Allocate a `PldRx` per `Context`, new pld-spec API

Since the state mgmt becomes quite messy with multiple sub-tasks inside
an IPC ctx, AND bc generally speaking the payload-type-spec should map
1-to-1 with the `Context`, it doesn't make a lot of sense to be using
`ContextVar`s to modify the `Context.pld_rx: PldRx` instance.

Instead, always allocate a full instance inside `mk_context()` with the
default `.pld_rx: PldRx` set to use the `msg._ops._def_any_pldec: MsgDec`

In support, simplify the `.msg._ops` impl and APIs:
- drop `_ctxvar_PldRx`, `_def_pld_rx` and `current_pldrx()`.
- rename `PldRx._pldec` -> `._pld_dec`.
- rename the unused `PldRx.apply_to_ipc()` -> `.wraps_ipc()`.
- add a required `PldRx._ctx: Context` attr since it is needed
  internally in some meths and each pld-rx now maps to a specific ctx.
- modify all recv methods to accept a `ipc: Context|MsgStream` (instead
  of a `ctx` arg) since both have a ref to the same `._rx_chan` and there
  are only a couple spots (in `.dec_msg()`) where we need the `ctx`
  explicitly (which can now be easily accessed via a new `MsgStream.ctx`
  property, see below).
- always show the `.dec_msg()` frame in tbs if there's a reference error
  when calling `_raise_from_unexpected_msg()` in the fallthrough case.
- implement `limit_plds()` as light wrapper around getting the
  `current_ipc_ctx()` and mutating its `MsgDec` via
  `Context.pld_rx.limit_plds()`.
- add a `maybe_limit_plds()` which just provides an `@acm` equivalent of
  `limit_plds()` handy for composing in a `async with ():` style block
  (avoiding additional indent levels in the body of async funcs).

Obvi extend the `Context` and `MsgStream` interfaces as needed
to match the above:
- add a `Context.pld_rx` pub prop.
- new private refs to `Context._started_msg: Started` and
  a `._started_pld` (mostly for internal debugging / testing / logging)
  and set inside `.open_context()` immediately after the syncing phase.
- a `Context.has_outcome() -> bool:` predicate which can be used to more
  easily determine if the ctx errored or has a final result.
- pub props for `MsgStream.ctx: Context` and `.chan: Channel` providing
  full `ipc`-arg compat with the `PldRx` method signatures.

tractor/_context.py

@@ -41,6 +41,7 @@ from typing import (
     Callable,
     Mapping,
     Type,
+    TypeAlias,
     TYPE_CHECKING,
     Union,
 )
@@ -155,6 +156,41 @@ class Context:
     # payload receiver
     _pld_rx: msgops.PldRx
 
+    @property
+    def pld_rx(self) -> msgops.PldRx:
+        '''
+        The current `tractor.Context`'s msg-payload-receiver.
+
+        A payload receiver is the IPC-msg processing sub-sys which
+        filters inter-actor-task communicated payload data, i.e. the
+        `PayloadMsg.pld: PayloadT` field value, AFTER its container
+        shuttlle msg (eg. `Started`/`Yield`/`Return) has been
+        delivered up from `tractor`'s transport layer but BEFORE the
+        data is yielded to `tractor` application code.
+
+        The "IPC-primitive API" is normally one of a `Context` (this)` or a `MsgStream`
+        or some higher level API using one of them.
+
+        For ex. `pld_data: PayloadT = MsgStream.receive()` implicitly
+        calls into the stream's parent `Context.pld_rx.recv_pld().` to
+        receive the latest `PayloadMsg.pld` value.
+
+        Modification of the current payload spec via `limit_plds()`
+        allows a `tractor` application to contextually filter IPC
+        payload content with a type specification as supported by the
+        interchange backend.
+
+        - for `msgspec` see <PUTLINKHERE>.
+
+        Note that the `PldRx` itself is a per-`Context` instance that
+        normally only changes when some (sub-)task, on a given "side"
+        of the IPC ctx (either a "child"-side RPC or inside
+        a "parent"-side `Portal.open_context()` block), modifies it
+        using the `.msg._ops.limit_plds()` API.
+
+        '''
+        return self._pld_rx
+
     # full "namespace-path" to target RPC function
     _nsf: NamespacePath
 
