fbsd-nat: Defer any ineligible events reported by wait.

If wait_1 finds an event for a thread or process that does not match
the set of threads and processes previously resumed, defer the event.
If the event is for a specific thread, suspend the thread and continue
the associated process before waiting for another event.

One specific example of such an event is if a thread is created while
another thread in the same process hits a breakpoint.  If the second
thread's event is reported first, the target resume method does not
yet "know" about the new thread and will not suspend it via
PT_SUSPEND.  When wait is called, it will probably return the event
from the first thread before the result of the step from second
thread.  This is the case reported in PR 21497.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=21497

Author: bsdjhb

gdb/fbsd-nat.c

@@ -1486,7 +1486,40 @@ fbsd_nat_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
   if (is_async_p ())
     async_file_flush ();
 
-  ptid_t wptid = wait_1 (ptid, ourstatus, target_options);
+  ptid_t wptid;
+  while (1)
+    {
+      wptid = wait_1 (ptid, ourstatus, target_options);
+
+      /* If no event was found, just return.  */
+      if (ourstatus->kind () == TARGET_WAITKIND_IGNORE
+	  || ourstatus->kind () == TARGET_WAITKIND_NO_RESUMED)
+	break;
+
+      /* If an event is reported for a thread or process while
+	 stepping some other thread, suspend the thread reporting the
+	 event and defer the event until it can be reported to the
+	 core.  */
+      if (!wptid.matches (m_resume_ptid))
+	{
+	  add_pending_event (wptid, *ourstatus);
+	  fbsd_nat_debug_printf ("deferring event [%s], [%s]",
+				 target_pid_to_str (wptid).c_str (),
+				 ourstatus->to_string ().c_str ());
+	  if (wptid.pid () == m_resume_ptid.pid ())
+	    {
+	      fbsd_nat_debug_printf ("suspending thread [%s]",
+				     target_pid_to_str (wptid).c_str ());
+	      if (ptrace (PT_SUSPEND, wptid.lwp (), NULL, 0) == -1)
+		perror_with_name (("ptrace (PT_SUSPEND)"));
+	      if (ptrace (PT_CONTINUE, wptid.pid (), (caddr_t) 1, 0) == -1)
+		perror_with_name (("ptrace (PT_CONTINUE)"));
+	    }
+	  continue;
+	}
+
+      break;
+    }
 
   /* If we are in async mode and found an event, there may still be
      another event pending.  Trigger the event pipe so that that the