Re: [PATCH 1/3] PR remote/19496, internal err forking-threads-plus-bkpt

[Pedro Alves <palves at redhat dot com>]

On 01/28/2016 12:48 AM, Don Breazeal wrote:
> This patch fixes an internal error that occurs in
> gdb.threads/forking-threads-plus-breakpoint.exp:
> 
> /blah/binutils-gdb/gdb/target.c:2723: internal-error: Can't determine the
> current address space of thread Thread 3170.3170
> 
> In default_thread_address_space, find_inferior_ptid couldn't find 3170.3170
> because it had been overwritten in inferior_appeared, called as follows:
> 
> inferior_appeared
>   remote_add_inferior
>     remote_notice_new_inferior
>       remote_update_thread_list
> 
> The cause of the problem was the following sequence of events:
> 
> * GDB knows only about the main thread
> 
> * the first fork event is reported to GDB, saved as pending_event
> 
> * qXfer:threads_read gets the threads from the remote.
>   remove_new_fork_children id's the fork child from the pending event
>   and removes it from the list reported to GDB.  All the rest of the
>   threads, including the fork parent, are added to the GDB thread list.
> 
> * GDB stops all the threads.  All the stop events are pushed onto the
>   stop reply queue behind the pending fork event.
> 
> * remote_wait_ns calls queued_stop_reply and process_stop_reply to
>   remove the fork event from the front of the stop reply queue and save
>   event information in the thread_info structure for the fork parent
>   thread.  Unfortunately, none of the information saved in this way is
>   the fork-specific information, so the actual fork event info is lost.
> 
> * A subsequent qXfer:threads:read packet gets the thread list including
>   the fork parent and fork child.  remove_new_fork_children checks the
>   thread list to see if there is a fork parent, doesn't find one, checks
>   the stop reply queue for a pending fork event, doesn't find one, and
>   allows the fork child thread to be reported to GDB before the fork
>   event has been handled.  remote_update_thread_list calls
>   remote_notice_new_thread and overwrites the current (main) thread in
>   inferior_appeared.  GDB has now lost all knowledge of the main thread,
>   and an internal error results.
> 
> The fix was to make sure that when the stop reply was removed from the
> stop reply queuei, all of the necessary fork event information was stored
> in the parent thread structure.  In process_stop_reply we call a new
> function, update_thread_if_fork_parent, to store the pending_follow
> information from the fork stop reply in the fork parent thread.
> 
> Tested on x86_64 and Nios II Linux.  No regressions, but more failures,
> which are addressed in subsequent patches in this patchset.
> 

Many thanks for the detective work!

> +static void
> +update_thread_if_fork_parent (struct stop_reply *stop_reply)
> +{
> +  ptid_t ptid;
> +
> +  ptid = stop_reply->ptid;
> +  if (stop_reply->ws.kind == TARGET_WAITKIND_FORKED
> +      || stop_reply->ws.kind == TARGET_WAITKIND_VFORKED)
> +    {
> +      struct thread_info *tp = find_thread_ptid (ptid);
> +
> +      tp->pending_follow = stop_reply->ws;
> +    }
> +}

So the fork event has been reported out of target_wait but it was
left pending on the infrun side (infrun.c:save_waitstatus)

IOW, the fork event hasn't been processed by handle_inferior_event
yet, so it hasn't made it to tp->pending_follow yet.

The information is not lost, we're just looking for it in the
wrong place.  I think this would be the right fix:

>From 211e4553a500f7a81d11860f9707db97b0a53c45 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Mon, 1 Feb 2016 20:25:00 +0000
Subject: [PATCH] Fix PR remote/19496, internal error

---
 gdb/remote.c                                                  | 7 ++++++-
 gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp | 6 ------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index 8831b50..5dffb98 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -6180,7 +6180,12 @@ remove_new_fork_children (struct threads_listing_context *context)
      fork child threads from the CONTEXT list.  */
   ALL_NON_EXITED_THREADS (thread)
     {
-      struct target_waitstatus *ws = &thread->pending_follow;
+      struct target_waitstatus *ws;
+
+      if (thread->suspend.waitstatus_pending_p)
+	ws = &thread->suspend.waitstatus;
+      else
+	ws = &thread->pending_follow;

       if (is_pending_fork_parent (ws, pid, thread->ptid))
 	{
diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
index 6c72061..ff3ca9a 100644
--- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
+++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
@@ -100,12 +100,6 @@ proc do_test { cond_bp_target detach_on_fork displaced } {
     set fork_count 0
     set ok 0

-    if {$displaced == "off"
-	&& [target_info exists gdb_protocol]
-	&& ([target_info gdb_protocol] == "remote"
-	    || [target_info gdb_protocol] == "extended-remote")} {
-	setup_kfail "remote/19496" *-*-*
-    }
     set test "inferior 1 exited"
     gdb_test_multiple "" $test {
 	-re "Inferior 1 \(\[^\r\n\]+\) exited normally" {
-- 
1.9.3