bpo-33041: Rework compiling an "async for" loop. (#6142)

* Added new opcode END_ASYNC_FOR.
* Setting global StopAsyncIteration no longer breaks "async for" loops.
* Jumping into an "async for" loop is now disabled.
* Jumping out of an "async for" loop no longer corrupts the stack.
* Simplify the compiler.

2 files changed, 15 insertions, 0 deletions

diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
index 47f226b..8f505e6 100644
--- a/Doc/library/dis.rst
+++ b/Doc/library/dis.rst
@@ -588,6 +588,17 @@ the original TOS1.
    .. versionadded:: 3.5
 
 
+.. opcode:: END_ASYNC_FOR
+
+   Terminates an :keyword:`async for` loop.  Handles an exception raised
+   when awaiting a next item.  If TOS is :exc:`StopAsyncIteration` pop 7
+   values from the stack and restore the exception state using the second
+   three of them.  Otherwise re-raise the exception using the three values
+   from the stack.  An exception handler block is removed from the block stack.
+
+   .. versionadded:: 3.8
+
+
 .. opcode:: BEFORE_ASYNC_WITH
 
    Resolves ``__aenter__`` and ``__aexit__`` from the object on top of the
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index fcc868b..4e6c851 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -157,3 +157,7 @@ CPython bytecode changes
 
   (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in
   :issue:`17611`.)
+
+* Added new opcode :opcode:`END_ASYNC_FOR` for handling exceptions raised
+  when awaiting a next item in an :keyword:`async for` loop.
+  (Contributed by Serhiy Storchaka in :issue:`33041`.)