@@ -67,6 +67,10 @@ _Py_ThreadCanHandleSignals(PyInterpreterState *interp)
 extern _Py_thread_local PyThreadState *_Py_tss_tstate;
 #endif
 
+#ifndef NDEBUG
+extern int _PyThreadState_CheckConsistency(PyThreadState *tstate);
+#endif
+
 // Export for most shared extensions, used via _PyThreadState_GET() static
 // inline function.
 PyAPI_FUNC(PyThreadState *) _PyThreadState_GetCurrent(void);

diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
@@ -1074,9 +1074,12 @@ static void
 thread_run(void *boot_raw)
 {
     struct bootstate *boot = (struct bootstate *) boot_raw;
-    PyThreadState *tstate;
+    PyThreadState *tstate = boot->tstate;
+
+    // gh-104690: If Python is being finalized and PyInterpreterState_Delete()
+    // was called, tstate becomes a dangling pointer.
+    assert(_PyThreadState_CheckConsistency(tstate));
 
-    tstate = boot->tstate;
     _PyThreadState_Bind(tstate);
     PyEval_AcquireThread(tstate);
     tstate->interp->threads.count++;

@@ -163,16 +163,6 @@ UNSIGNAL_ASYNC_EXC(PyInterpreterState *interp)
     COMPUTE_EVAL_BREAKER(interp, ceval, ceval2);
 }
 
-#ifndef NDEBUG
-/* Ensure that tstate is valid */
-static int
-is_tstate_valid(PyThreadState *tstate)
-{
-    assert(!_PyMem_IsPtrFreed(tstate));
-    assert(!_PyMem_IsPtrFreed(tstate->interp));
-    return 1;
-}
-#endif
 
 /*
  * Implementation of the Global Interpreter Lock (GIL).
@@ -325,7 +315,7 @@ drop_gil(struct _ceval_state *ceval, PyThreadState *tstate)
         /* Not switched yet => wait */
         if (((PyThreadState*)_Py_atomic_load_relaxed(&gil->last_holder)) == tstate)
         {
-            assert(is_tstate_valid(tstate));
+            assert(_PyThreadState_CheckConsistency(tstate));
             RESET_GIL_DROP_REQUEST(tstate->interp);
             /* NOTE: if COND_WAIT does not atomically start waiting when
                releasing the mutex, another thread can run through, take
@@ -386,7 +376,7 @@ take_gil(PyThreadState *tstate)
         PyThread_exit_thread();
     }
 
-    assert(is_tstate_valid(tstate));
+    assert(_PyThreadState_CheckConsistency(tstate));
     PyInterpreterState *interp = tstate->interp;
     struct _ceval_state *ceval = &interp->ceval;
     struct _gil_runtime_state *gil = ceval->gil;
@@ -427,7 +417,7 @@ take_gil(PyThreadState *tstate)
                 }
                 PyThread_exit_thread();
             }
-            assert(is_tstate_valid(tstate));
+            assert(_PyThreadState_CheckConsistency(tstate));
 
             SET_GIL_DROP_REQUEST(interp);
             drop_requested = 1;
@@ -466,7 +456,7 @@ take_gil(PyThreadState *tstate)
         drop_gil(ceval, tstate);
         PyThread_exit_thread();
     }
-    assert(is_tstate_valid(tstate));
+    assert(_PyThreadState_CheckConsistency(tstate));
 
     if (_Py_atomic_load_relaxed(&ceval->gil_drop_request)) {
         RESET_GIL_DROP_REQUEST(interp);
@@ -679,7 +669,7 @@ PyEval_AcquireThread(PyThreadState *tstate)
 void
 PyEval_ReleaseThread(PyThreadState *tstate)
 {
-    assert(is_tstate_valid(tstate));
+    assert(_PyThreadState_CheckConsistency(tstate));
 
     PyThreadState *new_tstate = _PyThreadState_SwapNoGIL(NULL);
     if (new_tstate != tstate) {
@@ -877,7 +867,7 @@ Py_AddPendingCall(int (*func)(void *), void *arg)
 static int
 handle_signals(PyThreadState *tstate)
 {
-    assert(is_tstate_valid(tstate));
+    assert(_PyThreadState_CheckConsistency(tstate));
     if (!_Py_ThreadCanHandleSignals(tstate->interp)) {
         return 0;
     }
@@ -983,7 +973,7 @@ void
 _Py_FinishPendingCalls(PyThreadState *tstate)
 {
     assert(PyGILState_Check());
-    assert(is_tstate_valid(tstate));
+    assert(_PyThreadState_CheckConsistency(tstate));
 
     if (make_pending_calls(tstate->interp) < 0) {
         PyObject *exc = _PyErr_GetRaisedException(tstate);
@@ -1024,7 +1014,7 @@ Py_MakePendingCalls(void)
     assert(PyGILState_Check());
 
     PyThreadState *tstate = _PyThreadState_GET();
-    assert(is_tstate_valid(tstate));
+    assert(_PyThreadState_CheckConsistency(tstate));
 
     /* Only execute pending calls on the main thread. */
     if (!_Py_IsMainThread() || !_Py_IsMainInterpreter(tstate->interp)) {

@@ -2889,6 +2889,24 @@ _PyThreadState_PopFrame(PyThreadState *tstate, _PyInterpreterFrame * frame)
 }
 
 
+#ifndef NDEBUG
+// Check that a Python thread state valid. In practice, this function is used
+// on a Python debug build to check if 'tstate' is a dangling pointer, if the
+// PyThreadState memory has been freed.
+//
+// Usage:
+//
+//     assert(_PyThreadState_CheckConsistency(tstate));
+int
+_PyThreadState_CheckConsistency(PyThreadState *tstate)
+{
+    assert(!_PyMem_IsPtrFreed(tstate));
+    assert(!_PyMem_IsPtrFreed(tstate->interp));
+    return 1;
+}
+#endif
+
+
 #ifdef __cplusplus
 }
 #endif