Re-think, `msgspec`-multi-typed msg dialogs

The greasy details are strewn throughout a `msgspec` issue:
https://github.com/jcrist/msgspec/issues/140

and specifically this code was mostly written as part of POC example in
this comment:
https://github.com/jcrist/msgspec/issues/140#issuecomment-1177850792

This work obviously pertains to our desire and prep for typed messaging
and capabilities aware msg-oriented-protocols in #196, caps sec nods in

I added a "wants to have" method to `Context` showing how I think we
could offer a pretty neat msg-type-set-as-capability-for-protocol
system.

tractor/_streaming.py

@@ -699,6 +699,18 @@ class Context:
         await self.chan.send({'started': value, 'cid': self.cid})
         self._started_called = True
 
+
+    # TODO: msg capability context api1
+    # @acm
+    # async def enable_msg_caps(
+    #     self,
+    #     msg_subtypes: Union[
+    #         list[list[Struct]],
+    #         Protocol,   # hypothetical type that wraps a msg set
+    #     ],
+    # ) -> tuple[Callable, Callable]:  # payload enc, dec pair
+    #     ...
+
     # TODO: do we need a restart api?
     # async def restart(self) -> None:
     #     pass

tractor/msg.py

@@ -40,10 +40,19 @@ Built-in messaging patterns, types, APIs and helpers.
 #   - https://jcristharif.com/msgspec/api.html#struct
 #   - https://jcristharif.com/msgspec/extending.html
 # via ``msgpack-python``:
-# - https://github.com/msgpack/msgpack-python#packingunpacking-of-custom-data-type
+# https://github.com/msgpack/msgpack-python#packingunpacking-of-custom-data-type
 
 from __future__ import annotations
+from contextlib import contextmanager as cm
 from pkgutil import resolve_name
+from typing import Union, Any, Optional
+
+
+from msgspec import Struct, Raw
+from msgspec.msgpack import (
+    Encoder,
+    Decoder,
+)
 
 
 class NamespacePath(str):
@@ -78,3 +87,151 @@ class NamespacePath(str):
             (ref.__module__,
              getattr(ref, '__name__', ''))
         ))
+
+
+# LIFO codec stack that is appended when the user opens the
+# ``configure_native_msgs()`` cm below to configure a new codec set
+# which will be applied to all new (msgspec relevant) IPC transports
+# that are spawned **after** the configure call is made.
+_lifo_codecs: list[
+    tuple[
+        Encoder,
+        Decoder,
+    ],
+] = [(Encoder(), Decoder())]
+
+
+def get_msg_codecs() -> tuple[
+    Encoder,
+    Decoder,
+]:
+    '''
+    Return the currently configured ``msgspec`` codec set.
+
+    The defaults are defined above.
+
+    '''
+    global _lifo_codecs
+    return _lifo_codecs[-1]
+
+
+@cm
+def configure_native_msgs(
+    tagged_structs: list[Struct],
+):
+    '''
+    Push a codec set that will natively decode
+    tagged structs provied in ``tagged_structs``
+    in all IPC transports and pop the codec on exit.
+
+    '''
+    # See "tagged unions" docs:
+    # https://jcristharif.com/msgspec/structs.html#tagged-unions
+
+    # "The quickest way to enable tagged unions is to set tag=True when
+    # defining every struct type in the union. In this case tag_field
+    # defaults to "type", and tag defaults to the struct class name
+    # (e.g. "Get")."
+    enc = Encoder()
+
+    types_union = Union[tagged_structs[0]] | Any
+    for struct in tagged_structs[1:]:
+        types_union |= struct
+
+    dec = Decoder(types_union)
+
+    _lifo_codecs.append((enc, dec))
+    try:
+        print("YOYOYOOYOYOYOY")
+        yield enc, dec
+    finally:
+        print("NONONONONON")
+        _lifo_codecs.pop()
+
+
+class Header(Struct, tag=True):
+    '''
+    A msg header which defines payload properties
+
+    '''
+    uid: str
+    msgtype: Optional[str] = None
+
+
+class Msg(Struct, tag=True):
+    '''
+    The "god" msg type, a box for task level msg types.
+
+    '''
+    header: Header
+    payload: Raw
+
+
+_root_dec = Decoder(Msg)
+_root_enc = Encoder()
+
+# sub-decoders for retreiving embedded
+# payload data and decoding to a sender
+# side defined (struct) type.
+_subdecs:  dict[
+    Optional[str],
+    Decoder] = {
+    None: Decoder(Any),
+}
+
+
+@cm
+def enable_context(
+    msg_subtypes: list[list[Struct]]
+) -> Decoder:
+
+    for types in msg_subtypes:
+        first = types[0]
+
+        # register using the default tag_field of "type"
+        # which seems to map to the class "name".
+        tags = [first.__name__]
+
+        # create a tagged union decoder for this type set
+        type_union = Union[first]
+        for typ in types[1:]:
+            type_union |= typ
+            tags.append(typ.__name__)
+
+        dec = Decoder(type_union)
+
+        # register all tags for this union sub-decoder
+        for tag in tags:
+            _subdecs[tag] = dec
+        try:
+            yield dec
+        finally:
+            for tag in tags:
+                _subdecs.pop(tag)
+
+
+def decmsg(msg: Msg) -> Any:
+    msg = _root_dec.decode(msg)
+    tag_field = msg.header.msgtype
+    dec = _subdecs[tag_field]
+    return dec.decode(msg.payload)
+
+
+def encmsg(
+    dialog_id: str | int,
+    payload: Any,
+) -> Msg:
+
+    tag_field = None
+
+    plbytes = _root_enc.encode(payload)
+    if b'type' in plbytes:
+        assert isinstance(payload, Struct)
+        tag_field = type(payload).__name__
+        payload = Raw(plbytes)
+
+    msg = Msg(
+        Header(dialog_id, tag_field),
+        payload,
+    )
+    return _root_enc.encode(msg)