Add a new one-way stream API

NB: this is a breaking change removing support for `Portal.run()` being
able to invoke remote streaming functions and instead replacing the
method call with an async context manager api `Portal.open_stream_from()`
This style explicitly defines stream teardown at the call site instead
of expecting the user to handle tricky things correctly themselves: eg.
`async_geneartor.aclosing()`. Going forward `Portal.run()` can be used
only for invoking async functions.

tractor/_portal.py

@@ -3,8 +3,10 @@ Portal api
 """
 import importlib
 import inspect
-import typing
+from typing import (
-from typing import Tuple, Any, Dict, Optional, Set
+    Tuple, Any, Dict, Optional, Set,
+    Callable, AsyncGenerator
+)
 from functools import partial
 from dataclasses import dataclass
 import warnings
@@ -26,7 +28,7 @@ log = get_logger(__name__)
 async def maybe_open_nursery(
     nursery: trio.Nursery = None,
     shield: bool = False,
-) -> typing.AsyncGenerator[trio.Nursery, Any]:
+) -> AsyncGenerator[trio.Nursery, Any]:
     """Create a new nursery if None provided.
 
     Blocks on exit as expected if no input nursery is provided.
@@ -39,6 +41,13 @@ async def maybe_open_nursery(
             yield nursery
 
 
+def func_deats(func: Callable) -> Tuple[str, str]:
+    return (
+        func.__module__,
+        func.__name__,
+    )
+
+
 class Portal:
     """A 'portal' to a(n) (remote) ``Actor``.
 
@@ -105,48 +114,15 @@ class Portal:
                 "A pending main result has already been submitted"
         self._expect_result = await self._submit(ns, func, kwargs)
 
-    async def run_from_ns(
+    async def _return_once(
-        self,
-        namespace_path: str,
-        function_name: str,
-        **kwargs,
-    ) -> Any:
-        """Run a function from a (remote) namespace in a new task on the far-end actor.
-
-        This is a more explitcit way to run tasks in a remote-process
-        actor using explicit object-path syntax. Hint: this is how
-        `.run()` works underneath.
-
-        Note::
-
-            A special namespace `self` can be used to invoke `Actor`
-            instance methods in the remote runtime. Currently this should only
-            be used for `tractor` internals.
-        """
-        return await self._return_from_resptype(
-            *(await self._submit(namespace_path, function_name, kwargs))
-        )
-
-    async def _return_from_resptype(
         self,
         cid: str,
         recv_chan: trio.abc.ReceiveChannel,
         resptype: str,
         first_msg: dict
     ) -> Any:
+        assert resptype == 'asyncfunc'  # single response
 
-        # receive only stream
-        if resptype == 'asyncgen':
-            ctx = Context(self.channel, cid, _portal=self)
-            rchan = ReceiveMsgStream(ctx, recv_chan, self)
-            self._streams.add(rchan)
-            return rchan
-
-        elif resptype == 'context':  # context manager style setup/teardown
-            # TODO likely not here though
-            raise NotImplementedError
-
-        elif resptype == 'asyncfunc':  # single response
         msg = await recv_chan.receive()
         try:
             return msg['return']
@@ -154,8 +130,6 @@ class Portal:
             # internal error should never get here
             assert msg.get('cid'), "Received internal error at portal?"
             raise unpack_error(msg, self.channel)
-        else:
-            raise ValueError(f"Unknown msg response type: {first_msg}")
 
     async def result(self) -> Any:
         """Return the result(s) from the remote actor's "main" task.
@@ -178,9 +152,7 @@ class Portal:
         assert self._expect_result
         if self._result is None:
             try:
-                self._result = await self._return_from_resptype(
+                self._result = await self._return_once(*self._expect_result)
-                    *self._expect_result
-                )
             except RemoteActorError as err:
                 self._result = err
 
@@ -247,6 +219,28 @@ class Portal:
                 f"{self.channel} for {self.channel.uid} was already closed?")
             return False
 
