[3.15] gh-161366: Fix deadlock with FT GC debug collectables print (G…

Lib/test/test_free_threading/test_gc.py

@@ -9,6 +9,7 @@
 
 from test import support
 from test.support import threading_helper
+from test.support.script_helper import assert_python_ok
 
 
 class MyObj:
@@ -118,6 +119,35 @@ def mutate():
             b.wait()
             time.sleep(0.2)
 
+    def test_debug_collectable_python_stderr(self):
+        # The DEBUG_COLLECTABLE output is written with sys.stderr.write(),
+        # which may run Python code or wait for another thread (e.g. for the
+        # lock of a buffered stream), so it must not be written while the
+        # world is stopped.  Debug builds assert that the world is not stopped
+        # when Python code runs, so use a sys.stderr written in Python.
+        code = """if 1:
+            import gc
+            import io
+            import sys
+
+            class Stderr(io.StringIO):
+                def write(self, s):
+                    return super().write(s)
+
+            sys.stderr = Stderr()
+            gc.set_debug(gc.DEBUG_COLLECTABLE)
+            a = []
+            a.append(a)
+            del a
+            gc.collect()
+            gc.set_debug(0)
+            output = sys.stderr.getvalue()
+            sys.stderr = sys.__stderr__
+            print(output)
+            """
+        _, out, _ = assert_python_ok("-c", code)
+        self.assertRegex(out, b"gc: collectable <list 0x[0-9A-Fa-f]+>")
+
     def test_merge_brc_queue_of_detached_thread(self):
         # GH-157838: objects queued for merging by a thread that is detached
         # (blocked in a lock acquire, sleep, etc.) are merged and freed on its

Misc/NEWS.d/next/Core_and_Builtins/2026-11-05-21-00-57.gh-issue-161366.ouxAvv.rst

@@ -0,0 +1,4 @@
+Fix a deadlock in the garbage collector of the :term:`free-threaded build`
+when :data:`gc.DEBUG_COLLECTABLE` is set and another thread is writing to
+:data:`sys.stderr`. The debug output is now written after the other threads
+are resumed.

Python/gc_free_threading.c

@@ -2137,18 +2137,22 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
     gc_visit_heaps(interp, &validate_alive_bits, &state->base);
 #endif
 
+    // The debug output is written with sys.stderr.write(), which may run
+    // Python code, so it is printed below after the world is started.
+    bool debug_print = (interp->gc.debug & _PyGC_DEBUG_COLLECTABLE) != 0;
+
+    // Find weakref callbacks we will honor (but do not call them).
+    find_weakref_callbacks(state);
+    _PyEval_StartTheWorld(interp);
+
     // Print debugging information.
-    if (interp->gc.debug & _PyGC_DEBUG_COLLECTABLE) {
+    if (debug_print) {
         PyObject *op;
         WORKSTACK_FOR_EACH(&state->unreachable, op) {
             debug_cycle("collectable", op);
         }
     }
 
-    // Find weakref callbacks we will honor (but do not call them).
-    find_weakref_callbacks(state);
-    _PyEval_StartTheWorld(interp);
-
     // Deallocate any object from the refcount merge step
     cleanup_worklist(&state->objs_to_decref);