python: backport 'Use @asynciocontextmanager'

This removes a non-idiomatic use of a "coroutine callback" in favor of
something a bit more standardized.

Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@commit 97f7ffa3be17a50544b52767d14b6fd478c07b9e
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

python/qemu/qmp/protocol.py

@@ -15,6 +15,7 @@ class.
 
 import asyncio
 from asyncio import StreamReader, StreamWriter
+from contextlib import asynccontextmanager
 from enum import Enum
 from functools import wraps
 import logging
@@ -22,6 +23,7 @@ import socket
 from ssl import SSLContext
 from typing import (
     Any,
+    AsyncGenerator,
     Awaitable,
     Callable,
     Generic,
@@ -337,9 +339,8 @@ class AsyncProtocol(Generic[T]):
             This exception will wrap a more concrete one. In most cases,
             the wrapped exception will be `OSError`.
         """
-        await self._session_guard(
-            self._do_start_server(address, ssl),
-            'Failed to establish connection')
+        async with self._session_guard('Failed to establish connection'):
+            await self._do_start_server(address, ssl)
         assert self.runstate == Runstate.CONNECTING
 
     @upper_half
@@ -362,12 +363,10 @@ class AsyncProtocol(Generic[T]):
         """
         if self._accepted is None:
             raise QMPError("Cannot call accept() before start_server().")
-        await self._session_guard(
-            self._do_accept(),
-            'Failed to establish connection')
-        await self._session_guard(
-            self._establish_session(),
-            'Failed to establish session')
+        async with self._session_guard('Failed to establish connection'):
+            await self._do_accept()
+        async with self._session_guard('Failed to establish session'):
+            await self._establish_session()
         assert self.runstate == Runstate.RUNNING
 
     @upper_half
@@ -392,12 +391,10 @@ class AsyncProtocol(Generic[T]):
             protocol-level failure occurs while establishing a new
             session, the wrapped error may also be an `QMPError`.
         """
-        await self._session_guard(
-            self._do_connect(address, ssl),
-            'Failed to establish connection')
-        await self._session_guard(
-            self._establish_session(),
-            'Failed to establish session')
+        async with self._session_guard('Failed to establish connection'):
+            await self._do_connect(address, ssl)
+        async with self._session_guard('Failed to establish session'):
+            await self._establish_session()
         assert self.runstate == Runstate.RUNNING
 
     @upper_half
@@ -418,7 +415,8 @@ class AsyncProtocol(Generic[T]):
     # Section: Session machinery
     # --------------------------
 
-    async def _session_guard(self, coro: Awaitable[None], emsg: str) -> None:
+    @asynccontextmanager
+    async def _session_guard(self, emsg: str) -> AsyncGenerator[None, None]:
         """
         Async guard function used to roll back to `IDLE` on any error.
 
@@ -435,10 +433,9 @@ class AsyncProtocol(Generic[T]):
         :raise ConnectError:
             When any other error is encountered in the guarded block.
         """
-        # Note: After Python 3.6 support is removed, this should be an
-        # @asynccontextmanager instead of accepting a callback.
         try:
-            await coro
+            # Caller's code runs here.
+            yield
         except BaseException as err:
             self.logger.error("%s: %s", emsg, exception_summary(err))
             self.logger.debug("%s:\n%s\n", emsg, pretty_traceback())