Support sync code breakpointing via built-in

Override `breakpoint()` for sync code making it work
properly with `trio` as per:

python-trio/trio#1155 (comment)

Relates to #193

tractor/_debug.py

@@ -275,14 +275,31 @@ def _mk_pdb():
     return pdb
 
 
-def _set_trace(actor):
-    log.runtime(f"\nAttaching pdb to actor: {actor.uid}\n")
-
+def _set_trace(actor=None):
     pdb = _mk_pdb()
-    pdb.set_trace(
-        # start 2 levels up in user code
-        frame=sys._getframe().f_back.f_back,
-    )
+
+    if actor is not None:
+        log.runtime(f"\nAttaching pdb to actor: {actor.uid}\n")
+
+        pdb.set_trace(
+            # start 2 levels up in user code
+            frame=sys._getframe().f_back.f_back,
+        )
+
+    else:
+        # we entered the global ``breakpoint()`` built-in from sync code
+        global _in_debug, _pdb_release_hook
+        _in_debug = 'sync'
+
+        def nuttin():
+            pass
+
+        _pdb_release_hook = nuttin
+
+        pdb.set_trace(
+            # start 2 levels up in user code
+            frame=sys._getframe().f_back,
+        )
 
 
 breakpoint = partial(

tractor/_entry.py

@@ -71,9 +71,6 @@ def _trio_main(
     # we don't need it thanks to our cancellation machinery.
     signal.signal(signal.SIGINT, signal.SIG_IGN)
 
-    # TODO: make a global func to set this or is it too hacky?
-    # os.environ['PYTHONBREAKPOINT'] = 'tractor._debug.breakpoint'
-
     log.info(f"Started new trio process for {actor.uid}")
 
     if actor.loglevel is not None:

tractor/_root.py

@@ -4,6 +4,7 @@
 from contextlib import asynccontextmanager
 from functools import partial
 import importlib
+import os
 from typing import Tuple, Optional, List, Any
 import typing
 import warnings
@@ -55,6 +56,11 @@ async def open_root_actor(
     """Async entry point for ``tractor``.
 
     """
+    # Override the global debugger hook to make it play nice with
+    # ``trio``, see:
+    # https://github.com/python-trio/trio/issues/1155#issuecomment-742964018
+    os.environ['PYTHONBREAKPOINT'] = 'tractor._debug._set_trace'
+
     # mark top most level process as root actor
     _state._runtime_vars['_is_root'] = True