__length_hint__ (#6636)

* Initial plan

* fix: prevent iterator length_hint deadlock

Co-authored-by: youknowone <69878+youknowone@users.noreply.github.com>

* fix

* Auto-format: cargo fmt --all

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

crates/vm/src/builtins/iter.rs

@@ -184,17 +184,27 @@ impl PySequenceIterator {
     }
 
     #[pymethod]
-    fn __length_hint__(&self, vm: &VirtualMachine) -> PyObjectRef {
-        let internal = self.internal.lock();
-        if let IterStatus::Active(obj) = &internal.status {
-            let seq = obj.sequence_unchecked();
-            seq.length(vm).map_or_else(
-                |_| vm.ctx.not_implemented(),
-                |x| PyInt::from(x).into_pyobject(vm),
-            )
-        } else {
-            PyInt::from(0).into_pyobject(vm)
-        }
+    fn __length_hint__(&self, vm: &VirtualMachine) -> PyResult<PyObjectRef> {
+        vm.with_recursion("in __length_hint__", || {
+            let (obj, position) = {
+                let internal = self.internal.lock();
+                match &internal.status {
+                    IterStatus::Active(obj) => (Some(obj.clone()), internal.position),
+                    IterStatus::Exhausted => (None, 0),
+                }
+            };
+            if let Some(obj) = obj {
+                let seq = obj.sequence_unchecked();
+                match seq.length_opt(vm) {
+                    Some(len) => {
+                        len.map(|len| PyInt::from(len.saturating_sub(position)).into_pyobject(vm))
+                    }
+                    None => Ok(vm.ctx.not_implemented()),
+                }
+            } else {
+                Ok(PyInt::from(0).into_pyobject(vm))
+            }
+        })
     }
 
     #[pymethod]

extra_tests/snippets/builtin_iter.py

@@ -0,0 +1,71 @@
+import queue
+import threading
+
+
+def make_iterator():
+    holder = {}
+
+    class Evil:
+        def __getitem__(self, index):
+            if index == 0:
+                return 0
+            raise IndexError
+
+        def __len__(self):
+            return holder["it"].__length_hint__()
+
+    obj = Evil()
+    holder["it"] = iter(obj)
+    return holder["it"]
+
+
+it = make_iterator()
+q = queue.Queue()
+
+
+def run():
+    try:
+        it.__length_hint__()
+    except Exception as exc:  # noqa: BLE001
+        q.put(exc)
+    else:
+        q.put(None)
+
+
+t = threading.Thread(target=run, daemon=True)
+t.start()
+t.join(1)
+
+assert not t.is_alive(), "iterator.__length_hint__ deadlocked"
+err = q.get_nowait()
+assert isinstance(err, RecursionError)
+
+
+class NoLen:
+    def __getitem__(self, index):
+        if index < 3:
+            return index
+        raise IndexError
+
+
+no_len_it = iter(NoLen())
+assert no_len_it.__length_hint__() is NotImplemented
+next(no_len_it)
+assert no_len_it.__length_hint__() is NotImplemented
+
+
+class Seq:
+    def __init__(self):
+        self.items = [1, 2, 3]
+
+    def __getitem__(self, index):
+        return self.items[index]
+
+    def __len__(self):
+        return len(self.items)
+
+
+seq_it = iter(Seq())
+assert seq_it.__length_hint__() == 3
+next(seq_it)
+assert seq_it.__length_hint__() == 2