gh-154701: prevent executor self-links in JIT cold exits

Include/internal/pycore_optimizer.h

@@ -206,7 +206,7 @@ typedef struct _PyExecutorObject {
 PyAPI_FUNC(_PyExecutorObject*) _Py_GetExecutor(PyCodeObject *code, int offset);
 
 int _Py_ExecutorInit(_PyExecutorObject *, const _PyBloomFilter *);
-void _Py_ExecutorDetach(_PyExecutorObject *);
+PyAPI_FUNC(void) _Py_ExecutorDetach(_PyExecutorObject *);
 PyAPI_FUNC(void) _Py_Executor_DependsOn(_PyExecutorObject *executor, void *obj);
 
 /* We use a bloomfilter with k = 6, m = 256

Lib/test/test_capi/test_opt.py

@@ -12,7 +12,7 @@
 
 from test.support import (script_helper, requires_specialization,
  import_helper, Py_GIL_DISABLED, requires_jit_enabled,
- reset_code)
+ reset_code, SHORT_TIMEOUT, isolation)
 
 _testinternalcapi = import_helper.import_module("_testinternalcapi")
 
@@ -6225,6 +6225,28 @@ def __exit__(self, e, v, t): ...
  f1()
  """), PYTHON_JIT="1")
 
+ @isolation.runInSubprocess(timeout=SHORT_TIMEOUT)
+ def test_for_iter_side_exit_does_not_self_link(self):
+ def exhaust(iterator):
+ for _ in iterator:
+ pass
+
+ values = range(TIER2_THRESHOLD)
+ # After the initial trace, MAX_CHAIN_DEPTH side exits cause the final
+ # executor to be installed at FOR_ITER.
+ warmup_iterators = (
+ iter(set(values)),
+ iter(dict.fromkeys(values)),
+ iter(values),
+ enumerate(values),
+ zip(values, values),
+ )
+ for iterator in warmup_iterators:
+ exhaust(iterator)
+
+ # A different iterator type must not link that executor to itself.
+ exhaust(map(bool, values))
+
 def global_identity(x):
  return x
 

Misc/NEWS.d/next/Core_and_Builtins/2026-08-07-10-14-56.gh-issue-154701.zulh2S.rst

@@ -0,0 +1 @@
+Fix an infinite loop in JIT when a ``FOR_ITER`` side exit links an executor back to itself.

Python/bytecodes.c

@@ -6262,6 +6262,10 @@ dummy_func(
  if (target->op.code == ENTER_EXECUTOR) {
  PyCodeObject *code = _PyFrame_GetCode(frame);
  executor = code->co_executors->executors[target->op.arg];
+ if (executor == _PyExecutor_FromExit(exit)) {
+ _Py_ExecutorDetach(executor);
+ GOTO_TIER_ONE(target);
+ }
  Py_INCREF(executor);
  assert(tstate->jit_exit == exit);
  exit->executor = executor;