python/aqmp: add runstate state machine to AsyncProtocol

This serves a few purposes:

1. Protect interfaces when it's not safe to call them (via @require)

2. Add an interface by which an async client can determine if the state
has changed, for the purposes of connection management.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20210915162955.333025-7-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

2 files changed, 160 insertions, 5 deletions

diff --git a/python/qemu/aqmp/__init__.py b/python/qemu/aqmp/__init__.py
index 5c0de72672..88ead4c023 100644
--- a/python/qemu/aqmp/__init__.py
+++ b/python/qemu/aqmp/__init__.py
@@ -22,12 +22,16 @@ managing QMP events.
 # the COPYING file in the top-level directory.
 
 from .error import AQMPError
-from .protocol import ConnectError
+from .protocol import ConnectError, Runstate, StateError
 
 
 # The order of these fields impact the Sphinx documentation order.
 __all__ = (
+    # Classes
+    'Runstate',
+
     # Exceptions, most generic to most explicit
     'AQMPError',
+    'StateError',
     'ConnectError',
 )
diff --git a/python/qemu/aqmp/protocol.py b/python/qemu/aqmp/protocol.py
index 2a93da791e..19460857bd 100644
--- a/python/qemu/aqmp/protocol.py
+++ b/python/qemu/aqmp/protocol.py
@@ -12,11 +12,10 @@ class.
 
 import asyncio
 from asyncio import StreamReader, StreamWriter
+from enum import Enum
+from functools import wraps
 from ssl import SSLContext
-# import exceptions will be removed in a forthcoming commit.
-# The problem stems from pylint/flake8 believing that 'Any'
-# is unused because of its only use in a string-quoted type.
-from typing import (  # pylint: disable=unused-import # noqa
+from typing import (
     Any,
     Awaitable,
     Callable,
@@ -26,6 +25,7 @@ from typing import (  # pylint: disable=unused-import # noqa
     Tuple,
     TypeVar,
     Union,
+    cast,
 )
 
 from .error import AQMPError
@@ -44,6 +44,20 @@ _TaskFN = Callable[[], Awaitable[None]]  # aka ``async def func() -> None``
 _FutureT = TypeVar('_FutureT', bound=Optional['asyncio.Future[Any]'])
 
 
+class Runstate(Enum):
+    """Protocol session runstate."""
+
+    #: Fully quiesced and disconnected.
+    IDLE = 0
+    #: In the process of connecting or establishing a session.
+    CONNECTING = 1
+    #: Fully connected and active session.
+    RUNNING = 2
+    #: In the process of disconnecting.
+    #: Runstate may be returned to `IDLE` by calling `disconnect()`.
+    DISCONNECTING = 3
+
+
 class ConnectError(AQMPError):
     """
     Raised when the initial connection process has failed.
@@ -65,6 +79,76 @@ class ConnectError(AQMPError):
         return f"{self.error_message}: {self.exc!s}"
 
 
+class StateError(AQMPError):
+    """
+    An API command (connect, execute, etc) was issued at an inappropriate time.
+
+    This error is raised when a command like
+    :py:meth:`~AsyncProtocol.connect()` is issued at an inappropriate
+    time.
+
+    :param error_message: Human-readable string describing the state violation.
+    :param state: The actual `Runstate` seen at the time of the violation.
+    :param required: The `Runstate` required to process this command.
+    """
+    def __init__(self, error_message: str,
+                 state: Runstate, required: Runstate):
+        super().__init__(error_message)
+        self.error_message = error_message
+        self.state = state
+        self.required = required
+
+
+F = TypeVar('F', bound=Callable[..., Any])  # pylint: disable=invalid-name
+
+
+# Don't Panic.
+def require(required_state: Runstate) -> Callable[[F], F]:
+    """
+    Decorator: protect a method so it can only be run in a certain `Runstate`.
+
+    :param required_state: The `Runstate` required to invoke this method.
+    :raise StateError: When the required `Runstate` is not met.
+    """
+    def _decorator(func: F) -> F:
+        # _decorator is the decorator that is built by calling the
+        # require() decorator factory; e.g.:
+        #
+        # @require(Runstate.IDLE) def foo(): ...
+        # will replace 'foo' with the result of '_decorator(foo)'.
+
+        @wraps(func)
+        def _wrapper(proto: 'AsyncProtocol[Any]',
+                     *args: Any, **kwargs: Any) -> Any:
+            # _wrapper is the function that gets executed prior to the
+            # decorated method.
+
+            name = type(proto).__name__
+
+            if proto.runstate != required_state:
+                if proto.runstate == Runstate.CONNECTING:
+                    emsg = f"{name} is currently connecting."
+                elif proto.runstate == Runstate.DISCONNECTING:
+                    emsg = (f"{name} is disconnecting."
+                            " Call disconnect() to return to IDLE state.")
+                elif proto.runstate == Runstate.RUNNING:
+                    emsg = f"{name} is already connected and running."
+                elif proto.runstate == Runstate.IDLE:
+                    emsg = f"{name} is disconnected and idle."
+                else:
+                    assert False
+                raise StateError(emsg, proto.runstate, required_state)
+            # No StateError, so call the wrapped method.
+            return func(proto, *args, **kwargs)
+
+        # Return the decorated method;
+        # Transforming Func to Decorated[Func].
+        return cast(F, _wrapper)
+
+    # Return the decorator instance from the decorator factory. Phew!
+    return _decorator
+
+
 class AsyncProtocol(Generic[T]):
     """
     AsyncProtocol implements a generic async message-based protocol.
@@ -118,7 +202,24 @@ class AsyncProtocol(Generic[T]):
         #: exit.
         self._dc_task: Optional[asyncio.Future[None]] = None
 
