bpo-32604: Swap threads only if the interpreter is different. (gh-5783)

The CPython runtime assumes that there is a one-to-one relationship (for a given interpreter) between PyThreadState and OS threads. Sending and receiving on a channel in the same interpreter was causing crashes because of this (specifically due to a check in PyThreadState_Swap()). The solution is to not switch threads if the interpreter is the same.
(cherry picked from commit f53d9f2778)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Lib/test/test__xxsubinterpreters.py

@@ -3,6 +3,7 @@ import os
 import pickle
 from textwrap import dedent, indent
 import threading
+import time
 import unittest
 
 from test import support
@@ -1147,6 +1148,54 @@ class ChannelTests(TestBase):
 
         self.assertEqual(obj, b'spam')
 
+    def test_send_recv_different_threads(self):
+        cid = interpreters.channel_create()
+
+        def f():
+            while True:
+                try:
+                    obj = interpreters.channel_recv(cid)
+                    break
+                except interpreters.ChannelEmptyError:
+                    time.sleep(0.1)
+            interpreters.channel_send(cid, obj)
+        t = threading.Thread(target=f)
+        t.start()
+
+        interpreters.channel_send(cid, b'spam')
+        t.join()
+        obj = interpreters.channel_recv(cid)
+
+        self.assertEqual(obj, b'spam')
+
+    def test_send_recv_different_interpreters_and_threads(self):
+        cid = interpreters.channel_create()
+        id1 = interpreters.create()
+        out = None
+
+        def f():
+            nonlocal out
+            out = _run_output(id1, dedent(f"""
+                import time
+                import _xxsubinterpreters as _interpreters
+                while True:
+                    try:
+                        obj = _interpreters.channel_recv({int(cid)})
+                        break
+                    except _interpreters.ChannelEmptyError:
+                        time.sleep(0.1)
+                assert(obj == b'spam')
+                _interpreters.channel_send({int(cid)}, b'eggs')
+                """))
+        t = threading.Thread(target=f)
+        t.start()
+
+        interpreters.channel_send(cid, b'spam')
+        t.join()
+        obj = interpreters.channel_recv(cid)
+
+        self.assertEqual(obj, b'eggs')
+
     def test_send_not_found(self):
         with self.assertRaises(interpreters.ChannelNotFoundError):
             interpreters.channel_send(10, b'spam')

Modules/_xxsubinterpretersmodule.c

@@ -1759,8 +1759,13 @@ _run_script_in_interpreter(PyInterpreterState *interp, const char *codestr,
     }
 
     // Switch to interpreter.
-    PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
-    PyThreadState *save_tstate = PyThreadState_Swap(tstate);
+    PyThreadState *save_tstate = NULL;
+    if (interp != PyThreadState_Get()->interp) {
+        // XXX Using the "head" thread isn't strictly correct.
+        PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
+        // XXX Possible GILState issues?
+        save_tstate = PyThreadState_Swap(tstate);
+    }
 
     // Run the script.
     _sharedexception *exc = NULL;
@@ -2079,9 +2084,9 @@ interp_create(PyObject *self, PyObject *args)
     }
 
     // Create and initialize the new interpreter.
-    PyThreadState *tstate, *save_tstate;
-    save_tstate = PyThreadState_Swap(NULL);
-    tstate = Py_NewInterpreter();
+    PyThreadState *save_tstate = PyThreadState_Swap(NULL);
+    // XXX Possible GILState issues?
+    PyThreadState *tstate = Py_NewInterpreter();
     PyThreadState_Swap(save_tstate);
     if (tstate == NULL) {
         /* Since no new thread state was created, there is no exception to
@@ -2096,6 +2101,7 @@ interp_create(PyObject *self, PyObject *args)
     return _get_id(tstate->interp);
 
 error:
+    // XXX Possible GILState issues?
     save_tstate = PyThreadState_Swap(tstate);
     Py_EndInterpreter(tstate);
     PyThreadState_Swap(save_tstate);
@@ -2146,9 +2152,9 @@ interp_destroy(PyObject *self, PyObject *args)
 
     // Destroy the interpreter.
     //PyInterpreterState_Delete(interp);
-    PyThreadState *tstate, *save_tstate;
-    tstate = PyInterpreterState_ThreadHead(interp);
-    save_tstate = PyThreadState_Swap(tstate);
+    PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
+    // XXX Possible GILState issues?
+    PyThreadState *save_tstate = PyThreadState_Swap(tstate);
     Py_EndInterpreter(tstate);
     PyThreadState_Swap(save_tstate);
 

Python/pystate.c

@@ -331,9 +331,10 @@ _PyInterpreterState_IDDecref(PyInterpreterState *interp)
     PyThread_release_lock(interp->id_mutex);
 
     if (refcount == 0) {
-        PyThreadState *tstate, *save_tstate;
-        tstate = PyInterpreterState_ThreadHead(interp);
-        save_tstate = PyThreadState_Swap(tstate);
+        // XXX Using the "head" thread isn't strictly correct.
+        PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
+        // XXX Possible GILState issues?
+        PyThreadState *save_tstate = PyThreadState_Swap(tstate);
         Py_EndInterpreter(tstate);
         PyThreadState_Swap(save_tstate);
     }
@@ -1213,8 +1214,14 @@ _PyCrossInterpreterData_Release(_PyCrossInterpreterData *data)
         }
         return;
     }
-    PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
-    PyThreadState *save_tstate = PyThreadState_Swap(tstate);
+
+    PyThreadState *save_tstate = NULL;
+    if (interp != PyThreadState_Get()->interp) {
+        // XXX Using the "head" thread isn't strictly correct.
+        PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
+        // XXX Possible GILState issues?
+        save_tstate = PyThreadState_Swap(tstate);
+    }
 
     // "Release" the data and/or the object.
     if (data->free != NULL) {
@@ -1223,8 +1230,9 @@ _PyCrossInterpreterData_Release(_PyCrossInterpreterData *data)
     Py_XDECREF(data->obj);
 
     // Switch back.
-    if (save_tstate != NULL)
+    if (save_tstate != NULL) {
         PyThreadState_Swap(save_tstate);
+    }
 }
 
 PyObject *