@@ -231,6 +267,8 @@ class Context:
 
     # init and streaming state
     _started_called: bool = False
+    _started_msg: MsgType|None = None
+    _started_pld: Any = None
     _stream_opened: bool = False
     _stream: MsgStream|None = None
 
@@ -623,7 +661,7 @@ class Context:
         log.runtime(
             'Setting remote error for ctx\n\n'
             f'<= {self.peer_side!r}: {self.chan.uid}\n'
-            f'=> {self.side!r}\n\n'
+            f'=> {self.side!r}: {self._actor.uid}\n\n'
             f'{error}'
         )
         self._remote_error: BaseException = error
@@ -678,7 +716,7 @@ class Context:
             log.error(
                 f'Remote context error:\n\n'
                 # f'{pformat(self)}\n'
-                f'{error}\n'
+                f'{error}'
             )
 
         if self._canceller is None:
@@ -724,8 +762,10 @@ class Context:
                 )
         else:
             message: str = 'NOT cancelling `Context._scope` !\n\n'
+            # from .devx import mk_pdb
+            # mk_pdb().set_trace()
 
-        fmt_str: str = 'No `self._scope: CancelScope` was set/used ?'
+        fmt_str: str = 'No `self._scope: CancelScope` was set/used ?\n'
         if (
             cs
             and
@@ -805,6 +845,7 @@ class Context:
         #         f'{ci.api_nsp}()\n'
         #     )
 
+        # TODO: use `.dev._frame_stack` scanning to find caller!
         return 'Portal.open_context()'
 
     async def cancel(
@@ -1304,17 +1345,6 @@ class Context:
                 ctx=self,
                 hide_tb=hide_tb,
             )
-            for msg in drained_msgs:
-
-                # TODO: mask this by default..
-                if isinstance(msg, Return):
-                    # from .devx import pause
-                    # await pause()
-                    # raise InternalError(
-                    log.warning(
-                        'Final `return` msg should never be drained !?!?\n\n'
-                        f'{msg}\n'
-                    )
 
             drained_status: str = (
                 'Ctx drained to final outcome msg\n\n'
@@ -1435,6 +1465,10 @@ class Context:
             self._result
         )
 
+    @property
+    def has_outcome(self) -> bool:
+        return bool(self.maybe_error) or self._final_result_is_set()
+
     # @property
     def repr_outcome(
         self,
@@ -1637,8 +1671,6 @@ class Context:
                     )
 
                 if rt_started != started_msg:
-                    # TODO: break these methods out from the struct subtype?
-
                     # TODO: make that one a mod func too..
                     diff = pretty_struct.Struct.__sub__(
                         rt_started,
@@ -1674,6 +1706,8 @@ class Context:
             ) from verr
 
         self._started_called = True
+        self._started_msg = started_msg
+        self._started_pld = value
 
     async def _drain_overflows(
         self,
@@ -1961,6 +1995,7 @@ async def open_context_from_portal(
     portal: Portal,
     func: Callable,
 
+    pld_spec: TypeAlias|None = None,
     allow_overruns: bool = False,
 
     # TODO: if we set this the wrapping `@acm` body will
@@ -2026,7 +2061,7 @@ async def open_context_from_portal(
     # XXX NOTE XXX: currenly we do NOT allow opening a contex
     # with "self" since the local feeder mem-chan processing
     # is not built for it.
-    if portal.channel.uid == portal.actor.uid:
+    if (uid := portal.channel.uid) == portal.actor.uid:
         raise RuntimeError(
             '** !! Invalid Operation !! **\n'
             'Can not open an IPC ctx with the local actor!\n'
@@ -2054,32 +2089,45 @@ async def open_context_from_portal(
     assert ctx._caller_info
     _ctxvar_Context.set(ctx)
 
