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Re: [PATCH] Fix 8.2 regression in gdb.python/py-evthreads.exp w/ gdbserver (PR gdb/23379)


On 2018-08-24 01:46 PM, Pedro Alves wrote:
> This commit fixes a 8.1->8.2 regression exposed by
> gdb.python/py-evthreads.exp when testing with
> --target_board=native-gdbserver.
> 
> gdb.log shows:
> 
>   src/gdb/thread.c:93: internal-error: thread_info* inferior_thread(): Assertion `tp' failed.
>   A problem internal to GDB has been detected,
>   further debugging may prove unreliable.
>   Quit this debugging session? (y or n) FAIL: gdb.python/py-evthreads.exp: run to breakpoint 1 (GDB internal error)
> 
> A backtrace shows (frames #2 and #10 highlighted) that the assertion
> fails when GDB is setting up the connection to the remote target, in
> non-stop mode:
> 
>   #0  0x0000000000622ff0 in internal_error(char const*, int, char const*, ...) (file=0xc1ad98 "src/gdb/thread.c", line=93, fmt=0xc1ad20 "%s: Assertion `%s' failed.") at src/gdb/common/errors.c:54
>   #1  0x000000000089567e in inferior_thread() () at src/gdb/thread.c:93
> = #2  0x00000000004da91d in get_event_thread() () at src/gdb/python/py-threadevent.c:38
>   #3  0x00000000004da9b7 in create_thread_event_object(_typeobject*, _object*) (py_type=0x11574c0 <continue_event_object_type>, thread=0x0)
>       at src/gdb/python/py-threadevent.c:60
>   #4  0x00000000004bf6fe in create_continue_event_object() () at src/gdb/python/py-continueevent.c:27
>   #5  0x00000000004bf738 in emit_continue_event(ptid_t) (ptid=...) at src/gdb/python/py-continueevent.c:40
>   #6  0x00000000004c7d47 in python_on_resume(ptid_t) (ptid=...) at src/gdb/python/py-inferior.c:108
>   #7  0x0000000000485bfb in std::_Function_handler<void (ptid_t), void (*)(ptid_t)>::_M_invoke(std::_Any_data const&, ptid_t&&) (__functor=..., __args#0=...) at /usr/include/c++/7/bits/std_function.h:316
>   #8  0x000000000089b416 in std::function<void (ptid_t)>::operator()(ptid_t) const (this=0x12aa600, __args#0=...)
>       at /usr/include/c++/7/bits/std_function.h:706
>   #9  0x000000000089aa0e in gdb::observers::observable<ptid_t>::notify(ptid_t) const (this=0x118a7a0 <gdb::observers::target_resumed>, args#0=...)
>       at src/gdb/common/observable.h:106
> = #10 0x0000000000896fbe in set_running(ptid_t, int) (ptid=..., running=1) at src/gdb/thread.c:880
>   #11 0x00000000007f750f in remote_target::remote_add_thread(ptid_t, bool, bool) (this=0x12c5440, ptid=..., running=true, executing=true) at src/gdb/remote.c:2434
>   #12 0x00000000007f779d in remote_target::remote_notice_new_inferior(ptid_t, int) (this=0x12c5440, currthread=..., executing=1)
>       at src/gdb/remote.c:2515
>   #13 0x00000000007f9c44 in remote_target::update_thread_list() (this=0x12c5440) at src/gdb/remote.c:3831
>   #14 0x00000000007fb922 in remote_target::start_remote(int, int) (this=0x12c5440, from_tty=0, extended_p=0)
>       at src/gdb/remote.c:4655
>   #15 0x00000000007fd102 in remote_target::open_1(char const*, int, int) (name=0x1a4f45e "localhost:2346", from_tty=0, extended_p=0)
>       at src/gdb/remote.c:5638
>   #16 0x00000000007fbec1 in remote_target::open(char const*, int) (name=0x1a4f45e "localhost:2346", from_tty=0)
>       at src/gdb/remote.c:4862
> 
> So on frame #10, we're marking a newly-discovered thread as running,
> and that causes the Python API to emit a gdb.ContinueEvent.
> gdb.ContinueEvent is a gdb.ThreadEvent, and as such includes the event
> thread as the "inferior_thread" attribute.  The problem is that when
> we get to frame #3/#4, we lost all references to the thread that is
> being marked as running.  create_continue_event_object assumes that it
> is the current thread, which is not true in this case.
> 
> Fix this by passing down the right thread in
> create_continue_event_object.  Also remove
> create_thread_event_object's default argument and have the only other
> caller left pass down the right thread explicitly too.
> 
> gdb/ChangeLog:
> yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
> 
> 	PR gdb/23379
> 	* python/py-continueevent.c: Include "gdbthread.h".
> 	(create_continue_event_object): Add intro comment.  Add 'ptid'
> 	parameter.  Use it to find thread to pass to
> 	create_thread_event_object.
> 	(emit_continue_event): Pass PTID down to
> 	create_continue_event_object.
> 	* python/py-event.h (py_get_event_thread): Declare.
> 	(create_thread_event_object): Remove default from 'thread'
> 	parameter.
> 	* python/py-stopevent.c (create_stop_event_object): Use
> 	py_get_event_thread.
> 	* python/py-threadevent.c (get_event_thread): Rename to ...
> 	(py_get_event_thread): ... this, and make extern.
> 	(create_thread_event_object): Assert that THREAD isn't null.
> 	Don't find the event thread here.
> ---
>  gdb/python/py-continueevent.c | 31 ++++++++++++++++++++++++++++---
>  gdb/python/py-event.h         | 12 +++++++++++-
>  gdb/python/py-stopevent.c     |  2 +-
>  gdb/python/py-threadevent.c   | 22 +++++-----------------
>  4 files changed, 45 insertions(+), 22 deletions(-)
> 
> diff --git a/gdb/python/py-continueevent.c b/gdb/python/py-continueevent.c
> index e5e922f0155..08d7da3cf2f 100644
> --- a/gdb/python/py-continueevent.c
> +++ b/gdb/python/py-continueevent.c
> @@ -20,11 +20,36 @@
>  #include "defs.h"
>  #include "py-event.h"
>  #include "py-ref.h"
> +#include "gdbthread.h"
> +
> +/* Create a gdb.ContinueEvent event.  gdb.ContinueEvent is-a
> +   gdb.ThreadEvent, and thread events can either be thread specific or
> +   process wide.  If gdb is running in non-stop mode then the event is
> +   thread specific (in which case the PTID thread is included in the
> +   event), otherwise it is process wide (in which case PTID is
> +   ignored).  In either case a borrowed reference is returned.  */

