Don't kill root's immediate children when in debug

If the root calls `trio.Process.kill()` on immediate child proc teardown
when the child is using pdb, we can get stdstreams clobbering that
results in a pdb++ repl where the user can't see what's been typed. Not
killing such children on cancellation / error seems to resolve this
issue whilst still giving reliable termination. For now, code that
special path until a time it becomes a problem for ensuring zombie
reaps.

tractor/_spawn.py

@@ -22,7 +22,13 @@ from multiprocessing import forkserver  # type: ignore
 from typing import Tuple
 
 from . import _forkserver_override
-from ._state import current_actor, is_main_process
+from ._state import (
+    current_actor,
+    is_main_process,
+    is_root_process,
+    _runtime_vars,
+)
+
 from .log import get_logger
 from ._portal import Portal
 from ._actor import Actor, ActorFailure
@@ -180,23 +186,45 @@ async def spawn_subactor(
     proc = await trio.open_process(spawn_cmd)
     try:
         yield proc
+
     finally:
+        log.debug(f"Attempting to kill {proc}")
+
         # XXX: do this **after** cancellation/tearfown
         # to avoid killing the process too early
         # since trio does this internally on ``__aexit__()``
 
-        # NOTE: we always "shield" join sub procs in
-        # the outer scope since no actor zombies are
-        # ever allowed. This ``__aexit__()`` also shields
-        # internally.
-        log.debug(f"Attempting to kill {proc}")
+        if (
+            is_root_process()
 
-        # NOTE: this timeout effectively does nothing right now since
-        # we are shielding the ``.wait()`` inside ``new_proc()`` which
-        # will pretty much never release until the process exits.
+            # XXX: basically the pre-closing of stdstreams in a
+            # root-processe's ``trio.Process.aclose()`` can clobber
+            # any existing debugger session so we avoid
+            and _runtime_vars['_debug_mode']
+        ):
+            # XXX: this is ``trio.Process.aclose()`` minus
+            # the std-streams pre-closing steps and ``Process.kill()``
+            # calls.
+            try:
+                await proc.wait()
+            finally:
+                if proc.returncode is None:
+                    # XXX: skip this when in debug and a session might
+                    # still be live
+                    # proc.kill()
+                    with trio.CancelScope(shield=True):
+                        await proc.wait()
+        else:
+            # NOTE: this timeout used to do nothing since we were shielding
+            # the ``.wait()`` inside ``new_proc()`` which will pretty much
+            # never release until the process exits, now it acts as
+            # a hard-kill time ultimatum.
             with trio.move_on_after(3) as cs:
-            async with proc:
+
+                # NOTE: This ``__aexit__()`` shields internally.
+                async with proc:  # calls ``trio.Process.aclose()``
                     log.debug(f"Terminating {proc}")
+
             if cs.cancelled_caught:
                 log.critical(f"HARD KILLING {proc}")
                 proc.kill()
@@ -212,7 +240,6 @@ async def new_proc(
     parent_addr: Tuple[str, int],
     _runtime_vars: Dict[str, Any],  # serialized and sent to _child
     *,
-    use_trio_run_in_process: bool = False,
     task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED
 ) -> None:
     """Create a new ``multiprocessing.Process`` using the
@@ -223,7 +250,7 @@ async def new_proc(
     # mark the new actor with the global spawn method
     subactor._spawn_method = _spawn_method
 
-    if use_trio_run_in_process or _spawn_method == 'trio':
+    if _spawn_method == 'trio':
         async with trio.open_nursery() as nursery:
             async with spawn_subactor(
                 subactor,
@@ -320,7 +347,6 @@ async def mp_new_proc(
     parent_addr: Tuple[str, int],
     _runtime_vars: Dict[str, Any],  # serialized and sent to _child
     *,
-    use_trio_run_in_process: bool = False,
     task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED
 
 ) -> None: