gdb/infrun: make fetch_inferior_event restore thread if exited or signalled

gdb/infrun: make fetch_inferior_event restore thread if exited or signalled

Commit 152a1749566 ("gdb: prune inferiors at end of
fetch_inferior_event, fix intermittent failure of
gdb.threads/fork-plus-threads.exp") introduced some follow-fork-related
test failures, such as:

    info inferiors^M
      Num  Description       Connection           Executable        ^M
    * 1    process 634972    1 (native)           /home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork ^M
      2    process 634975    1 (native)           /home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork ^M
    (gdb) PASS: gdb.base/foll-fork.exp: follow-fork-mode=parent: detach-on-fork=off: cmd=next 2: test_follow_fork: info inferiors
    inferior 2^M
    [Switching to inferior 2 [process 634975] (/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork)]^M
    [Switching to thread 2.1 (Thread 0x7ffff7c9a740 (LWP 634975))]^M
    #0  0x00007ffff7d7abf7 in _Fork () from /usr/lib/libc.so.6^M
    (gdb) PASS: gdb.base/foll-fork.exp: follow-fork-mode=parent: detach-on-fork=off: cmd=next 2: test_follow_fork: inferior 2
    continue^M
    Continuing.^M
    [Inferior 2 (process 634975) exited normally]^M
    [Switching to Thread 0x7ffff7c9a740 (LWP 634972)]^M
    (gdb) PASS: gdb.base/foll-fork.exp: follow-fork-mode=parent: detach-on-fork=off: cmd=next 2: test_follow_fork: continue until exit at continue unfollowed inferior to end
    break callee^M
    Breakpoint 2 at 0x555555555160: file /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/foll-fork.c, line 9.^M
    (gdb) FAIL: gdb.base/foll-fork.exp: follow-fork-mode=parent: detach-on-fork=off: cmd=next 2: test_follow_fork: break callee

What happens here is:

 - inferior 2 is selected
 - we continue, leading to inferior 2's exit
 - we set breakpoint, expect 2 locations, but only one location is
   resolved

Reading between the lines, we understand that inferior 2 got pruned,
when it shouldn't have been.

The issue can be reproduced by hand with:

