Fix a couple vStopped pending ack bugs

A following patch will add a testcase that has two processes with
threads stepping over a breakpoint continuously, and then detaches
from one of the processes while threads are running.  The other
process continues stepping over its breakpoint.  And then the testcase
sends a SIGUSR1, expecting that GDB reports it.  That would sometimes
hang against gdbserver, due to the bugs fixed here.  Both bugs are
related, in that they're about remote protocol asynchronous Stop
notifications.  There's a bug in GDB, and another in GDBserver.

The GDB bug:

- when we detach from a process, the remote target discards any
  pending RSP notification related to that process, including the
  in-flight, yet-unacked notification.  Discarding the in-flight
  notification is the problem.  Until the in-flight notification is
  acked with a vStopped packet, the server won't send another %Stop
  notification.  As a result, the debug session gets messed up.  In
  the new testcase's case, GDB would hang inside stop_all_threads,
  waiting for a stop for one of the process'es threads, which never
  arrived -- its stop reply was permanently stuck in the stop reply
  queue, waiting for a vStopped packet that never arrived.

  In summary:

   1. GDBserver sends stop notification about thread X, the remote
      target receives it and stores it
   2. At the same time, GDB detaches thread X's inferior
   3. The remote target discards the received stop notification
   4. GDBserver waits forever for the ack

The GDBserver bug:

  GDBserver has the opposite bug.  It also discards notifications for
  the process being detached.  If that discards the head of the
  notification queue, when gdb sends an ack, it ends up acking the
  _next_ notification.  Meaning, gdb loses one notification.  In the
  testcase, this results in a similar hang in stop_all_threads.

So we have two very similar bugs in GDB and GDBserver, both resulting
in a similar symptom.  That's why I'm fixing them both at the same
time.

gdb/ChangeLog:

	* remote.c (remote_notif_stop_ack): Don't error out on
	TARGET_WAITKIND_IGNORE; instead, just ignore the notification.
	(remote_target::discard_pending_stop_replies): Don't delete
	in-flight notification; instead, clear its contents.

gdbserver/ChangeLog:

	* server.cc (discard_queued_stop_replies): Don't ever discard the
	notification at the head of the list.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index b53cd2b68be0021a6e7e2ea5e780db042429af54..f11092d840c304a6a0d11485b77c64fdeb0214ae 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2021-02-03  Pedro Alves  <pedro@palves.net>
+
+       * remote.c (remote_notif_stop_ack): Don't error out on
+       TARGET_WAITKIND_IGNORE; instead, just ignore the notification.
+       (remote_target::discard_pending_stop_replies): Don't delete
+       in-flight notification; instead, clear its contents.
+
 2021-02-03  Pedro Alves  <pedro@palves.net>
 
        * remote.c (extended_remote_target::attach): Set target async in

diff --git a/gdb/remote.c b/gdb/remote.c
index f183b4f3b0e4a7fbbfe2bf3cc8fab9abde71b9ef..512bd9467f6b4928a0305befe07c82c9392120a3 100644 (file)
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -7006,13 +7006,11 @@ remote_notif_stop_ack (remote_target *remote,
   /* acknowledge */
   putpkt (remote, self->ack_command);
 
-  if (stop_reply->ws.kind == TARGET_WAITKIND_IGNORE)
-    {
-      /* We got an unknown stop reply.  */
-      error (_("Unknown stop reply"));
-    }
-
-  remote->push_stop_reply (stop_reply);
+  /* Kind can be TARGET_WAITKIND_IGNORE if we have meanwhile discarded
+     the notification.  It was left in the queue because we need to
+     acknowledge it and pull the rest of the notifications out.  */
+  if (stop_reply->ws.kind != TARGET_WAITKIND_IGNORE)
+    remote->push_stop_reply (stop_reply);
 }
 
 static int
@@ -7181,8 +7179,14 @@ remote_target::discard_pending_stop_replies (struct inferior *inf)
   /* Discard the in-flight notification.  */
   if (reply != NULL && reply->ptid.pid () == inf->pid)
     {
-      delete reply;
-      rns->pending_event[notif_client_stop.id] = NULL;
+      /* Leave the notification pending, since the server expects that
+        we acknowledge it with vStopped.  But clear its contents, so
+        that later on when we acknowledge it, we also discard it.  */
+      reply->ws.kind = TARGET_WAITKIND_IGNORE;
+
+      if (remote_debug)
+       fprintf_unfiltered (gdb_stdlog,
+                           "discarded in-flight notification\n");
     }
 
   /* Discard the stop replies we have already pulled with

diff --git a/gdbserver/ChangeLog b/gdbserver/ChangeLog
index 2f883e05b5cd4ea3ad7e416539e48eb1e05a7a8a..41154d9e1908a754ad895b08ff3714c64f547e5d 100644 (file)
--- a/gdbserver/ChangeLog
+++ b/gdbserver/ChangeLog
@@ -1,3 +1,8 @@
+2021-02-03  Pedro Alves  <pedro@palves.net>
+
+       * server.cc (discard_queued_stop_replies): Don't ever discard the
+       notification at the head of the list.
+
 2021-01-20  Simon Marchi  <simon.marchi@polymtl.ca>
 
        * ax.cc (bytecode_address_table): Make static.

diff --git a/gdbserver/server.cc b/gdbserver/server.cc
index 77e89fe6ed0e5e1d0c9719738e8238522328d7d5..a5497e93cee285ef5989ca9985f25d492407161b 100644 (file)
--- a/gdbserver/server.cc
+++ b/gdbserver/server.cc
@@ -203,6 +203,15 @@ discard_queued_stop_replies (ptid_t ptid)
       next = iter;
       ++next;
 
+      if (iter == notif_stop.queue.begin ())
+       {
+         /* The head of the list contains the notification that was
+            already sent to GDB.  So we can't remove it, otherwise
+            when GDB sends the vStopped, it would ack the _next_
+            notification, which hadn't been sent yet!  */
+         continue;
+       }
+
       if (remove_all_on_match_ptid (*iter, ptid))
        {
          delete *iter;