Address review comments: add CPython issue ref, trim test docstring

Author: arr2036

sdks/python/apache_beam/runners/worker/sdk_worker.py

@@ -1457,7 +1457,8 @@ def get(self, timeout=None):
     # List comprehension, not generator: *(gen) causes CPython to build the
     # argument tuple incrementally via _PyTuple_Resize, which asserts
     # Py_REFCNT(v)==1. A GC cycle between yields can increment that refcount,
-    # raising SystemError (Objects/tupleobject.c:927). *[list] allocates the
+    # raising SystemError (Objects/tupleobject.c:927). See cpython/issues/127058
+    # (fixed in 3.14: cpython@5a23994). *[list] allocates the
     # tuple once at its final size, avoiding the resize entirely.
     return self._func(*[arg.get(timeout) for arg in self._args])
 

sdks/python/apache_beam/runners/worker/sdk_worker_test.py

@@ -705,31 +705,7 @@ def testShortIdAssignment(self):
 
 
 class DeferredCallTest(unittest.TestCase):
-  """Tests for _DeferredCall.get().
-
-  Background: the original implementation used a generator expression in the
-  argument unpack position:
-
-      return self._func(*(arg.get(timeout) for arg in self._args))
-
-  CPython builds the argument tuple incrementally via _PyTuple_Resize as it
-  drains the generator.  _PyTuple_Resize guards that Py_REFCNT(v) == 1 before
-  resizing a non-empty tuple (Objects/tupleobject.c).  Under sustained load,
-  a GC cycle can run between generator yields and temporarily increment the
-  refcount on the partially-built tuple, causing _PyTuple_Resize to call
-  PyErr_BadInternalCall() and raise SystemError.  This was observed in
-  production workers (Python 3.11, Beam 2.73.0) at
-  Objects/tupleobject.c:927.
-
-  The partial tuple is a C-level local inside PySequence_Tuple, so the race
-  cannot be triggered deterministically from pure Python.  The tests here
-  verify correct behaviour; the crash itself requires CPython internal timing
-  or a debug build to reproduce reliably.
-
-  Fix: change the generator to a list comprehension.  CPython builds the list
-  first and passes it to CALL_FUNCTION_EX, which calls PySequence_Fast on a
-  list (a no-op path that does not call _PyTuple_Resize).
-  """
+  """Tests for _DeferredCall.get()."""
   def test_get_single_arg(self):
     f = sdk_worker._Future().set(42)
     call = sdk_worker._DeferredCall(lambda x: x, f)