Run first soft wait inside a task

Theoretically we can actually concurrently soft wait the process and the
nursery exit event, the only problem at the moment is some pubsub tests
puking. You're right in guessing this isn't really changing anything but
it is meant to be a reminder. If we can add this the spawn task can
report when a process dies earlier then expected and in the longer term
once we remove the `ActorNursery.run_in_actor()` API, we can probably do
away with both the nursery exit event and the portal result fetching.

tractor/_spawn.py

@@ -18,17 +18,30 @@
 Machinery for actor process spawning using multiple backends.
 
 """
-from __future__ import annotations
 import sys
+import multiprocessing as mp
 import platform
 from typing import (
-    Any, Optional, Callable, TypeVar, TYPE_CHECKING
+    Any, Dict, Optional, Callable,
+    TypeVar,
 )
 from collections.abc import Awaitable
 
 import trio
 from trio_typing import TaskStatus
 
+try:
+    from multiprocessing import semaphore_tracker  # type: ignore
+    resource_tracker = semaphore_tracker
+    resource_tracker._resource_tracker = resource_tracker._semaphore_tracker
+except ImportError:
+    # 3.8 introduces a more general version that also tracks shared mems
+    from multiprocessing import resource_tracker  # type: ignore
+
+from multiprocessing import forkserver  # type: ignore
+from typing import Tuple
+
+from . import _forkserver_override
 from ._debug import (
     maybe_wait_for_debugger,
     acquire_debug_lock,
@@ -47,11 +60,8 @@ from ._entry import _mp_main
 from ._exceptions import ActorFailure
 
 
-if TYPE_CHECKING:
-    import multiprocessing as mp
-    ProcessType = TypeVar('ProcessType', mp.Process, trio.Process)
-
 log = get_logger('tractor')
+ProcessType = TypeVar('ProcessType', mp.Process, trio.Process)
 
 # placeholder for an mp start context if so using that backend
 _ctx: Optional[mp.context.BaseContext] = None
@@ -60,7 +70,6 @@ _spawn_method: str = "trio"
 
 if platform.system() == 'Windows':
 
-    import multiprocessing as mp
     _ctx = mp.get_context("spawn")
 
     async def proc_waiter(proc: mp.Process) -> None:
@@ -83,7 +92,6 @@ def try_set_start_method(name: str) -> Optional[mp.context.BaseContext]:
     ``subprocess.Popen``.
 
     '''
-    import multiprocessing as mp
     global _ctx
     global _spawn_method
 
@@ -100,7 +108,6 @@ def try_set_start_method(name: str) -> Optional[mp.context.BaseContext]:
             f"Spawn method `{name}` is invalid please choose one of {methods}"
         )
     elif name == 'forkserver':
-        from . import _forkserver_override
         _forkserver_override.override_stdlib()
         _ctx = mp.get_context(name)
     elif name == 'trio':
@@ -148,7 +155,7 @@ async def cancel_on_completion(
 
     portal: Portal,
     actor: Actor,
-    errors: dict[tuple[str, str], Exception],
+    errors: Dict[Tuple[str, str], Exception],
 
 ) -> None:
     '''
@@ -251,12 +258,12 @@ async def new_proc(
     name: str,
     actor_nursery: 'ActorNursery',  # type: ignore  # noqa
     subactor: Actor,
-    errors: dict[tuple[str, str], Exception],
+    errors: Dict[Tuple[str, str], Exception],
 
     # passed through to actor main
-    bind_addr: tuple[str, int],
-    parent_addr: tuple[str, int],
-    _runtime_vars: dict[str, Any],  # serialized and sent to _child
+    bind_addr: Tuple[str, int],
+    parent_addr: Tuple[str, int],
+    _runtime_vars: Dict[str, Any],  # serialized and sent to _child
 
     *,
 
@@ -288,7 +295,7 @@ async def new_proc(
             # the OS; it otherwise can be passed via the parent channel if
             # we prefer in the future (for privacy).
             "--uid",
-            str(subactor.uid),
+            str(uid),
             # Address the child must connect to on startup
             "--parent_addr",
             str(parent_addr)
@@ -314,8 +321,7 @@ async def new_proc(
                 # wait for actor to spawn and connect back to us
                 # channel should have handshake completed by the
                 # local actor by the time we get a ref to it
-                event, chan = await actor_nursery._actor.wait_for_peer(
-                    subactor.uid)
+                event, chan = await actor_nursery._actor.wait_for_peer(uid)
 
             except trio.Cancelled:
                 cancelled_during_spawn = True
@@ -356,10 +362,54 @@ async def new_proc(
             task_status.started(portal)
 
