Use `enable_transports: list[str]` parameter

Actually applying the input it in the root as well as all sub-actors by
passing it down to sub-actors through runtime-vars as delivered by the
initial `SpawnSpec` msg during child runtime init.

Impl deats,
- add a new `_state._runtime_vars['_enable_tpts']: list[str]` field set
  by the input param (if provided) to `.open_root_actor()`.
- mk `current_ipc_protos()` return the runtime-var entry with instead
  the default in the `_runtime_vars: dict` set to `[_def_tpt_proto]`.
- in `.open_root_actor()`, still error on this being a >1 `list[str]`
  until we have more testing infra/suites to audit multi-protos per
  actor.
- return the new value (as 3rd element) from `Actor._from_parent()` as
  per the todo note; means `_runtime.async_main()` will allocate
  `accept_addrs` as tpt-specific `Address` entries and pass them to
  `IPCServer.listen_on()`.

Also,
- also add a new `_state._runtime_vars['_root_addrs']: list = []` field
  with the intent of fully replacing the `'_root_mailbox'` field since,
  * it will need to be a collection to support multi-tpt,
  * it's a more cohesive field name alongside `_registry_addrs`,
  * the root actor of every tree needs to have a dedicated addr set
    (separate from any host-singleton registry actor) so that all its
    subs can contact it for capabilities mgmt including debugger
    access/locking.
- in the root, populate the field in `._runtime.async_main()` and for
  now just set '_root_mailbox' to the first entry in that list in
  anticipation of future multi-homing/transport support.

tractor/_root.py

@@ -224,11 +224,16 @@ async def open_root_actor(
     ):
         if enable_transports is None:
             enable_transports: list[str] = _state.current_ipc_protos()
+        else:
+            _state._runtime_vars['_enable_tpts'] = enable_transports
 
-            # TODO! support multi-tpts per actor! Bo
-            assert (
-                len(enable_transports) == 1
-            ), 'No multi-tpt support yet!'
+        # TODO! support multi-tpts per actor!
+        # Bo
+        if not len(enable_transports) == 1:
+            raise RuntimeError(
+                f'No multi-tpt support yet!\n'
+                f'enable_transports={enable_transports!r}\n'
+            )
 
         _frame_stack.hide_runtime_frames()
         __tracebackhide__: bool = hide_tb

tractor/_runtime.py

@@ -914,9 +914,7 @@ class Actor:
             return (
                 chan,
                 accept_addrs,
-                None,
-                # ^TODO, preferred tpts list from rent!
-                # -[ ] need to extend the `SpawnSpec` tho!
+                _state._runtime_vars['_enable_tpts']
             )
 
         # failed to connect back?
@@ -1496,10 +1494,12 @@ async def async_main(
                 # all sub-actors should be able to speak to
                 # their root actor over that channel.
                 if _state._runtime_vars['_is_root']:
+                    raddrs: list[Address] = _state._runtime_vars['_root_addrs']
                     for addr in accept_addrs:
                         waddr: Address = wrap_address(addr)
-                        if waddr == waddr.get_root():
-                            _state._runtime_vars['_root_mailbox'] = addr
+                        raddrs.append(addr)
+                    else:
+                        _state._runtime_vars['_root_mailbox'] = raddrs[0]
 
                 # Register with the arbiter if we're told its addr
                 log.runtime(

tractor/_state.py

@@ -37,6 +37,13 @@ if TYPE_CHECKING:
     from ._context import Context
 
 
+# default IPC transport protocol settings
+TransportProtocolKey = Literal[
+    'tcp',
+    'uds',
+]
+_def_tpt_proto: TransportProtocolKey = 'tcp'
+
 _current_actor: Actor|None = None  # type: ignore # noqa
 _last_actor_terminated: Actor|None = None
 
@@ -47,6 +54,10 @@ _runtime_vars: dict[str, Any] = {
     # root of actor-process tree info
     '_is_root': False,  # bool
     '_root_mailbox': (None, None),  # tuple[str|None, str|None]
+    '_root_addrs': [],  # tuple[str|None, str|None]
+
+    # parent->chld ipc protocol caps
+    '_enable_tpts': [_def_tpt_proto],
 
     # registrar info
     '_registry_addrs': [],
@@ -180,14 +191,6 @@ def get_rt_dir(
     return rtdir
 
 
-# default IPC transport protocol settings
-TransportProtocolKey = Literal[
-    'tcp',
-    'uds',
-]
-_def_tpt_proto: TransportProtocolKey = 'tcp'
-
-
 def current_ipc_protos() -> list[str]:
     '''
     Return the list of IPC transport protocol keys currently
@@ -197,4 +200,4 @@ def current_ipc_protos() -> list[str]:
     concrete-backend sub-types defined throughout `tractor.ipc`.
 
     '''
-    return [_def_tpt_proto]
+    return _runtime_vars['_enable_tpts']

tractor/msg/types.py

@@ -170,6 +170,7 @@ class SpawnSpec(
     # a hard `Struct` def for all of these fields!
     _parent_main_data: dict
     _runtime_vars: dict[str, Any]
+    # ^NOTE see `._state._runtime_vars: dict`
 
     # module import capability
     enable_modules: dict[str, str]