Patch by Jonathan Giddy (with some cleanup by me) to always use the

thread state of the thread calling mainloop() (or another event
handling function) rather than the thread state of the function that
created the client data structure.

Modules/_tkinter.c

@@ -157,14 +157,14 @@ PERFORMANCE OF THIS SOFTWARE.
    the thread state) but doesn't release the Tcl lock; LEAVE_OVERLAP_TCL
    releases the Tcl lock.
 
-   By contrast, ENTER_PYTHON(tstate) and LEAVE_PYTHON are used in Tcl event
+   By contrast, ENTER_PYTHON and LEAVE_PYTHON are used in Tcl event
    handlers when the handler needs to use Python.  Such event handlers are
    entered while the lock for Tcl is held; the event handler presumably needs
-   to use Python.  ENTER_PYTHON(tstate) releases the lock for Tcl and acquires
+   to use Python.  ENTER_PYTHON releases the lock for Tcl and acquires
    the Python interpreter lock, restoring the appropriate thread state, and
    LEAVE_PYTHON releases the Python interpreter lock and re-acquires the lock
    for Tcl.  It is okay for ENTER_TCL/LEAVE_TCL pairs to be contained inside
-   the code between ENTER_PYTHON(tstate) and LEAVE_PYTHON.
+   the code between ENTER_PYTHON and LEAVE_PYTHON.
 
    These locks expand to several statements and brackets; they should not be
    used in branches of if statements and the like.
@@ -172,24 +172,28 @@ PERFORMANCE OF THIS SOFTWARE.
 */
 
 static type_lock tcl_lock = 0;
+static PyThreadState *tcl_tstate = NULL;
 
 #define ENTER_TCL \
-	Py_BEGIN_ALLOW_THREADS acquire_lock(tcl_lock, 1);
+	{ PyThreadState *tstate = PyThreadState_Get(); Py_BEGIN_ALLOW_THREADS \
+	    acquire_lock(tcl_lock, 1); tcl_tstate = tstate;
 
 #define LEAVE_TCL \
-	release_lock(tcl_lock); Py_END_ALLOW_THREADS
+	tcl_tstate = NULL; release_lock(tcl_lock); Py_END_ALLOW_THREADS}
 
 #define ENTER_OVERLAP \
 	Py_END_ALLOW_THREADS
 
 #define LEAVE_OVERLAP_TCL \
-	release_lock(tcl_lock);
+	tcl_tstate = NULL; release_lock(tcl_lock); }
 
-#define ENTER_PYTHON(tstate) \
-	release_lock(tcl_lock); PyEval_RestoreThread((tstate));
+#define ENTER_PYTHON \
+	{ PyThreadState *tstate = tcl_tstate; tcl_tstate = NULL; \
+            release_lock(tcl_lock); PyEval_RestoreThread((tstate)); }
 
 #define LEAVE_PYTHON \
-	PyEval_SaveThread(); acquire_lock(tcl_lock, 1);
+	{ PyThreadState *tstate = PyEval_SaveThread(); \
+            acquire_lock(tcl_lock, 1); tcl_tstate = tstate; }
 
 #else
 
@@ -197,7 +201,7 @@ static type_lock tcl_lock = 0;
 #define LEAVE_TCL
 #define ENTER_OVERLAP
 #define LEAVE_OVERLAP_TCL
-#define ENTER_PYTHON(tstate)
+#define ENTER_PYTHON
 #define LEAVE_PYTHON
 
 #endif
@@ -1160,7 +1164,6 @@ Tkapp_Merge(self, args)
 
 /* Client data struct */
 typedef struct {
-	PyThreadState *tstate;
 	PyObject *self;
 	PyObject *func;
 } PythonCmd_ClientData;
@@ -1189,8 +1192,7 @@ PythonCmd(clientData, interp, argc, argv)
 	PyObject *self, *func, *arg, *res, *tmp;
 	int i;
 
-	/* XXX Should create fresh thread state? */
-	ENTER_PYTHON(data->tstate)
+	ENTER_PYTHON
 
