gh-108987: Fix _thread.start_new_thread() race condition (#109135)

Fix _thread.start_new_thread() race condition. If a thread is created
during Python finalization, the newly spawned thread now exits
immediately instead of trying to access freed memory and lead to a
crash.

thread_run() calls PyEval_AcquireThread() which checks if the thread
must exit. The problem was that tstate was dereferenced earlier in
_PyThreadState_Bind() which leads to a crash most of the time.

Move _PyThreadState_CheckConsistency() from thread_run() to
_PyThreadState_Bind().

1 files changed, 29 insertions, 0 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index 09c3538..b5c4fd7 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1907,6 +1907,10 @@ PyThreadState_Swap(PyThreadState *newts)
 void
 _PyThreadState_Bind(PyThreadState *tstate)
 {
+    // gh-104690: If Python is being finalized and PyInterpreterState_Delete()
+    // was called, tstate becomes a dangling pointer.
+    assert(_PyThreadState_CheckConsistency(tstate));
+
     bind_tstate(tstate);
     // This makes sure there's a gilstate tstate bound
     // as soon as possible.
@@ -2908,6 +2912,31 @@ _PyThreadState_CheckConsistency(PyThreadState *tstate)
 #endif
 
 
+// Check if a Python thread must exit immediately, rather than taking the GIL
+// if Py_Finalize() has been called.
+//
+// When this function is called by a daemon thread after Py_Finalize() has been
+// called, the GIL does no longer exist.
+//
+// tstate can be a dangling pointer (point to freed memory): only tstate value
+// is used, the pointer is not deferenced.
+//
+// tstate must be non-NULL.
+int
+_PyThreadState_MustExit(PyThreadState *tstate)
+{
+    /* bpo-39877: Access _PyRuntime directly rather than using
+       tstate->interp->runtime to support calls from Python daemon threads.
+       After Py_Finalize() has been called, tstate can be a dangling pointer:
+       point to PyThreadState freed memory. */
+    PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(&_PyRuntime);
+    if (finalizing == NULL) {
+        finalizing = _PyInterpreterState_GetFinalizing(tstate->interp);
+    }
+    return (finalizing != NULL && finalizing != tstate);
+}
+
+
 #ifdef __cplusplus
 }
 #endif