gh-126016: Force crossinterpreter thread states to get cleaned up

Include/internal/pycore_pystate.h

@@ -244,6 +244,9 @@ extern PyThreadState * _PyThreadState_Swap(
 
 extern PyStatus _PyInterpreterState_Enable(_PyRuntimeState *runtime);
 
+extern PyThreadState *
+_PyThreadState_SwapAttached(PyThreadState *tstate);
+
 #ifdef HAVE_FORK
 extern PyStatus _PyInterpreterState_DeleteExceptMain(_PyRuntimeState *runtime);
 extern void _PySignal_AfterFork(void);

Lib/test/test_interpreters/test_lifecycle.py

@@ -184,6 +184,33 @@ def test_gh_109793(self):
             print("------")
         self.assertEqual(proc.returncode, 1)
 
+    @support.requires_subprocess()
+    @unittest.skipIf(sys.platform == "win32", "SIGINT does not work on Windows")
+    def test_interrupt_thread_with_interpreter(self):
+        # See GH-126016: Subinterpreters that are created
+        # in another thread might not have their thread states
+        # cleaned up if the user interrupted joining with CTRL+C.
+        import subprocess
+        import signal
+
+        with subprocess.Popen([
+            sys.executable, "-c",
+            "import threading, _interpreters\n"
+            "def run_interp(): _interpreters.run_string(_interpreters.create(),"
+            "'import time; print(1, flush=True); time.sleep(5)')\n"
+            "threading.Thread(target=run_interp).start()"
+        ], stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True) as proc:
+            with proc.stdout:
+                # Make sure that the thread has actually started
+                self.assertEqual(proc.stdout.read(1), "1")
+
+            # Send a KeyboardInterrupt to the process
+            proc.send_signal(signal.SIGINT)
+            with proc.stderr:
+                self.assertIn("KeyboardInterrupt", proc.stderr.read())
+            proc.wait()
+            # Make sure it didn't segfault
+            self.assertEqual(proc.returncode, 0)
 
 if __name__ == '__main__':
     # Test needs to be a package, so we can do relative imports.

Misc/NEWS.d/next/Library/2024-10-26-22-46-50.gh-issue-126016.gUWIIK.rst

@@ -0,0 +1 @@
+Fixed crash upon using CTRL+C while joining a thread containing a subinterpreter.

Python/pylifecycle.c

@@ -2477,7 +2477,28 @@ finalize_subinterpreters(void)
 
     /* Clean up all remaining subinterpreters. */
     while (interp != NULL) {
-        assert(!_PyInterpreterState_IsRunningMain(interp));
+        if (_PyInterpreterState_IsRunningMain(interp))
+        {
+            /*
+             * GH-126016: If a subinterpreter was running in another
+             * thread, and the user interrupted the joining of that thread
+             * with CTRL+C, then a thread state for this interpreter is still
+             * active. We need to destroy it before proceeding.
+             */
+            // XXX Are there more threads we have to worry about?
+            PyThreadState *to_destroy = interp->threads.main;
+
+            // We have to use SwapAttached to attach to an attached thread state.
+            // If the thread were really running, this would be a thread safety headache.
+            // Luckily, we know it isn't.
+            _PyThreadState_SwapAttached(to_destroy);
+            _PyInterpreterState_SetNotRunningMain(interp);
+            PyThreadState_Clear(to_destroy);
+
+            // Back to a NULL tstate
+            PyThreadState_Swap(NULL);
+            PyThreadState_Delete(to_destroy);
+        }
 
         /* Find the tstate to use for fini.  We assume the interpreter
            will have at most one tstate at this point. */

Python/pystate.c

@@ -2406,6 +2406,22 @@ PyThreadState_Swap(PyThreadState *newts)
     return _PyThreadState_Swap(&_PyRuntime, newts);
 }
 
+/*
+ * Calls PyThreadState_Swap() on the a bound thread state.
+ * This breaks the GIL, so it should only be used if certain that
+ * it's impossible for the thread to be running code.
+ */
+PyThreadState *
+_PyThreadState_SwapAttached(PyThreadState *tstate)
+{
+    // Evil thread state magic: we manually
+    // mark it as unbound so we can re-attach it
+    // to current thread.
+    tstate->_status.bound_gilstate = 0;
+    tstate->_status.holds_gil = 0;
+    tstate->_status.active = 0;
+    return PyThreadState_Swap(tstate);
+}
 
 void
 _PyThreadState_Bind(PyThreadState *tstate)