gdb: handle main thread exiting during detach

gdb: handle main thread exiting during detach

Overview
========

Consider the following situation, GDB is in non-stop mode, the main
thread is running while a second thread is stopped.  The user has the
second thread selected as the current thread and asks GDB to detach.
At the exact moment of detach the main thread exits.

This situation currently causes crashes, assertion failures, and
unexpected errors to be reported from GDB for both native and remote
targets.

This commit addresses this situation for native and remote targets.
There are a number of different fixes, but all are required in order
to get this functionality working correct for native and remote
targets.

Native Linux Target
===================

For the native Linux target, detaching is handled in the function
linux_nat_target::detach.  In here we call stop_wait_callback for each
thread, and it is this callback that will spot that the main thread
has exited.

GDB then detaches from everything except the main thread by calling
detach_callback.

After this the first problem is this assert:

  /* Only the initial process should be left right now.  */
  gdb_assert (num_lwps (pid) == 1);

The num_lwps call will return 0 as the main thread has exited and all
of the other threads have now been detached.  I fix this by changing
the assert to allow for 0 or 1 lwps at this point.  As the 0 case can
only happen in non-stop mode, the assert becomes:

  gdb_assert (num_lwps (pid) == 1
	      || (target_is_non_stop_p () && num_lwps (pid) == 0));

The next problem is that we do:

  main_lwp = find_lwp_pid (ptid_t (pid));

and then proceed assuming that main_lwp is not nullptr.  In the case
that the main thread has exited though, main_lwp will be nullptr.

However, we only need main_lwp so that GDB can detach from the
thread.  If the main thread has exited, and GDB has already detached
from every other thread, then GDB has finished detaching, GDB can skip
the calls that try to detach from the main thread, and then tell the
user that the detach was a success.

For Remote Targets
==================

On remote targets there are two problems.

First is that when the exit occurs during the early phase of the
detach, we see the stop notification arrive while GDB is removing the
breakpoints ahead of the detach.  The 'set debug remote on' trace
looks like this:

  [remote] Sending packet: $z0,7f1648fe0241,1#35
  [remote]   Notification received: Stop:W0;process:2a0ac8
  # At this point an unpatched gdbserver segfaults, and the connection
  # is broken.  A patched gdbserver continues as below...
  [remote] Packet received: E01
  [remote] Sending packet: $z0,7f1648ff00a8,1#68
  [remote] Packet received: E01
  [remote] Sending packet: $z0,7f1648ff132f,1#6b
  [remote] Packet received: E01
  [remote] Sending packet: $D;2a0ac8#3e
  [remote] Packet received: E01

I was originally running into Segmentation Faults, from within
gdbserver/mem-break.cc, in the function find_gdb_breakpoint.  This
function calls current_process() and then dereferences the result to
find the breakpoint list.

However, in our case, the current process has already exited, and so
the current_process() call returns nullptr.  At the point of failure,
the gdbserver backtrace looks like this:

  #0  0x00000000004190e4 in find_gdb_breakpoint (z_type=48 '0', addr=4198762, kind=1) at ../../src/gdbserver/mem-break.cc:982
  #1  0x000000000041930d in delete_gdb_breakpoint (z_type=48 '0', addr=4198762, kind=1) at ../../src/gdbserver/mem-break.cc:1093
  #2  0x000000000042d8db in process_serial_event () at ../../src/gdbserver/server.cc:4372
  #3  0x000000000042dcab in handle_serial_event (err=0, client_data=0x0) at ../../src/gdbserver/server.cc:4498
  ...

The problem is that, as a result non-stop being on, the process
exiting is only reported back to GDB after the request to remove a
breakpoint has been sent.  Clearly gdbserver can't actually remove
this breakpoint -- the process has already exited -- so I think the
best solution is for gdbserver just to report an error, which is what
I've done.

The second problem I ran into was on the gdb side, as the process has
already exited, but GDB has not yet acknowledged the exit event, the
detach -- the 'D' packet in the above trace -- fails.  This was being
reported to the user with a 'Can't detach process' error.  As the test
actually calls detach from Python code, this error was then becoming a
Python exception.

Though clearly the detach has returned an error, and so, maybe, having
GDB throw an error would be fine, I think in this case, there's a good
argument that the remote error can be ignored -- if GDB tries to
detach and gets back an error, and if there's a pending exit event for
the pid we tried to detach, then just ignore the error and pretend the
detach worked fine.

We could possibly check for a pending exit event before sending the
detach packet, however, I believe that it might be possible (in
non-stop mode) for the stop notification to arrive after the detach is
sent, but before gdbserver has started processing the detach.  In this
case we would still need to check for pending stop events after seeing
the detach fail, so I figure there's no point having two checks -- we
just send the detach request, and if it fails, check to see if the
process has already exited.

Testing
=======

In order to test this issue I needed to ensure that the exit event
arrives at the same time as the detach call.  The window of
opportunity for getting the exit to arrive is so small I've never
managed to trigger this in real use -- I originally spotted this issue
while working on another patch, which did manage to trigger this
issue.

However, if we trigger both the exit and the detach from a single
Python function then we never return to GDB's event loop, as such GDB
never processes the exit event, and so the first time GDB gets a
chance to see the exit is during the detach call.  And so that is the
approach I've taken for testing this patch.

Tested-By: Kevin Buettner <kevinb@redhat.com>
Approved-By: Kevin Buettner <kevinb@redhat.com>