-    # XXX NOTE since `._scope` is NOT set BEFORE we retreive the
-    # `Started`-msg any cancellation triggered
-    # in `._maybe_cancel_and_set_remote_error()` will
-    # NOT actually cancel the below line!
-    # -> it's expected that if there is an error in this phase of
-    # the dialog, the `Error` msg should be raised from the `msg`
-    # handling block below.
-    first: Any = await ctx._pld_rx.recv_pld(
-        ctx=ctx,
-        expect_msg=Started,
-    )
-    ctx._started_called: bool = True
-
-    uid: tuple = portal.channel.uid
-    cid: str = ctx.cid
-
     # placeholder for any exception raised in the runtime
     # or by user tasks which cause this context's closure.
     scope_err: BaseException|None = None
     ctxc_from_callee: ContextCancelled|None = None
     try:
-        async with trio.open_nursery() as nurse:
+        async with (
+            trio.open_nursery() as tn,
+            msgops.maybe_limit_plds(
+                ctx=ctx,
+                spec=pld_spec,
+            ) as maybe_msgdec,
+        ):
+            if maybe_msgdec:
+                assert maybe_msgdec.pld_spec == pld_spec
 
-            # NOTE: used to start overrun queuing tasks
+            # XXX NOTE since `._scope` is NOT set BEFORE we retreive the
-            ctx._scope_nursery: trio.Nursery = nurse
+            # `Started`-msg any cancellation triggered
-            ctx._scope: trio.CancelScope = nurse.cancel_scope
+            # in `._maybe_cancel_and_set_remote_error()` will
+            # NOT actually cancel the below line!
+            # -> it's expected that if there is an error in this phase of
+            # the dialog, the `Error` msg should be raised from the `msg`
+            # handling block below.
+            started_msg, first = await ctx._pld_rx.recv_msg_w_pld(
+                ipc=ctx,
+                expect_msg=Started,
+                passthrough_non_pld_msgs=False,
+            )
+
+            # from .devx import pause
+            # await pause()
+            ctx._started_called: bool = True
+            ctx._started_msg: bool = started_msg
+            ctx._started_pld: bool = first
+
+            # NOTE: this in an implicit runtime nursery used to,
+            # - start overrun queuing tasks when as well as
+            # for cancellation of the scope opened by the user.
+            ctx._scope_nursery: trio.Nursery = tn
+            ctx._scope: trio.CancelScope = tn.cancel_scope
 
             # deliver context instance and .started() msg value
             # in enter tuple.
@@ -2126,13 +2174,13 @@ async def open_context_from_portal(
 
             # when in allow_overruns mode there may be
             # lingering overflow sender tasks remaining?
-            if nurse.child_tasks:
+            if tn.child_tasks:
                 # XXX: ensure we are in overrun state
                 # with ``._allow_overruns=True`` bc otherwise
                 # there should be no tasks in this nursery!
                 if (
                     not ctx._allow_overruns
-                    or len(nurse.child_tasks) > 1
+                    or len(tn.child_tasks) > 1
                 ):
                     raise InternalError(
                         'Context has sub-tasks but is '
@@ -2304,8 +2352,8 @@ async def open_context_from_portal(
             ):
                 log.warning(
                     'IPC connection for context is broken?\n'
-                    f'task:{cid}\n'
+                    f'task: {ctx.cid}\n'
-                    f'actor:{uid}'
+                    f'actor: {uid}'
                 )
 
         raise  # duh
@@ -2455,9 +2503,8 @@ async def open_context_from_portal(
                 and ctx.cancel_acked
             ):
                 log.cancel(
-                    'Context cancelled by {ctx.side!r}-side task\n'
+                    f'Context cancelled by {ctx.side!r}-side task\n'
                     f'|_{ctx._task}\n\n'
-
                     f'{repr(scope_err)}\n'
                 )
 
@@ -2485,7 +2532,7 @@ async def open_context_from_portal(
             f'cid: {ctx.cid}\n'
         )
         portal.actor._contexts.pop(
-            (uid, cid),
+            (uid, ctx.cid),
             None,
         )
 
