Factor `breakpoint()` blocking into `@acm`

Call it `maybe_block_bp()` can wrap the `open_root_actor()` body with
it. Main reason is to guarantee we can bp inside actor runtime bootup as
needed when debugging internals! Prolly should factor this to another
module tho?

ALSO, ensure we RTE on recurrent entries to `open_root_actor()` from
within an existing tree! There was actually `test_spawning` test somehow
getting away with this!? Should never be possible or allowed!

tractor/_root.py

@@ -18,7 +18,9 @@
 Root actor runtime ignition(s).
 
 '''
-from contextlib import asynccontextmanager as acm
+from contextlib import (
+    asynccontextmanager as acm,
+)
 from functools import partial
 import importlib
 import inspect
@@ -26,7 +28,10 @@ import logging
 import os
 import signal
 import sys
-from typing import Callable
+from typing import (
+    Any,
+    Callable,
+)
 import warnings
 
 
@@ -47,21 +52,95 @@ from .ipc import (
     _connect_chan,
 )
 from ._addr import (
+    Address,
     UnwrappedAddress,
     default_lo_addrs,
     mk_uuid,
     preferred_transport,
     wrap_address,
 )
-from ._exceptions import is_multi_cancelled
+from ._exceptions import (
+    ActorFailure,
+    is_multi_cancelled,
+)
 
 
 logger = log.get_logger('tractor')
 
 
+# TODO: stick this in a `@acm` defined in `devx._debug`?
+# -[ ] also maybe consider making this a `wrapt`-deco to
+#     save an indent level?
+#
+@acm
+async def maybe_block_bp(
+    debug_mode: bool,
+    maybe_enable_greenback: bool,
+) -> bool:
+    # Override the global debugger hook to make it play nice with
+    # ``trio``, see much discussion in:
+    # https://github.com/python-trio/trio/issues/1155#issuecomment-742964018
+    builtin_bp_handler: Callable = sys.breakpointhook
+    orig_bp_path: str|None = os.environ.get(
+        'PYTHONBREAKPOINT',
+        None,
+    )
+    bp_blocked: bool
+    if (
+        debug_mode
+        and maybe_enable_greenback
+        and (
+            maybe_mod := await _debug.maybe_init_greenback(
+                raise_not_found=False,
+            )
+        )
+    ):
+        logger.info(
+            f'Found `greenback` installed @ {maybe_mod}\n'
+            'Enabling `tractor.pause_from_sync()` support!\n'
+        )
+        os.environ['PYTHONBREAKPOINT'] = (
+            'tractor.devx._debug._sync_pause_from_builtin'
+        )
+        _state._runtime_vars['use_greenback'] = True
+        bp_blocked = False
+
+    else:
+        # TODO: disable `breakpoint()` by default (without
+        # `greenback`) since it will break any multi-actor
+        # usage by a clobbered TTY's stdstreams!
+        def block_bps(*args, **kwargs):
+            raise RuntimeError(
+                'Trying to use `breakpoint()` eh?\n\n'
+                'Welp, `tractor` blocks `breakpoint()` built-in calls by default!\n'
+                'If you need to use it please install `greenback` and set '
+                '`debug_mode=True` when opening the runtime '
+                '(either via `.open_nursery()` or `open_root_actor()`)\n'
+            )
+
+        sys.breakpointhook = block_bps
+        # lol ok,
+        # https://docs.python.org/3/library/sys.html#sys.breakpointhook
+        os.environ['PYTHONBREAKPOINT'] = "0"
+        bp_blocked = True
+
+    try:
+        yield bp_blocked
+    finally:
+        # restore any prior built-in `breakpoint()` hook state
+        if builtin_bp_handler is not None:
+            sys.breakpointhook = builtin_bp_handler
+
+        if orig_bp_path is not None:
+            os.environ['PYTHONBREAKPOINT'] = orig_bp_path
+
+        else:
+            # clear env back to having no entry
+            os.environ.pop('PYTHONBREAKPOINT', None)
+
+
 @acm
 async def open_root_actor(
-
     *,
     # defaults are above
     registry_addrs: list[UnwrappedAddress]|None = None,
@@ -111,55 +190,30 @@ async def open_root_actor(
     Runtime init entry point for ``tractor``.
 
