Fix 8.2 regression in gdb.python/py-evthreads.exp w/ gdbserver (PR gdb/23379)

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:
2018-08-24  Pedro Alves  <palves@redhat.com>
	    Simon Marchi  <simon.marchi@ericsson.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, make extern, add 'ptid' parameter
	and use it to find the thread.
	(create_thread_event_object): Assert that THREAD isn't null.
	Don't find the event thread here.

4 files changed, 43 insertions, 27 deletions

diff --git a/gdb/python/py-continueevent.c b/gdb/python/py-continueevent.c
index e5e922f..b047a83 100644
--- a/gdb/python/py-continueevent.c
+++ b/gdb/python/py-continueevent.c
@@ -20,11 +20,24 @@
 #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 new reference is returned.  */
 
 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 = py_get_event_thread (ptid);
+
+  if (py_thr == nullptr)
+    return nullptr;
+
+  return create_thread_event_object (&continue_event_object_type, py_thr);
 }
 
 /* Callback function which notifies observers when a continue event occurs.
@@ -37,7 +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 ());
+  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 22bab10..56003e8 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 (ptid_t ptid)
+  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 884e6e3..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);
+  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 d6aa946..1bc2fb0 100644
--- a/gdb/python/py-threadevent.c
+++ b/gdb/python/py-threadevent.c
@@ -20,48 +20,40 @@
 #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 (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<>
 create_thread_event_object (PyTypeObject *py_type, PyObject *thread)
 {
+  gdb_assert (thread != NULL);
+
   gdbpy_ref<> thread_event_obj (create_event_object (py_type));
   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)