I think the last sentence is wrong.  If we are returning a gdbpy_ref, doesn't
mean we are returning an owning reference?  From what I understand, a borrowed
reference would be a naked pointer that you don't need to decref.

py_get_event_thread, from which this seems to have been copied from, does indeed
return a borrowed reference.

So it should say "a new reference is returned".

Ref: https://docs.python.org/3.7/c-api/intro.html, ctrl-f "borrow".

>  static gdbpy_ref<>
> -create_continue_event_object (void)
> +create_continue_event_object (ptid_t ptid)
>  {
> -  return create_thread_event_object (&continue_event_object_type);
> +  PyObject *py_thr = nullptr;
> +
> +  if (non_stop)
> +    {
> +      thread_info *thr = find_thread_ptid (ptid);
> +      if (thr != nullptr)
> +	py_thr = (PyObject *) thread_to_thread_object (thr);
> +    }
> +  else
> +    py_thr = Py_None;
> +
> +  if (py_thr == nullptr)
> +    {
> +      PyErr_SetString (PyExc_RuntimeError, _("Could not find event thread"));
> +      return nullptr;
> +    }
> +
> +  return create_thread_event_object (&continue_event_object_type, py_thr);

This and py_get_thread_event look very similar, could we use py_get_thread_event
from here?  See the patch at the end of the message.