@@ -2516,8 +2563,9 @@ def mk_context(
     from .devx._frame_stack import find_caller_info
     caller_info: CallerInfo|None = find_caller_info()
 
-    # TODO: when/how do we apply `.limit_plds()` from here?
+    pld_rx = msgops.PldRx(
-    pld_rx: msgops.PldRx = msgops.current_pldrx()
+        _pld_dec=msgops._def_any_pldec,
+    )
 
     ctx = Context(
         chan=chan,
@@ -2531,13 +2579,16 @@ def mk_context(
         _caller_info=caller_info,
         **kwargs,
     )
+    pld_rx._ctx = ctx
     ctx._result = Unresolved
     return ctx
 
 
 # TODO: use the new type-parameters to annotate this in 3.13?
 # -[ ] https://peps.python.org/pep-0718/#unknown-types
-def context(func: Callable) -> Callable:
+def context(
+    func: Callable,
+) -> Callable:
     '''
     Mark an (async) function as an SC-supervised, inter-`Actor`,
     child-`trio.Task`, IPC endpoint otherwise known more

tractor/_streaming.py

@@ -52,6 +52,7 @@ from tractor.msg import (
 
 if TYPE_CHECKING:
     from ._context import Context
+    from ._ipc import Channel
 
 
 log = get_logger(__name__)
@@ -65,10 +66,10 @@ log = get_logger(__name__)
 class MsgStream(trio.abc.Channel):
     '''
     A bidirectional message stream for receiving logically sequenced
-    values over an inter-actor IPC ``Channel``.
+    values over an inter-actor IPC `Channel`.
 
     This is the type returned to a local task which entered either
-    ``Portal.open_stream_from()`` or ``Context.open_stream()``.
+    `Portal.open_stream_from()` or `Context.open_stream()`.
 
     Termination rules:
 
@@ -95,6 +96,22 @@ class MsgStream(trio.abc.Channel):
         self._eoc: bool|trio.EndOfChannel = False
         self._closed: bool|trio.ClosedResourceError = False
 
+    @property
+    def ctx(self) -> Context:
+        '''
+        This stream's IPC `Context` ref.
+
+        '''
+        return self._ctx
+
+    @property
+    def chan(self) -> Channel:
+        '''
+        Ref to the containing `Context`'s transport `Channel`.
+
+        '''
+        return self._ctx.chan
+
     # TODO: could we make this a direct method bind to `PldRx`?
     # -> receive_nowait = PldRx.recv_pld
     # |_ means latter would have to accept `MsgStream`-as-`self`?
@@ -109,7 +126,7 @@ class MsgStream(trio.abc.Channel):
     ):
         ctx: Context = self._ctx
         return ctx._pld_rx.recv_pld_nowait(
-            ctx=ctx,
+            ipc=self,
             expect_msg=expect_msg,
         )
 
@@ -148,7 +165,7 @@ class MsgStream(trio.abc.Channel):
         try:
 
             ctx: Context = self._ctx
-            return await ctx._pld_rx.recv_pld(ctx=ctx)
+            return await ctx._pld_rx.recv_pld(ipc=self)
 
         # XXX: the stream terminates on either of:
         # - via `self._rx_chan.receive()` raising  after manual closure

tractor/msg/_ops.py

@@ -22,10 +22,9 @@ operational helpers for processing transaction flows.
 '''
 from __future__ import annotations
 from contextlib import (
-    # asynccontextmanager as acm,
+    asynccontextmanager as acm,
     contextmanager as cm,
 )
-from contextvars import ContextVar
 from typing import (
     Any,
     Type,
@@ -50,6 +49,7 @@ from tractor._exceptions import (
     _mk_msg_type_err,
     pack_from_raise,
 )
+from tractor._state import current_ipc_ctx
 from ._codec import (
     mk_dec,
     MsgDec,
@@ -75,7 +75,7 @@ if TYPE_CHECKING:
 log = get_logger(__name__)
 
