querying the remote side (which would say "no, not
alive").  After inspecting all the target_thread_alive calls in the
codebase, it seems that only the one from prune_threads could result
in that thread being accidentally deleted.  There's only one call to
prune_threads in GDB's common code, so this patch handles this by
replacing the prune_threads call with a delete_exited_threads call.
This seems like an improvement anyway, because we'll still be doing
what the comment suggests we want to do, and, we avoid remote protocol
traffic.

Regression-tested on X86_64 Linux.

gdb/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
	    Tom de Vries  <tdevries@suse.de>
	    Pedro Alves  <palves@redhat.com>

	PR threads/25478
	* infrun.c (stop_all_threads): Do NOT ignore
	TARGET_WAITKIND_NO_RESUMED, TARGET_WAITKIND_THREAD_EXITED,
	TARGET_WAITKIND_EXITED, TARGET_WAITKIND_SIGNALLED wait statuses
	received.
	(handle_no_resumed): Remove code handling a live inferior with no
	threads.
	* remote.c (has_single_non_exited_thread): New.
	(remote_target::update_thread_list): Do not delete a thread if is
	the last thread of the process.
	* thread.c (thread_select): Call delete_exited_threads instead of
	prune_threads.

gdb/testsuite/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.multi/multi-exit.c: New file.
	* gdb.multi/multi-exit.exp: New file.
	* gdb.multi/multi-kill.c: New file.
	* gdb.multi/multi-kill.exp: New file.