             # wait for ActorNursery.wait() to be called
+            n_exited = actor_nursery._join_procs
             with trio.CancelScope(shield=True):
-                await actor_nursery._join_procs.wait()
+                await n_exited.wait()
 
             async with trio.open_nursery() as nursery:
+
+                async def soft_wait_and_maybe_cancel_ria_task():
+                    # This is a "soft" (cancellable) join/reap which
+                    # will remote cancel the actor on a ``trio.Cancelled``
+                    # condition.
+                    await soft_wait(
+                        proc,
+                        trio.Process.wait,
+                        portal
+                    )
+
+                    if n_exited.is_set():
+                        # cancel result waiter that may have been spawned in
+                        # tandem if not done already
+                        log.warning(
+                            "Cancelling existing result waiter task for "
+                            f"{subactor.uid}"
+                        )
+                        nursery.cancel_scope.cancel()
+
+                    else:
+                        log.warning(
+                            f'Process for actor {uid} terminated before'
+                            'nursery exit. ' 'This may mean an IPC'
+                            'connection failed!'
+                        )
+
+                nursery.start_soon(soft_wait_and_maybe_cancel_ria_task)
+
+                # TODO: when we finally remove the `.run_in_actor()` api
+                # we should be able to entirely drop these 2 blocking calls:
+                # - we don't need to wait on nursery exit to capture
+                #   process-spawn-machinery level errors (and propagate them).
+                # - we don't need to wait on final results from ria portals
+                #   since this will be done in some higher level wrapper API.
+
+                # XXX: interestingly we can't put this here bc it causes
+                # the pub-sub tests to fail? wth.. should probably drop
+                # those XD
+                # wait for ActorNursery.wait() to be called
+                # with trio.CancelScope(shield=True):
+                #     await n_exited.wait()
+
                 if portal in actor_nursery._cancel_after_result_on_exit:
                     nursery.start_soon(
                         cancel_on_completion,
@@ -368,22 +418,6 @@ async def new_proc(
                         errors
                     )
 
-                # This is a "soft" (cancellable) join/reap which
-                # will remote cancel the actor on a ``trio.Cancelled``
-                # condition.
-                await soft_wait(
-                    proc,
-                    trio.Process.wait,
-                    portal
-                )
-
-                # cancel result waiter that may have been spawned in
-                # tandem if not done already
-                log.warning(
-                    "Cancelling existing result waiter task for "
-                    f"{subactor.uid}")
-                nursery.cancel_scope.cancel()
-
         finally:
             # The "hard" reap since no actor zombies are allowed!
             # XXX: do this **after** cancellation/tearfown to avoid
@@ -400,9 +434,10 @@ async def new_proc(
                                 await proc.wait()
 
                     if is_root_process():
+
                         await maybe_wait_for_debugger(
                             child_in_debug=_runtime_vars.get(
-                                '_debug_mode', False),
+                                '_debug_mode', False)
                         )
 
                     if proc.poll() is None:
@@ -441,30 +476,20 @@ async def mp_new_proc(
     name: str,
     actor_nursery: 'ActorNursery',  # type: ignore  # noqa
     subactor: Actor,
-    errors: dict[tuple[str, str], Exception],
+    errors: Dict[Tuple[str, str], Exception],
     # passed through to actor main
-    bind_addr: tuple[str, int],
-    parent_addr: tuple[str, int],
-    _runtime_vars: dict[str, Any],  # serialized and sent to _child
+    bind_addr: Tuple[str, int],
+    parent_addr: Tuple[str, int],
+    _runtime_vars: Dict[str, Any],  # serialized and sent to _child
     *,
     infect_asyncio: bool = False,
     task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED
 
 ) -> None:
 
-    # uggh zone
-    try:
-        from multiprocessing import semaphore_tracker  # type: ignore
-        resource_tracker = semaphore_tracker
-        resource_tracker._resource_tracker = resource_tracker._semaphore_tracker  # noqa
-    except ImportError:
-        # 3.8 introduces a more general version that also tracks shared mems
-        from multiprocessing import resource_tracker  # type: ignore
-
     assert _ctx
     start_method = _ctx.get_start_method()
     if start_method == 'forkserver':
-        from multiprocessing import forkserver  # type: ignore
         # XXX do our hackery on the stdlib to avoid multiple
         # forkservers (one at each subproc layer).
         fs = forkserver._forkserver