>  }
>  
>  /* Callback function which notifies observers when a continue event occurs.
> @@ -37,7 +62,7 @@ emit_continue_event (ptid_t ptid)
>    if (evregpy_no_listeners_p (gdb_py_events.cont))
>      return 0;
>  
> -  gdbpy_ref<> event (create_continue_event_object ());
> +  gdbpy_ref<> event (create_continue_event_object (ptid));
>    if (event != NULL)
>      return evpy_emit_event (event.get (), gdb_py_events.cont);
>    return -1;
> diff --git a/gdb/python/py-event.h b/gdb/python/py-event.h
> index 22bab108ed1..7a9a51ddb1c 100644
> --- a/gdb/python/py-event.h
> +++ b/gdb/python/py-event.h
> @@ -63,8 +63,18 @@ extern int evpy_emit_event (PyObject *event,
>                              eventregistry_object *registry);
>  
>  extern gdbpy_ref<> create_event_object (PyTypeObject *py_type);
> +
> +/* thread events can either be thread specific or process wide.  If gdb is
> +   running in non-stop mode then the event is thread specific, otherwise
> +   it is process wide.
> +   This function returns the currently stopped thread in non-stop mode and
> +   Py_None otherwise.  In each case it returns a borrowed reference.  */
> +extern PyObject *py_get_event_thread ()
> +  CPYCHECKER_RETURNS_BORROWED_REF;
> +
>  extern gdbpy_ref<> create_thread_event_object (PyTypeObject *py_type,
> -					       PyObject *thread = nullptr);
> +					       PyObject *thread);
> +
>  extern int emit_new_objfile_event (struct objfile *objfile);
>  extern int emit_clear_objfiles_event (void);
>  
> diff --git a/gdb/python/py-stopevent.c b/gdb/python/py-stopevent.c
> index 884e6e3ef8f..33b65f99b80 100644
> --- a/gdb/python/py-stopevent.c
> +++ b/gdb/python/py-stopevent.c
> @@ -23,7 +23,7 @@
>  gdbpy_ref<>
>  create_stop_event_object (PyTypeObject *py_type)
>  {
> -  return create_thread_event_object (py_type);
> +  return create_thread_event_object (py_type, py_get_event_thread ());
>  }
>  
>  /* Callback observers when a stop event occurs.  This function will create a
> diff --git a/gdb/python/py-threadevent.c b/gdb/python/py-threadevent.c
> index d6aa9467b9a..1a115bad801 100644
> --- a/gdb/python/py-threadevent.c
> +++ b/gdb/python/py-threadevent.c
> @@ -20,17 +20,10 @@
>  #include "infrun.h"
>  #include "gdbthread.h"
>  
> -/* thread events can either be thread specific or process wide.  If gdb is
> -   running in non-stop mode then the event is thread specific, otherwise
> -   it is process wide.
> -   This function returns the currently stopped thread in non-stop mode and
> -   Py_None otherwise.  In each case it returns a borrowed reference.  */
> +/* See py-event.h.  */
>  
> -static PyObject *get_event_thread (void)
> -  CPYCHECKER_RETURNS_BORROWED_REF;
> -
> -static PyObject *
> -get_event_thread (void)
> +PyObject *
> +py_get_event_thread ()
>  {
>    PyObject *thread;
>  
> @@ -51,17 +44,12 @@ get_event_thread (void)
>  gdbpy_ref<>
>  create_thread_event_object (PyTypeObject *py_type, PyObject *thread)
>  {
> +  gdb_assert (thread != NULL);
> +
>    gdbpy_ref<> thread_event_obj (create_event_object (py_type));

You mentioned at some point that you preferred

  gdbpy_ref<> thread_event_obj = create_event_object (py_type);

over

  gdbpy_ref<> thread_event_obj (create_event_object (py_type));

(which I agree with).

>    if (thread_event_obj == NULL)
>      return NULL;
>  
> -  if (thread == NULL)
> -    {
> -      thread = get_event_thread ();
> -      if (!thread)
> -	return NULL;
> -    }
> -
>    if (evpy_add_attribute (thread_event_obj.get (),
>                            "inferior_thread",
>                            thread) < 0)
> 

Proposed changes:

>From 6b789cb151b5160fca3e3c90d47fb921e3d7697e Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@ericsson.com>
Date: Fri, 24 Aug 2018 14:26:23 -0400
Subject: [PATCH] Use py_get_event_thread in create_continue_event_object

