gh-118727: Don't drop the GIL in `drop_gil()` unless the current thread holds it (#118745)

`drop_gil()` assumes that its caller is attached, which means that the current
thread holds the GIL if and only if the GIL is enabled, and the enabled-state
of the GIL won't change. This isn't true, though, because `detach_thread()`
calls `_PyEval_ReleaseLock()` after detaching and
`_PyThreadState_DeleteCurrent()` calls it after removing the current thread
from consideration for stop-the-world requests (effectively detaching it).

Fix this by remembering whether or not a thread acquired the GIL when it last
attached, in `PyThreadState._status.holds_gil`, and check this in `drop_gil()`
instead of `gil->enabled`.

This fixes a crash in `test_multiprocessing_pool_circular_import()`, so I've
reenabled it.

1 files changed, 4 insertions, 7 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index 0832b37..1ea1ad9 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1843,7 +1843,7 @@ _PyThreadState_DeleteCurrent(PyThreadState *tstate)
 #endif
     current_fast_clear(tstate->interp->runtime);
     tstate_delete_common(tstate);
-    _PyEval_ReleaseLock(tstate->interp, NULL);
+    _PyEval_ReleaseLock(tstate->interp, tstate, 1);
     free_threadstate((_PyThreadStateImpl *)tstate);
 }
 
@@ -2068,7 +2068,7 @@ _PyThreadState_Attach(PyThreadState *tstate)
 
 
     while (1) {
-        int acquired_gil = _PyEval_AcquireLock(tstate);
+        _PyEval_AcquireLock(tstate);
 
         // XXX assert(tstate_is_alive(tstate));
         current_fast_set(&_PyRuntime, tstate);
@@ -2079,20 +2079,17 @@ _PyThreadState_Attach(PyThreadState *tstate)
         }
 
 #ifdef Py_GIL_DISABLED
-        if (_PyEval_IsGILEnabled(tstate) != acquired_gil) {
+        if (_PyEval_IsGILEnabled(tstate) && !tstate->_status.holds_gil) {
             // The GIL was enabled between our call to _PyEval_AcquireLock()
             // and when we attached (the GIL can't go from enabled to disabled
             // here because only a thread holding the GIL can disable
             // it). Detach and try again.
-            assert(!acquired_gil);
             tstate_set_detached(tstate, _Py_THREAD_DETACHED);
             tstate_deactivate(tstate);
             current_fast_clear(&_PyRuntime);
             continue;
         }
         _Py_qsbr_attach(((_PyThreadStateImpl *)tstate)->qsbr);
-#else
-        (void)acquired_gil;
 #endif
         break;
     }
@@ -2123,7 +2120,7 @@ detach_thread(PyThreadState *tstate, int detached_state)
     tstate_deactivate(tstate);
     tstate_set_detached(tstate, detached_state);
     current_fast_clear(&_PyRuntime);
-    _PyEval_ReleaseLock(tstate->interp, tstate);
+    _PyEval_ReleaseLock(tstate->interp, tstate, 0);
 }
 
 void