+    async def run_from_ns(
+        self,
+        namespace_path: str,
+        function_name: str,
+        **kwargs,
+    ) -> Any:
+        """Run a function from a (remote) namespace in a new task on the far-end actor.
+
+        This is a more explitcit way to run tasks in a remote-process
+        actor using explicit object-path syntax. Hint: this is how
+        `.run()` works underneath.
+
+        Note::
+
+            A special namespace `self` can be used to invoke `Actor`
+            instance methods in the remote runtime. Currently this should only
+            be used for `tractor` internals.
+        """
+        return await self._return_once(
+            *(await self._submit(namespace_path, function_name, kwargs))
+        )
+
     async def run(
         self,
         func: str,
@@ -272,30 +266,57 @@ class Portal:
             assert isinstance(fn_name, str)
 
         else:  # function reference was passed directly
-
-            # TODO: ensure async
             if not (
-                inspect.isasyncgenfunction(func) or
                 inspect.iscoroutinefunction(func)
             ):
                 raise TypeError(f'{func} must be an async function!')
 
-            fn = func
+            fn_mod_path, fn_name = func_deats(func)
-            fn_mod_path = fn.__module__
-            fn_name = fn.__name__
 
-        return await self._return_from_resptype(
+        return await self._return_once(
             *(await self._submit(fn_mod_path, fn_name, kwargs))
         )
 
-    # @asynccontextmanager
+    @asynccontextmanager
-    # async def open_stream_from(
+    async def open_stream_from(
-    #     self,
+        self,
-    #     async_gen: 'AsyncGeneratorFunction',
+        async_gen_func: Callable,  # typing: ignore
-    #     **kwargs,
+        **kwargs,
-    # ) -> ReceiveMsgStream:
+    ) -> AsyncGenerator[ReceiveMsgStream, None]:
-    #     # TODO
+
-    #     pass
+        if not inspect.isasyncgenfunction(async_gen_func):
+            if not inspect.iscoroutinefunction(async_gen_func) or (
+                not getattr(async_gen_func, '_tractor_stream_function', False)
+            ):
+                raise TypeError(
+                    f'{async_gen_func} must be an async generator function!')
+
+        fn_mod_path, fn_name = func_deats(async_gen_func)
+        (
+            cid,
+            recv_chan,
+            functype,
+            first_msg
+        ) = await self._submit(fn_mod_path, fn_name, kwargs)
+
+        # receive only stream
+        assert functype == 'asyncgen'
+
+        ctx = Context(self.channel, cid, _portal=self)
+        try:
+            async with ReceiveMsgStream(ctx, recv_chan, self) as rchan:
+                self._streams.add(rchan)
+                yield rchan
+        finally:
+            # cancel the far end task on consumer close
+            try:
+                await ctx.cancel()
+            except trio.ClosedResourceError:
+                # if the far end terminates before we send a cancel the
+                # underlying transport-channel may already be closed.
+                log.debug(f'Context {ctx} was already closed?')
+
+            self._streams.remove(rchan)
 
     # @asynccontextmanager
     # async def open_context(
@@ -304,7 +325,9 @@ class Portal:
     #     **kwargs,
     # ) -> Context:
     #     # TODO
-    #     pass
+    #     elif resptype == 'context':  # context manager style setup/teardown
+    #         # TODO likely not here though
+    #         raise NotImplementedError
 
 
 @dataclass
@@ -324,10 +347,7 @@ class LocalPortal:
         """
         obj = self.actor if ns == 'self' else importlib.import_module(ns)
         func = getattr(obj, func_name)
-        if inspect.iscoroutinefunction(func):
         return await func(**kwargs)
-        else:
-            return func(**kwargs)
 
 
 @asynccontextmanager
@@ -336,7 +356,7 @@ async def open_portal(
     nursery: Optional[trio.Nursery] = None,
     start_msg_loop: bool = True,
     shield: bool = False,
-) -> typing.AsyncGenerator[Portal, None]:
+) -> AsyncGenerator[Portal, None]:
     """Open a ``Portal`` through the provided ``channel``.
 
     Spawns a background task to handle message processing.