---
 gdb/python/py-continueevent.c | 18 +++---------------
 gdb/python/py-event.h         |  2 +-
 gdb/python/py-stopevent.c     |  3 ++-
 gdb/python/py-threadevent.c   | 16 ++++++++++------
 4 files changed, 16 insertions(+), 23 deletions(-)

diff --git a/gdb/python/py-continueevent.c b/gdb/python/py-continueevent.c
index 08d7da3..c011b71 100644
--- a/gdb/python/py-continueevent.c
+++ b/gdb/python/py-continueevent.c
@@ -32,22 +32,10 @@
 static gdbpy_ref<>
 create_continue_event_object (ptid_t ptid)
 {
-  PyObject *py_thr = nullptr;
-
-  if (non_stop)
-    {
-      thread_info *thr = find_thread_ptid (ptid);
-      if (thr != nullptr)
-	py_thr = (PyObject *) thread_to_thread_object (thr);
-    }
-  else
-    py_thr = Py_None;
+  PyObject *py_thr = py_get_event_thread (ptid);

   if (py_thr == nullptr)
-    {
-      PyErr_SetString (PyExc_RuntimeError, _("Could not find event thread"));
-      return nullptr;
-    }
+    return nullptr;

   return create_thread_event_object (&continue_event_object_type, py_thr);
 }
@@ -62,7 +50,7 @@ emit_continue_event (ptid_t ptid)
   if (evregpy_no_listeners_p (gdb_py_events.cont))
     return 0;

-  gdbpy_ref<> event (create_continue_event_object (ptid));
+  gdbpy_ref<> event = create_continue_event_object (ptid);
   if (event != NULL)
     return evpy_emit_event (event.get (), gdb_py_events.cont);
   return -1;
diff --git a/gdb/python/py-event.h b/gdb/python/py-event.h
index 7a9a51d..56003e8 100644
--- a/gdb/python/py-event.h
+++ b/gdb/python/py-event.h
@@ -69,7 +69,7 @@ extern gdbpy_ref<> create_event_object (PyTypeObject *py_type);
    it is process wide.
    This function returns the currently stopped thread in non-stop mode and
    Py_None otherwise.  In each case it returns a borrowed reference.  */
-extern PyObject *py_get_event_thread ()
+extern PyObject *py_get_event_thread (ptid_t ptid)
   CPYCHECKER_RETURNS_BORROWED_REF;

 extern gdbpy_ref<> create_thread_event_object (PyTypeObject *py_type,
diff --git a/gdb/python/py-stopevent.c b/gdb/python/py-stopevent.c
index 33b65f9..f6bd207 100644
--- a/gdb/python/py-stopevent.c
+++ b/gdb/python/py-stopevent.c
@@ -23,7 +23,8 @@
 gdbpy_ref<>
 create_stop_event_object (PyTypeObject *py_type)
 {
-  return create_thread_event_object (py_type, py_get_event_thread ());
+  return create_thread_event_object (py_type,
+				     py_get_event_thread (inferior_ptid));
 }

 /* Callback observers when a stop event occurs.  This function will create a
diff --git a/gdb/python/py-threadevent.c b/gdb/python/py-threadevent.c
index 1a115ba..1bc2fb0 100644
--- a/gdb/python/py-threadevent.c
+++ b/gdb/python/py-threadevent.c
@@ -23,22 +23,26 @@
 /* See py-event.h.  */

 PyObject *
-py_get_event_thread ()
+py_get_event_thread (ptid_t ptid)
 {
-  PyObject *thread;
+  PyObject *pythread;

   if (non_stop)
-    thread = (PyObject *) thread_to_thread_object (inferior_thread ());
+    {
+      thread_info *thread = find_thread_ptid (ptid);
+      if (thread != nullptr)
+	pythread = (PyObject *) thread_to_thread_object (thread);
+    }
   else
-    thread = Py_None;
+    pythread = Py_None;

-  if (!thread)
+  if (!pythread)
     {
       PyErr_SetString (PyExc_RuntimeError, "Could not find event thread");
       return NULL;
     }

-  return thread;
+  return pythread;
 }

 gdbpy_ref<>
-- 
2.7.4


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