 
-_def_any_pldec: MsgDec = mk_dec()
+_def_any_pldec: MsgDec[Any] = mk_dec()
 
 
 class PldRx(Struct):
@@ -104,15 +104,19 @@ class PldRx(Struct):
     '''
     # TODO: better to bind it here?
     # _rx_mc: trio.MemoryReceiveChannel
-    _pldec: MsgDec
+    _pld_dec: MsgDec
+    _ctx: Context|None = None
     _ipc: Context|MsgStream|None = None
 
     @property
     def pld_dec(self) -> MsgDec:
-        return self._pldec
+        return self._pld_dec
 
+    # TODO: a better name?
+    # -[ ] when would this be used as it avoids needingn to pass the
+    #   ipc prim to every method
     @cm
-    def apply_to_ipc(
+    def wraps_ipc(
         self,
         ipc_prim: Context|MsgStream,
 
@@ -140,49 +144,50 @@ class PldRx(Struct):
         exit.
 
         '''
-        orig_dec: MsgDec = self._pldec
+        orig_dec: MsgDec = self._pld_dec
         limit_dec: MsgDec = mk_dec(spec=spec)
         try:
-            self._pldec = limit_dec
+            self._pld_dec = limit_dec
             yield limit_dec
         finally:
-            self._pldec = orig_dec
+            self._pld_dec = orig_dec
 
     @property
     def dec(self) -> msgpack.Decoder:
-        return self._pldec.dec
+        return self._pld_dec.dec
 
     def recv_pld_nowait(
         self,
         # TODO: make this `MsgStream` compat as well, see above^
         # ipc_prim: Context|MsgStream,
-        ctx: Context,
+        ipc: Context|MsgStream,
 
         ipc_msg: MsgType|None = None,
         expect_msg: Type[MsgType]|None = None,
-
+        hide_tb: bool = False,
         **dec_msg_kwargs,
 
     ) -> Any|Raw:
-        __tracebackhide__: bool = True
+        __tracebackhide__: bool = hide_tb
 
         msg: MsgType = (
             ipc_msg
             or
 
             # sync-rx msg from underlying IPC feeder (mem-)chan
-            ctx._rx_chan.receive_nowait()
+            ipc._rx_chan.receive_nowait()
         )
         return self.dec_msg(
             msg,
-            ctx=ctx,
+            ipc=ipc,
             expect_msg=expect_msg,
+            hide_tb=hide_tb,
             **dec_msg_kwargs,
         )
 
     async def recv_pld(
         self,
-        ctx: Context,
+        ipc: Context|MsgStream,
         ipc_msg: MsgType|None = None,
         expect_msg: Type[MsgType]|None = None,
         hide_tb: bool = True,
@@ -200,11 +205,11 @@ class PldRx(Struct):
             or
 
             # async-rx msg from underlying IPC feeder (mem-)chan
-            await ctx._rx_chan.receive()
+            await ipc._rx_chan.receive()
         )
         return self.dec_msg(
             msg=msg,
-            ctx=ctx,
+            ipc=ipc,
             expect_msg=expect_msg,
             **dec_msg_kwargs,
         )
@@ -212,7 +217,7 @@ class PldRx(Struct):
     def dec_msg(
         self,
         msg: MsgType,
-        ctx: Context,
+        ipc: Context|MsgStream,
         expect_msg: Type[MsgType]|None,
 
         raise_error: bool = True,
@@ -225,6 +230,9 @@ class PldRx(Struct):
 
         '''
         __tracebackhide__: bool = hide_tb
+
+        _src_err = None
+        src_err: BaseException|None = None
         match msg:
             # payload-data shuttle msg; deliver the `.pld` value
             # directly to IPC (primitive) client-consumer code.
@@ -234,7 +242,7 @@ class PldRx(Struct):
                 |Return(pld=pld)  # termination phase
             ):
                 try:
-                    pld: PayloadT = self._pldec.decode(pld)
+                    pld: PayloadT = self._pld_dec.decode(pld)
                     log.runtime(
                         'Decoded msg payload\n\n'
                         f'{msg}\n\n'
@@ -243,25 +251,30 @@ class PldRx(Struct):
                     )
                     return pld
 