 	/* TBD: no error checking here since we know, via the
 	 * Tkapp_CreateCommand() that the client data is a two-tuple
@@ -1235,7 +1237,7 @@ PythonCmdDelete(clientData)
 {
 	PythonCmd_ClientData *data = (PythonCmd_ClientData *)clientData;
 
-	ENTER_PYTHON(data->tstate)
+	ENTER_PYTHON
 	Py_XDECREF(data->self);
 	Py_XDECREF(data->func);
 	PyMem_DEL(data);
@@ -1264,7 +1266,6 @@ Tkapp_CreateCommand(self, args)
 	data = PyMem_NEW(PythonCmd_ClientData, 1);
 	if (!data)
 		return NULL;
-	data->tstate = PyThreadState_Get();
 	Py_XINCREF(self);
 	Py_XINCREF(func);
 	data->self = self;
@@ -1313,7 +1314,6 @@ Tkapp_DeleteCommand(self, args)
 /** File Handler **/
 
 typedef struct _fhcdata {
-	PyThreadState *tstate;
 	PyObject *func;
 	PyObject *file;
 	int id;
@@ -1333,7 +1333,6 @@ NewFHCD(func, file, id)
 	if (p != NULL) {
 		Py_XINCREF(func);
 		Py_XINCREF(file);
-		p->tstate = PyThreadState_Get();
 		p->func = func;
 		p->file = file;
 		p->id = id;
@@ -1370,8 +1369,7 @@ FileHandler(clientData, mask)
 	FileHandler_ClientData *data = (FileHandler_ClientData *)clientData;
 	PyObject *func, *file, *arg, *res;
 
-	/* XXX Should create fresh thread state? */
-	ENTER_PYTHON(data->tstate)
+	ENTER_PYTHON
 	func = data->func;
 	file = data->file;
 
@@ -1500,7 +1498,6 @@ typedef struct {
 	PyObject_HEAD
 	Tcl_TimerToken token;
 	PyObject *func;
-	PyThreadState *tstate;
 } TkttObject;
 
 static PyObject *
@@ -1545,7 +1542,6 @@ Tktt_New(func)
 	Py_INCREF(func);
 	v->token = NULL;
 	v->func = func;
-	v->tstate = PyThreadState_Get();
 
 	/* Extra reference, deleted when called or when handler is deleted */
 	Py_INCREF(v);
@@ -1620,7 +1616,7 @@ TimerHandler(clientData)
 
 	v->func = NULL;
 
-	ENTER_PYTHON(v->tstate)
+	ENTER_PYTHON
 
 	res  = PyEval_CallObject(func, NULL);
 	Py_DECREF(func);
@@ -1667,6 +1663,9 @@ Tkapp_MainLoop(self, args)
 	PyObject *args;
 {
 	int threshold = 0;
+#ifdef WITH_THREAD
+	PyThreadState *tstate = PyThreadState_Get();
+#endif
 
 	if (!PyArg_ParseTuple(args, "|i", &threshold))
 		return NULL;
@@ -1681,7 +1680,9 @@ Tkapp_MainLoop(self, args)
 #ifdef WITH_THREAD
 		Py_BEGIN_ALLOW_THREADS
 		acquire_lock(tcl_lock, 1);
+		tcl_tstate = tstate;
 		result = Tcl_DoOneEvent(TCL_DONT_WAIT);
+		tcl_tstate = NULL;
 		release_lock(tcl_lock);
 		if (result == 0)
 			Sleep(20);
@@ -1898,6 +1899,9 @@ static PyThreadState *event_tstate = NULL;
 static int
 EventHook()
 {
+#if defined(WITH_THREAD) || defined(MS_WINDOWS)
+	PyThreadState *tstate = PyThreadState_Get();
+#endif
 #ifndef MS_WINDOWS
 	FHANDLE tfile;
 #endif
@@ -1919,7 +1923,11 @@ EventHook()
 #if defined(WITH_THREAD) || defined(MS_WINDOWS)
 		Py_BEGIN_ALLOW_THREADS
 		acquire_lock(tcl_lock, 1);
+		tcl_tstate = tstate;
+
 		result = Tcl_DoOneEvent(TCL_DONT_WAIT);
+
+		tcl_tstate = NULL;
 		release_lock(tcl_lock);
 		if (result == 0)
 			Sleep(20);