+        self._runstate = Runstate.IDLE
+        self._runstate_changed: Optional[asyncio.Event] = None
+
+    @property  # @upper_half
+    def runstate(self) -> Runstate:
+        """The current `Runstate` of the connection."""
+        return self._runstate
+
     @upper_half
+    async def runstate_changed(self) -> Runstate:
+        """
+        Wait for the `runstate` to change, then return that runstate.
+        """
+        await self._runstate_event.wait()
+        return self.runstate
+
+    @upper_half
+    @require(Runstate.IDLE)
     async def connect(self, address: Union[str, Tuple[str, int]],
                       ssl: Optional[SSLContext] = None) -> None:
         """
@@ -152,6 +253,30 @@ class AsyncProtocol(Generic[T]):
     # Section: Session machinery
     # --------------------------
 
+    @property
+    def _runstate_event(self) -> asyncio.Event:
+        # asyncio.Event() objects should not be created prior to entrance into
+        # an event loop, so we can ensure we create it in the correct context.
+        # Create it on-demand *only* at the behest of an 'async def' method.
+        if not self._runstate_changed:
+            self._runstate_changed = asyncio.Event()
+        return self._runstate_changed
+
+    @upper_half
+    @bottom_half
+    def _set_state(self, state: Runstate) -> None:
+        """
+        Change the `Runstate` of the protocol connection.
+
+        Signals the `runstate_changed` event.
+        """
+        if state == self._runstate:
+            return
+
+        self._runstate = state
+        self._runstate_event.set()
+        self._runstate_event.clear()
+
     @upper_half
     async def _new_session(self,
                            address: Union[str, Tuple[str, int]],
@@ -176,6 +301,8 @@ class AsyncProtocol(Generic[T]):
             protocol-level failure occurs while establishing a new
             session, the wrapped error may also be an `AQMPError`.
         """
+        assert self.runstate == Runstate.IDLE
+
         try:
             phase = "connection"
             await self._establish_connection(address, ssl)
@@ -185,6 +312,7 @@ class AsyncProtocol(Generic[T]):
 
         except BaseException as err:
             emsg = f"Failed to establish {phase}"
+            # Reset from CONNECTING back to IDLE.
             await self.disconnect()
 
             # NB: CancelledError is not a BaseException before Python 3.8
@@ -197,6 +325,8 @@ class AsyncProtocol(Generic[T]):
             # Raise BaseExceptions un-wrapped, they're more important.
             raise
 
+        assert self.runstate == Runstate.RUNNING
+
     @upper_half
     async def _establish_connection(
             self,
@@ -211,6 +341,14 @@ class AsyncProtocol(Generic[T]):
             UNIX socket path or TCP address/port.
         :param ssl: SSL context to use, if any.
         """
+        assert self.runstate == Runstate.IDLE
+        self._set_state(Runstate.CONNECTING)
+
+        # Allow runstate watchers to witness 'CONNECTING' state; some
+        # failures in the streaming layer are synchronous and will not
+        # otherwise yield.
+        await asyncio.sleep(0)
+
         await self._do_connect(address, ssl)
 
     @upper_half
@@ -240,6 +378,8 @@ class AsyncProtocol(Generic[T]):
         own negotiations here. The Runstate will be RUNNING upon
         successful conclusion.
         """
+        assert self.runstate == Runstate.CONNECTING
+
         self._outgoing = asyncio.Queue()
 
         reader_coro = self._bh_loop_forever(self._bh_recv_message)
@@ -253,6 +393,9 @@ class AsyncProtocol(Generic[T]):
             self._writer_task,
         )
 
+        self._set_state(Runstate.RUNNING)
+        await asyncio.sleep(0)  # Allow runstate_event to process
+
     @upper_half
     @bottom_half
     def _schedule_disconnect(self) -> None:
@@ -266,6 +409,7 @@ class AsyncProtocol(Generic[T]):
         It can be invoked no matter what the `runstate` is.
         """
         if not self._dc_task:
+            self._set_state(Runstate.DISCONNECTING)
             self._dc_task = create_task(self._bh_disconnect())
 
     @upper_half
@@ -281,6 +425,7 @@ class AsyncProtocol(Generic[T]):
         :raise Exception:
             Arbitrary exception re-raised on behalf of the reader/writer.
         """
+        assert self.runstate == Runstate.DISCONNECTING
         assert self._dc_task
 
         aws: List[Awaitable[object]] = [self._dc_task]
@@ -295,6 +440,7 @@ class AsyncProtocol(Generic[T]):
             await all_defined_tasks  # Raise Exceptions from the bottom half.
         finally:
             self._cleanup()
+            self._set_state(Runstate.IDLE)
 
     @upper_half
     def _cleanup(self) -> None:
@@ -306,6 +452,7 @@ class AsyncProtocol(Generic[T]):
             assert (task is None) or task.done()
             return None if (task and task.done()) else task
 
+        assert self.runstate == Runstate.DISCONNECTING
         self._dc_task = _paranoid_task_erase(self._dc_task)
         self._reader_task = _paranoid_task_erase(self._reader_task)
         self._writer_task = _paranoid_task_erase(self._writer_task)
@@ -314,6 +461,9 @@ class AsyncProtocol(Generic[T]):
         self._reader = None
         self._writer = None
 
+        # NB: _runstate_changed cannot be cleared because we still need it to
+        # send the final runstate changed event ...!
+
     # ----------------------------
     # Section: Bottom Half methods
     # ----------------------------
@@ -328,6 +478,7 @@ class AsyncProtocol(Generic[T]):
         it is free to wait on any pending actions that may still need to
         occur in either the reader or writer tasks.
         """
+        assert self.runstate == Runstate.DISCONNECTING
 
         def _done(task: Optional['asyncio.Future[Any]']) -> bool:
             return task is not None and task.done()