-                # XXX pld-type failure
+                # XXX pld-value type failure
-                except ValidationError as src_err:
+                except ValidationError as valerr:
+                    # pack mgterr into error-msg for
+                    # reraise below; ensure remote-actor-err
+                    # info is displayed nicely?
                     msgterr: MsgTypeError = _mk_msg_type_err(
                         msg=msg,
                         codec=self.pld_dec,
-                        src_validation_error=src_err,
+                        src_validation_error=valerr,
                         is_invalid_payload=True,
                     )
                     msg: Error = pack_from_raise(
                         local_err=msgterr,
                         cid=msg.cid,
-                        src_uid=ctx.chan.uid,
+                        src_uid=ipc.chan.uid,
                     )
+                    src_err = valerr
 
                 # XXX some other decoder specific failure?
                 # except TypeError as src_error:
                 #     from .devx import mk_pdb
                 #     mk_pdb().set_trace()
                 #     raise src_error
+                # ^-TODO-^ can remove?
 
             # a runtime-internal RPC endpoint response.
             # always passthrough since (internal) runtime
@@ -299,6 +312,7 @@ class PldRx(Struct):
                     return src_err
 
             case Stop(cid=cid):
+                ctx: Context = getattr(ipc, 'ctx', ipc)
                 message: str = (
                     f'{ctx.side!r}-side of ctx received stream-`Stop` from '
                     f'{ctx.peer_side!r} peer ?\n'
@@ -341,14 +355,21 @@ class PldRx(Struct):
         # |_https://docs.python.org/3.11/library/exceptions.html#BaseException.add_note
         #
         # fallthrough and raise from `src_err`
-        _raise_from_unexpected_msg(
+        try:
-            ctx=ctx,
+            _raise_from_unexpected_msg(
-            msg=msg,
+                ctx=getattr(ipc, 'ctx', ipc),
-            src_err=src_err,
+                msg=msg,
-            log=log,
+                src_err=src_err,
-            expect_msg=expect_msg,
+                log=log,
-            hide_tb=hide_tb,
+                expect_msg=expect_msg,
-        )
+                hide_tb=hide_tb,
+            )
+        except UnboundLocalError:
+            # XXX if there's an internal lookup error in the above
+            # code (prolly on `src_err`) we want to show this frame
+            # in the tb!
+            __tracebackhide__: bool = False
+            raise
 
     async def recv_msg_w_pld(
         self,
@@ -378,52 +399,13 @@ class PldRx(Struct):
         # msg instance?
         pld: PayloadT = self.dec_msg(
             msg,
-            ctx=ipc,
+            ipc=ipc,
             expect_msg=expect_msg,
             **kwargs,
         )
         return msg, pld
 
 
-# Always maintain a task-context-global `PldRx`
-_def_pld_rx: PldRx = PldRx(
-    _pldec=_def_any_pldec,
-)
-_ctxvar_PldRx: ContextVar[PldRx] = ContextVar(
-    'pld_rx',
-    default=_def_pld_rx,
-)
-
-
-def current_pldrx() -> PldRx:
-    '''
-    Return the current `trio.Task.context`'s msg-payload-receiver.
-
-    A payload receiver is the IPC-msg processing sub-sys which
-    filters inter-actor-task communicated payload data, i.e. the
-    `PayloadMsg.pld: PayloadT` field value, AFTER it's container
-    shuttlle msg (eg. `Started`/`Yield`/`Return) has been delivered
-    up from `tractor`'s transport layer but BEFORE the data is
-    yielded to application code, normally via an IPC primitive API
-    like, for ex., `pld_data: PayloadT = MsgStream.receive()`.
-
-    Modification of the current payload spec via `limit_plds()`
-    allows a `tractor` application to contextually filter IPC
-    payload content with a type specification as supported by
-    the interchange backend.
-
-    - for `msgspec` see <PUTLINKHERE>.
-
-    NOTE that the `PldRx` itself is a per-`Context` global sub-system
-    that normally does not change other then the applied pld-spec
-    for the current `trio.Task`.
-
-    '''
-    # ctx: context = current_ipc_ctx()
-    # return ctx._pld_rx
-    return _ctxvar_PldRx.get()
-
-
 @cm
 def limit_plds(
     spec: Union[Type[Struct]],
@@ -439,29 +421,55 @@ def limit_plds(
     '''
     __tracebackhide__: bool = True
     try:
-        # sanity on orig settings
+        curr_ctx: Context = current_ipc_ctx()
-        orig_pldrx: PldRx = current_pldrx()
+        rx: PldRx = curr_ctx._pld_rx
-        orig_pldec: MsgDec = orig_pldrx.pld_dec
+        orig_pldec: MsgDec = rx.pld_dec
 
