gh-111964: Implement stop-the-world pauses (gh-112471)

The `--disable-gil` builds occasionally need to pause all but one thread.  Some
examples include:

* Cyclic garbage collection, where this is often called a "stop the world event"
* Before calling `fork()`, to ensure a consistent state for internal data structures
* During interpreter shutdown, to ensure that daemon threads aren't accessing Python objects

This adds the following functions to implement global and per-interpreter pauses:

* `_PyEval_StopTheWorldAll()` and `_PyEval_StartTheWorldAll()` (for the global runtime)
* `_PyEval_StopTheWorld()` and `_PyEval_StartTheWorld()` (per-interpreter)

(The function names may change.)

These functions are no-ops outside of the `--disable-gil` build.

1 files changed, 9 insertions, 0 deletions

diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c
index d70abbc..f3b1692 100644
--- a/Python/ceval_gil.c
+++ b/Python/ceval_gil.c
@@ -949,6 +949,15 @@ _Py_HandlePending(PyThreadState *tstate)
 {
     PyInterpreterState *interp = tstate->interp;
 
+    /* Stop-the-world */
+    if (_Py_eval_breaker_bit_is_set(interp, _PY_EVAL_PLEASE_STOP_BIT)) {
+        _Py_set_eval_breaker_bit(interp, _PY_EVAL_PLEASE_STOP_BIT, 0);
+        _PyThreadState_Suspend(tstate);
+
+        /* The attach blocks until the stop-the-world event is complete. */
+        _PyThreadState_Attach(tstate);
+    }
+
     /* Pending signals */
     if (_Py_eval_breaker_bit_is_set(interp, _PY_SIGNALS_PENDING_BIT)) {
         if (handle_signals(tstate) != 0) {