gh-151518: Strengthen the STW attach fairness regression

Hold each test pause for 10 ms and repeat pauses in C to reduce the
opportunities for a waiting thread to run between pause requests.

Clarify that parking rechecks the thread state before sleeping.

Author: tpn

Modules/_testinternalcapi.c

@@ -30,6 +30,7 @@
 #include "pycore_instruction_sequence.h"  // _PyInstructionSequence_New()
 #include "pycore_interpframe.h"   // _PyFrame_GetFunction()
 #include "pycore_jit.h"           // _PyJIT_AddressInJitCode()
+#include "pycore_lock.h"          // PyEvent_WaitTimed()
 #include "pycore_object.h"        // _PyObject_IsFreed()
 #include "pycore_optimizer.h"     // _Py_Executor_DependsOn
 #include "pycore_pathconfig.h"    // _PyPathConfig_ClearGlobal()
@@ -212,8 +213,14 @@ static PyObject *
 test_stop_the_world(PyObject *self, PyObject *Py_UNUSED(args))
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    _PyEval_StopTheWorld(interp);
-    _PyEval_StartTheWorld(interp);
+    // Request consecutive pauses without running Python code between them.
+    for (int i = 0; i < 100; i++) {
+        _PyEval_StopTheWorld(interp);
+        // Give detached threads time to try to reattach during the pause.
+        PyEvent event = {0};
+        PyEvent_WaitTimed(&event, 10 * 1000 * 1000, /*detach=*/0);
+        _PyEval_StartTheWorld(interp);
+    }
     Py_RETURN_NONE;
 }
 

Python/pystate.c

@@ -2269,7 +2269,7 @@ tstate_wait_attach(PyThreadState *tstate)
             state = _Py_THREAD_SUSPENDED_WAITING;
         }
         if (state == _Py_THREAD_SUSPENDED_WAITING) {
-            // Wait until the stop-the-world thread lets us attach.
+            // Park rechecks the state before sleeping, in case we were resumed.
             _PyParkingLot_Park(&tstate->state, &state, sizeof(tstate->state),
                                /*timeout=*/-1, NULL, /*detach=*/0);
         }