gh-117300: Use stop the world to make `sys._current_frames` and `sys._current_exceptions` thread-safe. (#117301)

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.

2 files changed, 6 insertions, 1 deletions

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 37c16cd..f6f23b0 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -562,7 +562,8 @@ class SysModuleTest(unittest.TestCase):
             # 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()
diff --git a/Python/pystate.c b/Python/pystate.c
index 8489f53..8bec727 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -2408,6 +2408,7 @@ _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) {
@@ -2441,6 +2442,7 @@ fail:
 
 done:
     HEAD_UNLOCK(runtime);
+    _PyEval_StartTheWorldAll(runtime);
     return result;
 }
 
@@ -2472,6 +2474,7 @@ _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) {
@@ -2504,6 +2507,7 @@ fail:
 
 done:
     HEAD_UNLOCK(runtime);
+    _PyEval_StartTheWorldAll(runtime);
     return result;
 }