RFA[threads]: Fork event updates, part the fifteenth

[Daniel Jacobowitz <drow at mvista dot com>]

And here's the really exciting one for today.  The patch looks a little
silly because I'm not quite ready to introduce the body of
linux_record_stopped_pid yet; it will go in linux-nat.c, which means the
patch to add it will be noisy and generally uninteresting.  Let me explain
what's going on and why it is necessary.

When we get a fork event, the kernel reports two things.  The parent is
stopped and reports a new "extended wait status", indicating that a fork has
occured.  The child reports a SIGSTOP and the fact that we've never seen its
PID before is supposed to tip us off.  Unfortunately there was no practical
way for these two events to be received in a deterministic order, given the
other design constraints on the interface.  Particularly, by the time that
the event is reported I wanted to be able to wait for and PTRACE_DETACH from
the child without having to resume the parent, so the child already had to
be on the runqueue.

This SIGSTOP from an unknown PID is the same mechanism used by CLONE_PTRACE.

This patch provides a hook to just add the PIDs to a list and go back to
waiting for the fork/vfork/clone event.  Right now, this will further mess
up debugging of a CLONE_PTRACE application.  However, while there is some
code lying around to support it I just verified that we can't debug a simple
CLONE_PTRACE application, so I don't feel bad about that :)  We receive
events from an unknown process and get confused.

Is this OK?  Would you rather it go in with the real linux_record_stopped_pid?
I'm just submitting it separately in order to make the patch clearer.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

2002-12-15  Daniel Jacobowitz  <drow@mvista.com>

	* lin-lwp.c (linux_record_stopped_pid): New function and
	prototype.
	(child_wait): Call linux_record_stopped_pid.
	(lin_lwp_wait): Likewise.

--- lin-lwp.c.orig	2002-12-15 16:44:49.000000000 -0500
+++ lin-lwp.c	2002-12-15 17:00:27.000000000 -0500
@@ -156,6 +156,14 @@ static sigset_t blocked_mask;
 
 /* Prototypes for local functions.  */
 static int stop_wait_callback (struct lwp_info *lp, void *data);
+
+static int linux_record_stopped_pid (int pid);
+
+static int
+linux_record_stopped_pid (int pid)
+{
+  return 0;
+}
 
 /* Convert wait status STATUS to a string.  Used for printing debug
    messages only.  */
@@ -972,6 +980,19 @@ child_wait (ptid_t ptid, struct target_w
 	  save_errno = EINTR;
 	}
 
+      /* If we expect to receive stopped processes after fork, vfork, and
+	 clone events, then just add them to the list and go back to
+	 waiting for the event to be reported.  The stopped process can
+	 be returned from waitpid before or after the event is.
+	 If we want to handle debugging of CLONE_PTRACE processes we need
+	 to do more here, for instance enabling lin_lwp_ops.  */
+      if (pid != -1 && WIFSTOPPED (status) && pid != GET_PID (inferior_ptid))
+	if (linux_record_stopped_pid (pid))
+	  {
+	    pid = -1;
+	    save_errno = EINTR;
+	  }
+
       clear_sigio_trap ();
       clear_sigint_trap ();
     }
@@ -1110,6 +1131,22 @@ lin_lwp_wait (ptid_t ptid, struct target
 	      continue;
 	    }
 
+	  /* If we expect to receive stopped processes after fork, vfork, and
+	     clone events, then just add them to the list and go back to
+	     waiting for the event to be reported.  The stopped process can
+	     be returned from waitpid before or after the event is.  */
+	  if (WIFSTOPPED (status) && ! lp && linux_record_stopped_pid (lwpid))
+	    {
+	      status = 0;
+	      continue;
+	    }
+
+	  /* NOTE drow/2002-12-15: This code seems to be for debugging
+	     CLONE_PTRACE processes which do not use the thread library.
+	     That's currently disabled; if it is enabled then this should
+	     be broken out into a function so that we can also handle
+	     new LWPs if they are reported by the extended events
+	     interface.  */
 	  if (! lp)
 	    {
 	      lp = add_lwp (BUILD_LWP (lwpid, GET_PID (inferior_ptid)));