    $ ./gdb -q --data-directory=data-directory testsuite/outputs/gdb.base/foll-fork/foll-fork -ex "set detach-on-fork off" -ex start -ex "next 2" -ex "inferior 2" -ex "set debug infrun"
    ...
    Temporary breakpoint 1, main () at /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/foll-fork.c:14
    14        int  v = 5;
    [New inferior 2 (process 637627)]
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".
    17        if (pid == 0) /* set breakpoint here */
    [Switching to inferior 2 [process 637627] (/home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork)]
    [Switching to thread 2.1 (Thread 0x7ffff7c9a740 (LWP 637627))]
    #0  0x00007ffff7d7abf7 in _Fork () from /usr/lib/libc.so.6
    (gdb) continue
    Continuing.
    [infrun] clear_proceed_status_thread: 637627.637627.0
    [infrun] proceed: enter
      [infrun] proceed: addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT
      [infrun] scoped_disable_commit_resumed: reason=proceeding
      [infrun] start_step_over: enter
        [infrun] start_step_over: stealing global queue of threads to step, length = 0
        [infrun] operator(): step-over queue now empty
      [infrun] start_step_over: exit
      [infrun] proceed: start: resuming threads, all-stop-on-top-of-non-stop
        [infrun] proceed: resuming 637627.637627.0
        [infrun] resume_1: step=0, signal=GDB_SIGNAL_0, trap_expected=0, current thread [637627.637627.0] at 0x7ffff7d7abf7
        [infrun] do_target_resume: resume_ptid=637627.637627.0, step=0, sig=GDB_SIGNAL_0
        [infrun] infrun_async: enable=1
        [infrun] prepare_to_wait: prepare_to_wait
      [infrun] proceed: end: resuming threads, all-stop-on-top-of-non-stop
      [infrun] reset: reason=proceeding
      [infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed for target native
      [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed for target native
      [infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed for target native
    [infrun] proceed: exit
    [infrun] fetch_inferior_event: enter
      [infrun] scoped_disable_commit_resumed: reason=handling event
      [infrun] do_target_wait: Found 2 inferiors, starting at #1
      [infrun] random_pending_event_thread: None found.
      [infrun] print_target_wait_results: target_wait (-1.0.0 [process -1], status) =
      [infrun] print_target_wait_results:   637627.637627.0 [process 637627],
      [infrun] print_target_wait_results:   status->kind = EXITED, exit_status = 0
      [infrun] handle_inferior_event: status->kind = EXITED, exit_status = 0
    [Inferior 2 (process 637627) exited normally]
      [infrun] stop_waiting: stop_waiting
      [infrun] stop_all_threads: start: reason=presenting stop to user in all-stop, inf=-1
        [infrun] stop_all_threads: pass=0, iterations=0
        [infrun] stop_all_threads:   637624.637624.0 not executing
        [infrun] stop_all_threads: pass=1, iterations=1
        [infrun] stop_all_threads:   637624.637624.0 not executing
        [infrun] stop_all_threads: done
      [infrun] stop_all_threads: end: reason=presenting stop to user in all-stop, inf=-1
    [Switching to Thread 0x7ffff7c9a740 (LWP 637624)]
      [infrun] infrun_async: enable=0
      [infrun] reset: reason=handling event
      [infrun] maybe_set_commit_resumed_all_targets: not requesting commit-resumed for target native, no resumed threads
    (gdb) [infrun] fetch_inferior_event: exit
    (gdb) info inferiors
      Num  Description       Connection           Executable
    * 1    process 637624    1 (native)           /home/simark/build/binutils-gdb-one-target/gdb/testsuite/outputs/gdb.base/foll-fork/foll-fork
    (gdb) i th
      Id   Target Id                                      Frame
    * 1    Thread 0x7ffff7c9a740 (LWP 637624) "foll-fork" main () at /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/foll-fork.c:17

After handling the EXITED event for inferior 2, inferior 2 should have
stayed the current inferior, which should have prevented it from getting
pruned.  When debugging, we find that when getting at the
prune_inferiors call, the current inferior is inferior 1.  Further
debugging shows that prior to the call to
clean_up_just_stopped_threads_fsms, the current inferior is inferior 2,
and after, it's inferior 1.  Then, back in fetch_inferior_event, the
restore_thread object is disabled, due to:

	    /* If we got a TARGET_WAITKIND_NO_RESUMED event, then the
	       previously selected thread is gone.  We have two
	       choices - switch to no thread selected, or restore the
	       previously selected thread (now exited).  We chose the
	       later, just because that's what GDB used to do.  After
	       this, "info threads" says "The current thread <Thread
	       ID 2> has terminated." instead of "No thread
	       selected.".  */
	    if (!non_stop
		&& cmd_done
		&& ecs->ws.kind () != TARGET_WAITKIND_NO_RESUMED)
	      restore_thread.dont_restore ();

So in the end, inferior 1 stays current, and inferior 2 gets wrongfully
pruned.

I'd say clean_up_just_stopped_threads_fsms is the culprit here.  It
actually attempts to restore the event_thread to be current at the end,
after the loop (I presume the current thread on entry is always supposed
to be the event thread).  But in this case, the event is of kind EXITED,
and ecs->event_thread is not set, so the current inferior isn't
restored.

Fix that by using scoped_restore_current_thread.  If there is no current
thread, scoped_restore_current_thread will still restore the current
inferior, and that's what we want.

Random note: the thread_info object for inferior 2's thread is never
freed.  It is held (by refcount) by the restore_thread object in
fetch_inferior_event, while the inferior's thread list gets cleared, in
the exit event processing.  When the refcount reaches 0 (when the
restore_thread object is destroyed), there's nothing that actually
deletes the thread_info object.  And I think that nothing in GDB points
to it anymore, so it leaks.  I don't want to fix that in this patch, but
thought it would be good to mention it, in case somebody has an idea for
how to fix that.

Change-Id: Ibc7df543e2c46aad5f3b9250b28c3fb5912be4e8