gh-87135: Raise PythonFinalizationError when joining a blocked daemon thread (gh-130402)

If `Py_IsFinalizing()` is true, non-daemon threads (other than the current one)
are done, and daemon threads are prevented from running, so they
cannot finalize themselves and become done. Joining them (without timeout)
would block forever.

Raise PythonFinalizationError instead of hanging.

Raise even when a timeout is given, for consistency with trying to join your own thread.

See gh-123940 for a use case: calling `join()` from `__del__`. This is
ill-advised, but an exception should at least make it easier to diagnose.

1 files changed, 15 insertions, 5 deletions

diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index 6967f7ef42f..9776a32755d 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -511,11 +511,21 @@ ThreadHandle_join(ThreadHandle *self, PyTime_t timeout_ns)
     // To work around this, we set `thread_is_exiting` immediately before
     // `thread_run` returns.  We can be sure that we are not attempting to join
     // ourselves if the handle's thread is about to exit.
-    if (!_PyEvent_IsSet(&self->thread_is_exiting) &&
-        ThreadHandle_ident(self) == PyThread_get_thread_ident_ex()) {
-        // PyThread_join_thread() would deadlock or error out.
-        PyErr_SetString(ThreadError, "Cannot join current thread");
-        return -1;
+    if (!_PyEvent_IsSet(&self->thread_is_exiting)) {
+        if (ThreadHandle_ident(self) == PyThread_get_thread_ident_ex()) {
+            // PyThread_join_thread() would deadlock or error out.
+            PyErr_SetString(ThreadError, "Cannot join current thread");
+            return -1;
+        }
+        if (Py_IsFinalizing()) {
+            // gh-123940: On finalization, other threads are prevented from
+            // running Python code. They cannot finalize themselves,
+            // so join() would hang forever (or until timeout).
+            // We raise instead.
+            PyErr_SetString(PyExc_PythonFinalizationError,
+                            "cannot join thread at interpreter shutdown");
+            return -1;
+        }
     }
 
     // Wait until the deadline for the thread to exit.