Add `enable_transports`/`registry_addrs` proto guard

Raise `ValueError` from `open_root_actor()` when any
`registry_addrs` entry uses a transport proto not in
`enable_transports` — historically this caused a
silent indefinite hang during the registrar handshake
(the actor could never connect to register/discover).

Also,
- update `test_root_passes_tpt_to_sub` to detect a
  proto mismatch between parametrized `tpt_proto_key`
  and CLI `tpt_proto`, asserting the new guard raises
  `ValueError` with expected msg content.
- replace old commented-out notes with a clearer
  explanation of the mismatch foot-gun.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/ipc/test_multi_tpt.py

@@ -59,15 +59,18 @@ async def chk_tpts(
 )
 def test_root_passes_tpt_to_sub(
     tpt_proto_key: str,
+    tpt_proto: str,
     reg_addr: tuple,
     debug_mode: bool,
 ):
-    # XXX NOTE, the `reg_addr` addr won't be the same type as the
-    # `tpt_proto_key` would deliver here unless you pass `--tpt-proto
-    # <tpt_proto_key>` on the CLI.
-    #
-    # if tpt_proto_key == 'uds':
-    #     breakpoint()
+    # `reg_addr` is sourced from the CLI `--tpt-proto={tpt_proto}`,
+    # so when the parametrized `tpt_proto_key` differs, the test
+    # asks the runtime to `enable_transports=[<other_proto>]` while
+    # pointing `registry_addrs` at a `reg_addr` of the wrong proto.
+    # The layer-2 guard in `open_root_actor` is expected to fail
+    # fast with `ValueError` on this mismatch (rather than the prior
+    # silent hang during the registrar handshake).
+    proto_mismatch: bool = (tpt_proto_key != tpt_proto)
 
     async def main():
         async with tractor.open_nursery(
@@ -99,4 +102,14 @@ def test_root_passes_tpt_to_sub(
             # shudown sub-actor(s)
             await an.cancel()
 
+    if proto_mismatch:
+        # mismatched proto must raise `ValueError` from the
+        # `open_root_actor` runtime guard before any subactor spawn.
+        with pytest.raises(ValueError) as excinfo:
+            trio.run(main)
+        msg: str = str(excinfo.value)
+        assert 'enable_transports' in msg
+        assert 'registry_addrs' in msg
+        assert tpt_proto_key in msg or tpt_proto in msg
+    else:
         trio.run(main)

tractor/_root.py

@@ -371,6 +371,34 @@ async def open_root_actor(
             for uw_addr in uw_reg_addrs
         ]
 
+        # fail-fast on `enable_transports` / `registry_addrs` proto
+        # mismatch — historically this caused a silent indefinite
+        # hang during the registrar handshake (registry was reachable
+        # only via a transport not in `enable_transports`, so the
+        # actor could never connect to register/discover). See
+        # `tests/ipc/test_multi_tpt.py::test_root_passes_tpt_to_sub`
+        # for the foot-gun case + its layer-1 skip-guard.
+        bad_addrs: list[tuple[str, Address]] = [
+            (addr.proto_key, addr)
+            for addr in registry_addrs
+            if addr.proto_key not in enable_transports
+        ]
+        if bad_addrs:
+            raise ValueError(
+                f'`registry_addrs` contains addr(s) whose proto is '
+                f'not in `enable_transports`!\n'
+                f'enable_transports: {enable_transports!r}\n'
+                f'mismatched_addrs:\n'
+                + '\n'.join(
+                    f'  - proto_key={pk!r}  addr={a!r}'
+                    for pk, a in bad_addrs
+                )
+                + '\n\n'
+                f'Either add the missing proto to '
+                f'`enable_transports`, or remove the addr from '
+                f'`registry_addrs`.'
+            )
+
         # Debug-mode is currently only supported for backends whose
         # subactor root runtime is trio-native (so `tractor.devx.
         # debug._tty_lock` works). See `_DEBUG_COMPATIBLE_BACKENDS`