     '''
+    # XXX NEVER allow nested actor-trees!
+    if already_actor := _state.current_actor(err_on_no_runtime=False):
+        rtvs: dict[str, Any] = _state._runtime_vars
+        root_mailbox: list[str, int] = rtvs['_root_mailbox']
+        registry_addrs: list[list[str, int]] = rtvs['_registry_addrs']
+        raise ActorFailure(
+            f'A current actor already exists !?\n'
+            f'({already_actor}\n'
+            f'\n'
+            f'You can NOT open a second root actor from within '
+            f'an existing tree and the current root of this '
+            f'already exists !!\n'
+            f'\n'
+            f'_root_mailbox: {root_mailbox!r}\n'
+            f'_registry_addrs: {registry_addrs!r}\n'
+        )
+
+    async with maybe_block_bp(
+        debug_mode=debug_mode,
+        maybe_enable_greenback=maybe_enable_greenback,
+    ):
         _debug.hide_runtime_frames()
         __tracebackhide__: bool = hide_tb
 
-    # TODO: stick this in a `@cm` defined in `devx._debug`?
-    #
-    # Override the global debugger hook to make it play nice with
-    # ``trio``, see much discussion in:
-    # https://github.com/python-trio/trio/issues/1155#issuecomment-742964018
-    builtin_bp_handler: Callable = sys.breakpointhook
-    orig_bp_path: str|None = os.environ.get(
-        'PYTHONBREAKPOINT',
-        None,
-    )
-    if (
-        debug_mode
-        and maybe_enable_greenback
-        and (
-            maybe_mod := await _debug.maybe_init_greenback(
-                raise_not_found=False,
-            )
-        )
-    ):
-        logger.info(
-            f'Found `greenback` installed @ {maybe_mod}\n'
-            'Enabling `tractor.pause_from_sync()` support!\n'
-        )
-        os.environ['PYTHONBREAKPOINT'] = (
-            'tractor.devx._debug._sync_pause_from_builtin'
-        )
-        _state._runtime_vars['use_greenback'] = True
-
-    else:
-        # TODO: disable `breakpoint()` by default (without
-        # `greenback`) since it will break any multi-actor
-        # usage by a clobbered TTY's stdstreams!
-        def block_bps(*args, **kwargs):
-            raise RuntimeError(
-                'Trying to use `breakpoint()` eh?\n\n'
-                'Welp, `tractor` blocks `breakpoint()` built-in calls by default!\n'
-                'If you need to use it please install `greenback` and set '
-                '`debug_mode=True` when opening the runtime '
-                '(either via `.open_nursery()` or `open_root_actor()`)\n'
-            )
-
-        sys.breakpointhook = block_bps
-        # lol ok,
-        # https://docs.python.org/3/library/sys.html#sys.breakpointhook
-        os.environ['PYTHONBREAKPOINT'] = "0"
-
         # attempt to retreive ``trio``'s sigint handler and stash it
         # on our debugger lock state.
         _debug.DebugStatus._trio_handler = signal.getsignal(signal.SIGINT)
@@ -186,6 +240,7 @@ async def open_root_actor(
         if start_method is not None:
             _spawn.try_set_start_method(start_method)
 
+        # TODO! remove this ASAP!
         if arbiter_addr is not None:
             warnings.warn(
                 '`arbiter_addr` is now deprecated\n'
@@ -313,10 +368,9 @@ async def open_root_actor(
             # DO NOT use the registry_addrs as the transport server
             # addrs for this new non-registar, root-actor.
             for addr in ponged_addrs:
-            waddr = wrap_address(addr)
+                waddr: Address = wrap_address(addr)
-            print(waddr)
                 trans_bind_addrs.append(
-                waddr.get_random(namespace=waddr.namespace)
+                    waddr.get_random(bindspace=waddr.bindspace)
                 )
 
         # Start this local actor as the "registrar", aka a regular
@@ -419,7 +473,11 @@ async def open_root_actor(
                             err,
                         )
                     ):
-                    logger.exception('Root actor crashed\n')
+                        logger.exception(
+                            'Root actor crashed\n'
+                            f'>x)\n'
+                            f' |_{actor}\n'
+                        )
 
                     # ALWAYS re-raise any error bubbled up from the
                     # runtime!
@@ -436,30 +494,19 @@ async def open_root_actor(
                     #         tempn.start_soon(an.exited.wait)
 
                     logger.info(
-                    'Closing down root actor'
+                        f'Closing down root actor\n'
+                        f'>)\n'
+                        f'|_{actor}\n'
                     )
                     await actor.cancel(None)  # self cancel
         finally:
             _state._current_actor = None
             _state._last_actor_terminated = actor
-
+            logger.runtime(
-        # restore built-in `breakpoint()` hook state
+                f'Root actor terminated\n'
-        if (
+                f')>\n'
-            debug_mode
+                f' |_{actor}\n'
-            and
+            )
-            maybe_enable_greenback
-        ):
-            if builtin_bp_handler is not None:
-                sys.breakpointhook = builtin_bp_handler
-
-            if orig_bp_path is not None:
-                os.environ['PYTHONBREAKPOINT'] = orig_bp_path
-
-            else:
-                # clear env back to having no entry
-                os.environ.pop('PYTHONBREAKPOINT', None)
-
-        logger.runtime("Root actor terminated")
 
 
 def run_daemon(