Fix itertools.pairwise (RustPython#5884)

* Fix pairwise hangs

* Don't skip failing tests;)

* Drop lock of read, not write

Author: ShaharNaveh

Lib/test/test_itertools.py

@@ -1182,8 +1182,6 @@ def test_pairwise(self):
         with self.assertRaises(TypeError):
             pairwise(None)                                  # non-iterable argument
 
-    # TODO: RUSTPYTHON
-    @unittest.skip("TODO: RUSTPYTHON, hangs")
     def test_pairwise_reenter(self):
         def check(reenter_at, expected):
             class I:
@@ -1234,8 +1232,6 @@ def __next__(self):
             ([5], [6]),
         ])
 
-    # TODO: RUSTPYTHON
-    @unittest.skip("TODO: RUSTPYTHON, hangs")
     def test_pairwise_reenter2(self):
         def check(maxcount, expected):
             class I:

vm/src/stdlib/itertools.rs

@@ -1943,7 +1943,7 @@ mod decl {
         type Args = PyIter;
 
         fn py_new(cls: PyTypeRef, iterator: Self::Args, vm: &VirtualMachine) -> PyResult {
-            PyItertoolsPairwise {
+            Self {
                 iterator,
                 old: PyRwLock::new(None),
             }
@@ -1959,18 +1959,29 @@ mod decl {
 
     impl IterNext for PyItertoolsPairwise {
         fn next(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyIterReturn> {
-            let old = match zelf.old.read().clone() {
+            let old_clone = {
+                let guard = zelf.old.read();
+                guard.clone()
+            };
+            let old = match old_clone {
                 None => match zelf.iterator.next(vm)? {
-                    PyIterReturn::Return(obj) => obj,
+                    PyIterReturn::Return(obj) => {
+                        // Needed for when we reenter
+                        *zelf.old.write() = Some(obj.clone());
+                        obj
+                    }
                     PyIterReturn::StopIteration(v) => return Ok(PyIterReturn::StopIteration(v)),
                 },
                 Some(obj) => obj,
             };
+
             let new = match zelf.iterator.next(vm)? {
                 PyIterReturn::Return(obj) => obj,
                 PyIterReturn::StopIteration(v) => return Ok(PyIterReturn::StopIteration(v)),
             };
+
             *zelf.old.write() = Some(new.clone());
+
             Ok(PyIterReturn::Return(vm.new_tuple((old, new)).into()))
         }
     }