gdbserver: report correct status in thread stop race condition

gdbserver: report correct status in thread stop race condition

The test introduced by the following patch would sometimes fail in this
configuration:

    FAIL: gdb.threads/next-fork-other-thread.exp: fork_func=vfork: target-non-stop=on: non-stop=off: displaced-stepping=auto: i=14: next to for loop

The test has multiple threads constantly forking or vforking while the
main thread keep doing "next"s.

(After writing the commit message, I realized this also fixes a similar
failure in gdb.threads/forking-threads-plus-breakpoint.exp with the
native-gdbserver and native-extended-gdbserver boards.)

As stop_all_threads is called, because the main thread finished its
"next", it inevitably happens at some point that we ask the remote
target to stop a thread and wait() reports that this thread stopped with
a fork or vfork event, instead of the SIGSTOP we sent to try to stop it.

While running this test, I attached to GDBserver and stopped at
linux-low.cc:3626.  We can see that the status pulled from the kernel
for 2742805 is indeed a vfork event:

    (gdb) p/x w
    $3 = 0x2057f
    (gdb) p WIFSTOPPED(w)
    $4 = true
    (gdb) p WSTOPSIG(w)
    $5 = 5
    (gdb) p/x (w >> 8) & (PTRACE_EVENT_VFORK << 8)
    $6 = 0x200

However, the statement at line 3626 overrides that:

    ourstatus->set_stopped (gdb_signal_from_host (WSTOPSIG (w)));

OURSTATUS becomes "stopped by a SIGTRAP".  The information about the
fork or vfork is lost.

It's then all downhill from there, stop_all_threads eventually asks for
a thread list update.  That thread list includes the child of that
forgotten fork or vfork, the remote target goes "oh cool, a new process,
let's attach to it!", when in fact that vfork child's destiny was to be
detached.

My reverse-engineered understanding of the code around there is that the
if/else between lines 3562 and 3583 (in the original code) makes sure
OURSTATUS is always initialized (not "ignore").  Either the details are
already in event_child->waitstatus (in the case of fork/vfork, for
example), in which case we just copy event_child->waitstatus to
ourstatus.  Or, if the event is a plain "stopped by a signal" or a
syscall event, OURSTATUS is set to "stopped", but without a signal
number.  Lines 3601 to 3629 (in the original code) serve to fill in that
last bit of information.

The problem is that when `w` holds the vfork status, the code wrongfully
takes this branch, because WSTOPSIG(w) returns SIGTRAP:

  else if (current_thread->last_resume_kind == resume_stop
       && WSTOPSIG (w) != SIGSTOP)

The intent of this branch is, for example, when we sent SIGSTOP to try
to stop a thread, but wait() reports that it stopped with another signal
(that it must have received from somewhere else simultaneously), say
SIGWINCH.  In that case, we want to report the SIGWINCH.  But in our
fork/vfork case, we don't want to take this branch, as the thread didn't
really stop because it received a signal.  For the non "stopped by a
signal" and non "syscall signal" cases, we would ideally skip over all
that snippet that fills in the signal or syscall number.

The fix I propose is to move this snipppet of the else branch of the
if/else above.  In addition to moving the code, the last two "else if"
branches:

  else if (current_thread->last_resume_kind == resume_stop
	   && WSTOPSIG (w) != SIGSTOP)
    {
      /* A thread that has been requested to stop by GDB with vCont;t,
	 but, it stopped for other reasons.  */
      ourstatus->set_stopped (gdb_signal_from_host (WSTOPSIG (w)));
    }
  else if (ourstatus->kind () == TARGET_WAITKIND_STOPPED)
    ourstatus->set_stopped (gdb_signal_from_host (WSTOPSIG (w)));

are changed into a single else:

  else
    ourstatus->set_stopped (gdb_signal_from_host (WSTOPSIG (w)));

This is the default path we take if:

 - W is not a syscall status
 - W does not represent a SIGSTOP that have sent to stop the thread and
   therefore want to suppress it

Change-Id: If2dc1f0537a549c293f7fa3c53efd00e3e194e79