-        with orig_pldrx.limit_plds(
+        with rx.limit_plds(
             spec=spec,
             **kwargs,
         ) as pldec:
-            log.info(
+            log.runtime(
                 'Applying payload-decoder\n\n'
                 f'{pldec}\n'
             )
             yield pldec
     finally:
-        log.info(
+        log.runtime(
             'Reverted to previous payload-decoder\n\n'
             f'{orig_pldec}\n'
         )
-        assert (
+        # sanity on orig settings
-            (pldrx := current_pldrx()) is orig_pldrx
+        assert rx.pld_dec is orig_pldec
-            and
+
-            pldrx.pld_dec is orig_pldec
+
-        )
+@acm
+async def maybe_limit_plds(
+    ctx: Context,
+    spec: Union[Type[Struct]]|None = None,
+    **kwargs,
+) -> MsgDec|None:
+    '''
+    Async compat maybe-payload type limiter.
+
+    Mostly for use inside other internal `@acm`s such that a separate
+    indent block isn't needed when an async one is already being
+    used.
+
+    '''
+    if spec is None:
+        yield None
+        return
+
+    # sanity on scoping
+    curr_ctx: Context = current_ipc_ctx()
+    assert ctx is curr_ctx
+
+    with ctx._pld_rx.limit_plds(spec=spec) as msgdec:
+        yield msgdec
+
+    curr_ctx: Context = current_ipc_ctx()
+    assert ctx is curr_ctx
 
 
 async def drain_to_final_msg(
@@ -543,21 +551,12 @@ async def drain_to_final_msg(
         match msg:
 
             # final result arrived!
-            case Return(
+            case Return():
-                # cid=cid,
-                # pld=res,
-            ):
-                # ctx._result: Any = res
-                ctx._result: Any = pld
                 log.runtime(
                     'Context delivered final draining msg:\n'
                     f'{pretty_struct.pformat(msg)}'
                 )
-                # XXX: only close the rx mem chan AFTER
+                ctx._result: Any = pld
-                # a final result is retreived.
-                # if ctx._rx_chan:
-                #     await ctx._rx_chan.aclose()
-                # TODO: ^ we don't need it right?
                 result_msg = msg
                 break
 
@@ -664,24 +663,6 @@ async def drain_to_final_msg(
                     result_msg = msg
                     break  # OOOOOF, yeah obvi we need this..
 
-                # XXX we should never really get here
-                # right! since `._deliver_msg()` should
-                # always have detected an {'error': ..}
-                # msg and already called this right!?!
-                # elif error := unpack_error(
-                #     msg=msg,
-                #     chan=ctx._portal.channel,
-                #     hide_tb=False,
-                # ):
-                #     log.critical('SHOULD NEVER GET HERE!?')
-                #     assert msg is ctx._cancel_msg
-                #     assert error.msgdata == ctx._remote_error.msgdata
-                #     assert error.ipc_msg == ctx._remote_error.ipc_msg
-                #     from .devx._debug import pause
-                #     await pause()
-                #     ctx._maybe_cancel_and_set_remote_error(error)
-                #     ctx._maybe_raise_remote_err(error)
-
                 else:
                     # bubble the original src key error
                     raise