gh-117300: Use stop the world to make sys._current_frames and

`sys._current_exceptions` thread-safe.

This adds a stop the world pause to make the two functions thread-safe
when the GIL is disabled in the free-threaded build.

Additionally, the main test thread may call `sys._current_exceptions()`
as soon as `g_raised.set()` is called. The background thread may not yet
reach the `leave_g.wait()` line.

Lib/test/test_sys.py

@@ -562,7 +562,8 @@ def g456():
             # And the next record must be for g456().
             filename, lineno, funcname, sourceline = stack[i+1]
             self.assertEqual(funcname, "g456")
-            self.assertTrue(sourceline.startswith("if leave_g.wait("))
+            self.assertTrue((sourceline.startswith("if leave_g.wait(") or
+                             sourceline.startswith("g_raised.set()")))
         finally:
             # Reap the spawned thread.
             leave_g.set()

Python/pystate.c

@@ -2406,6 +2406,7 @@ _PyThread_CurrentFrames(void)
      * Because these lists can mutate even when the GIL is held, we
      * need to grab head_mutex for the duration.
      */
+    _PyEval_StopTheWorldAll(runtime);
     HEAD_LOCK(runtime);
     PyInterpreterState *i;
     for (i = runtime->interpreters.head; i != NULL; i = i->next) {
@@ -2439,6 +2440,7 @@ _PyThread_CurrentFrames(void)
 
 done:
     HEAD_UNLOCK(runtime);
+    _PyEval_StartTheWorldAll(runtime);
     return result;
 }
 
@@ -2470,6 +2472,7 @@ _PyThread_CurrentExceptions(void)
      * Because these lists can mutate even when the GIL is held, we
      * need to grab head_mutex for the duration.
      */
+    _PyEval_StopTheWorldAll(runtime);
     HEAD_LOCK(runtime);
     PyInterpreterState *i;
     for (i = runtime->interpreters.head; i != NULL; i = i->next) {
@@ -2502,6 +2505,7 @@ _PyThread_CurrentExceptions(void)
 
 done:
     HEAD_UNLOCK(runtime);
+    _PyEval_StartTheWorldAll(runtime);
     return result;
 }