Expose `MsgType` and extend `MsgCodec` API a bit

Make a new `MsgType: TypeAlias` for the union of all msg types such that
it can be used in annots throughout the code base; just make
`.msg.__msg_spec__` delegate to it.

Add some new codec methods:
- `pld_spec_str`: for the `str`-casted value of the payload spec,
  generally useful in logging content.
- `msg_spec_items()`: to render a `dict` of msg types to their
  `str()`-casted values with support for singling out a specific
  `MsgType`, type by input `msg` instance.
- `pformat_msg_spec()`: for rendering the (partial) `.msg_spec` as
  a formatted `str` useful in logging.

Oh right, add a `Error._msg_dict: dict` in support of the previous
commit (for `MsgTypeError` packing as RAEs) such that our error msg type
can house a non-type-spec decoded wire-bytes for error
reporting/analysis purposes.

tractor/msg/__init__.py

@@ -19,7 +19,6 @@ Built-in messaging patterns, types, APIs and helpers.
 
 '''
 from typing import (
-    Union,
     TypeAlias,
 )
 from .ptr import (
@@ -56,8 +55,9 @@ from .types import (
 
     # full msg class set from above as list
     __msg_types__ as __msg_types__,
+
+    # type-alias for union of all msgs
+    MsgType as MsgType,
 )
-# TODO: use new type declaration syntax for msg-type-spec
-# https://docs.python.org/3/library/typing.html#type-aliases
-# https://docs.python.org/3/reference/simple_stmts.html#type
-__msg_spec__: TypeAlias = Union[*__msg_types__]
+
+__msg_spec__: TypeAlias = MsgType

tractor/msg/_codec.py

@@ -57,7 +57,7 @@ from trio.lowlevel import (
 from tractor.msg.pretty_struct import Struct
 from tractor.msg.types import (
     mk_msg_spec,
-    Msg,
+    MsgType,
 )
 
 
@@ -87,12 +87,50 @@ class MsgCodec(Struct):
 
     pld_spec: Union[Type[Struct]]|None
 
+    @property
+    def pld_spec_str(self) -> str:
+        spec: Union[Type]|Type = self.pld_spec
+
+        # TODO: could also use match: instead?
+        if getattr(spec, '__args__', False):
+            # `typing.Union` case
+            return str(spec)
+        else:
+            return spec.__name__
+
     # struct type unions
     # https://jcristharif.com/msgspec/structs.html#tagged-unions
     @property
     def msg_spec(self) -> Union[Type[Struct]]:
         return self._dec.type
 
+    def msg_spec_items(
+        self,
+        msg: MsgType|None = None,
+
+    ) -> dict[str, MsgType]|str:
+
+        msgt_table: dict[str, MsgType] = {
+            msgt: str(msgt)
+            for msgt in self.msg_spec.__args__
+        }
+        if msg:
+            msgt: MsgType = type(msg)
+            str_repr: str = msgt_table[msgt]
+            return {msgt: str_repr}
+
+        return msgt_table
+
+    # TODO: some way to make `pretty_struct.Struct` use this
+    # wrapped field over the `.msg_spec` one?
+    def pformat_msg_spec(
+        self,
+        msg: MsgType|None = None,
+    ) -> str:
+        return '\n'.join(
+            self.msg_spec_items(msg=msg).values()
+        )
+
     lib: ModuleType = msgspec
 
     # TODO: a sub-decoder system as well?
@@ -108,7 +146,7 @@ class MsgCodec(Struct):
     # OR
     # ) = {
     #     # pre-seed decoders for std-py-type-set for use when
-    #     # `Msg.pld == None|Any`.
+    #     # `MsgType.pld == None|Any`.
     #     None: msgpack.Decoder(Any),
     #     Any: msgpack.Decoder(Any),
     # }
@@ -303,7 +341,7 @@ def mk_codec(
     # by `tag_field: str` value key?
     # payload_msg_specs: dict[
     #     str,  # tag_field value as sub-decoder key
-    #     Union[Type[Struct]]  # `Msg.pld` type spec
+    #     Union[Type[Struct]]  # `MsgType.pld` type spec
     # ]|None = None,
 
     libname: str = 'msgspec',
@@ -336,7 +374,7 @@ def mk_codec(
         raise RuntimeError(
             f'If a payload spec is provided,\n'
             "the builtin SC-shuttle-protocol's msg set\n"
-            f'(i.e. `{Msg}`) MUST be used!\n\n'
+            f'(i.e. a `{MsgType}`) MUST be used!\n\n'
             f'However both values were passed as => mk_codec(\n'
             f'   ipc_msg_spec={ipc_msg_spec}`\n'
             f'   ipc_pld_spec={ipc_pld_spec}`\n)\n'

tractor/msg/types.py

@@ -31,6 +31,7 @@ from typing import (
     Literal,
     Type,
     TypeVar,
+    TypeAlias,
     Union,
 )
 
@@ -400,16 +401,29 @@ class CancelAck(
     pld: bool
 
 
+# TODO: unify this with `._exceptions.RemoteActorError`
+# such that we can have a msg which is both raisable and
+# IPC-wire ready?
+# B~o
 class Error(
     Struct,
     tag=True,
     tag_field='msg_type',
+
+    # TODO may omit defaults?
+    # https://jcristharif.com/msgspec/structs.html#omitting-default-values
+    # omit_defaults=True,
 ):
     '''
     A pkt that wraps `RemoteActorError`s for relay and raising.
 
     Fields are 1-to-1 meta-data as needed originally by
-    `RemoteActorError.msgdata: dict`.
+    `RemoteActorError.msgdata: dict` but now are defined here.
+
+    Note: this msg shuttles `ContextCancelled` and `StreamOverrun`
+    as well is used to rewrap any `MsgTypeError` for relay-reponse
+    to bad `Yield.pld` senders during an IPC ctx's streaming dialog
+    phase.
 
     '''
     src_uid: tuple[str, str]
@@ -428,6 +442,10 @@ class Error(
     # `StreamOverrun`
     sender: tuple[str, str]|None = None
 
+    # for the `MsgTypeError` case where the receiver side
+    # decodes the underlying original `Msg`-subtype
+    _msg_dict: dict|None = None
+
 
 # TODO: should be make a msg version of `ContextCancelled?`
 # and/or with a scope field or a full `ActorCancelled`?
@@ -486,6 +504,11 @@ __msg_types__: list[Msg] = (
     _payload_msgs
 )
 
+# TODO: use new type declaration syntax for msg-type-spec
+# https://docs.python.org/3/library/typing.html#type-aliases
+# https://docs.python.org/3/reference/simple_stmts.html#type
+MsgType: TypeAlias = Union[*__msg_types__]
+
 
 def mk_msg_spec(
     payload_type_union: Union[Type] = Any,