py/compile: Fix async for's stack handling of iterator expression.

Prior to this fix, async for assumed the iterator expression was a simple
identifier, and used that identifier as a local to store the intermediate
iterator object.  This is incorrect behaviour.

This commit fixes the issue by keeping the iterator object on the stack as
an anonymous local variable.

Fixes issue #11511.

Signed-off-by: Damien George <damien@micropython.org>

py/compile.c

@@ -1768,18 +1768,21 @@ STATIC void compile_await_object_method(compiler_t *comp, qstr method) {
 }
 
 STATIC void compile_async_for_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
-    // comp->break_label |= MP_EMIT_BREAK_FROM_FOR;
-
-    qstr context = MP_PARSE_NODE_LEAF_ARG(pns->nodes[1]);
+    // Allocate labels.
     uint while_else_label = comp_next_label(comp);
     uint try_exception_label = comp_next_label(comp);
     uint try_else_label = comp_next_label(comp);
     uint try_finally_label = comp_next_label(comp);
 
+    // Stack: (...)
+
+    // Compile the iterator expression and load and call its __aiter__ method.
     compile_node(comp, pns->nodes[1]); // iterator
+    // Stack: (..., iterator)
     EMIT_ARG(load_method, MP_QSTR___aiter__, false);
+    // Stack: (..., iterator, __aiter__)
     EMIT_ARG(call_method, 0, 0, 0);
-    compile_store_id(comp, context);
+    // Stack: (..., iterable)
 
     START_BREAK_CONTINUE_BLOCK
 
@@ -1787,9 +1790,15 @@ STATIC void compile_async_for_stmt(compiler_t *comp, mp_parse_node_struct_t *pns
 
     compile_increase_except_level(comp, try_exception_label, MP_EMIT_SETUP_BLOCK_EXCEPT);
 
-    compile_load_id(comp, context);
+    EMIT(dup_top);
+    // Stack: (..., iterable, iterable)
+
+    // Compile: yield from iterable.__anext__()
     compile_await_object_method(comp, MP_QSTR___anext__);
+    // Stack: (..., iterable, yielded_value)
+
     c_assign(comp, pns->nodes[0], ASSIGN_STORE); // variable
+    // Stack: (..., iterable)
     EMIT_ARG(pop_except_jump, try_else_label, false);
 
     EMIT_ARG(label_assign, try_exception_label);
@@ -1806,6 +1815,8 @@ STATIC void compile_async_for_stmt(compiler_t *comp, mp_parse_node_struct_t *pns
     compile_decrease_except_level(comp);
     EMIT(end_except_handler);
 
+    // Stack: (..., iterable)
+
     EMIT_ARG(label_assign, try_else_label);
     compile_node(comp, pns->nodes[2]); // body
 
@@ -1817,6 +1828,10 @@ STATIC void compile_async_for_stmt(compiler_t *comp, mp_parse_node_struct_t *pns
     compile_node(comp, pns->nodes[3]); // else
 
     EMIT_ARG(label_assign, break_label);
+    // Stack: (..., iterable)
+
+    EMIT(pop_top);
+    // Stack: (...)
 }
 
 STATIC void compile_async_with_stmt_helper(compiler_t *comp, size_t n, mp_parse_node_t *nodes, mp_parse_node_t body) {

tests/basics/async_for.py

@@ -1,29 +1,75 @@
 # test basic async for execution
 # example taken from PEP0492
 
+
 class AsyncIteratorWrapper:
     def __init__(self, obj):
-        print('init')
-        self._it = iter(obj)
+        print("init")
+        self._obj = obj
+
+    def __repr__(self):
+        return "AsyncIteratorWrapper-" + self._obj
 
     def __aiter__(self):
-        print('aiter')
-        return self
+        print("aiter")
+        return AsyncIteratorWrapperIterator(self._obj)
+
+
+class AsyncIteratorWrapperIterator:
+    def __init__(self, obj):
+        print("init")
+        self._it = iter(obj)
 
     async def __anext__(self):
-        print('anext')
+        print("anext")
         try:
             value = next(self._it)
         except StopIteration:
             raise StopAsyncIteration
         return value
 
-async def coro():
-    async for letter in AsyncIteratorWrapper('abc'):
+
+def run_coro(c):
+    print("== start ==")
+    try:
+        c.send(None)
+    except StopIteration:
+        print("== finish ==")
+
+
+async def coro0():
+    async for letter in AsyncIteratorWrapper("abc"):
         print(letter)
 
-o = coro()
-try:
-    o.send(None)
-except StopIteration:
-    print('finished')
+
+run_coro(coro0())
+
+
+async def coro1():
+    a = AsyncIteratorWrapper("def")
+    async for letter in a:
+        print(letter)
+    print(a)
+
+
+run_coro(coro1())
+
+a_global = AsyncIteratorWrapper("ghi")
+
+
+async def coro2():
+    async for letter in a_global:
+        print(letter)
+    print(a_global)
+
+
+run_coro(coro2())
+
+
+async def coro3(a):
+    async for letter in a:
+        print(letter)
+    print(a)
+
+
+run_coro(coro3(AsyncIteratorWrapper("jkl")))

tests/basics/async_for.py.exp

@@ -1,5 +1,7 @@
+== start ==
 init
 aiter
+init
 anext
 a
 anext
@@ -7,4 +9,43 @@ b
 anext
 c
 anext
-finished
+== finish ==
+== start ==
+init
+aiter
+init
+anext
+d
+anext
+e
+anext
+f
+anext
+AsyncIteratorWrapper-def
+== finish ==
+init
+== start ==
+aiter
+init
+anext
+g
+anext
+h
+anext
+i
+anext
+AsyncIteratorWrapper-ghi
+== finish ==
+init
+== start ==
+aiter
+init
+anext
+j
+anext
+k
+anext
+l
+anext
+AsyncIteratorWrapper-jkl
+== finish ==