gh-159739: Keep SimpleQueue valid after buffer allocation fails (#160165)

If allocating a SimpleQueue buffer fails, the partially constructed object reports nonzero capacity despite having no storage. A subclass finalizer that calls put() then writes through a NULL pointer and crashes; publish the capacity only after allocation succeeds. Backport to 3.15 and 3.14.

Author: jelle-openai

Misc/NEWS.d/next/Library/2026-10-21-19-55-00.gh-issue-159739.sque01.rst

@@ -0,0 +1,2 @@
+Avoid a crash when a :class:`queue.SimpleQueue` subclass finalizer adds an
+item after construction fails.

Modules/_queuemodule.c

@@ -51,13 +51,14 @@ RingBuf_Init(RingBuf *buf)
 {
     buf->put_idx = 0;
     buf->get_idx = 0;
-    buf->items_cap = INITIAL_RING_BUF_CAPACITY;
+    buf->items_cap = 0;
     buf->num_items = 0;
-    buf->items = PyMem_Calloc(buf->items_cap, sizeof(PyObject *));
+    buf->items = PyMem_Calloc(INITIAL_RING_BUF_CAPACITY, sizeof(PyObject *));
     if (buf->items == NULL) {
         PyErr_NoMemory();
         return -1;
     }
+    buf->items_cap = INITIAL_RING_BUF_CAPACITY;
     return 0;
 }