Solve the root-cancels-child-in-tty-lock race

Finally this makes a cancelled root actor nursery not clobber child
tasks which request and lock the root's tty for the debugger repl.

Using an edge triggered event which is set after all fifo-lock-queued
tasks are complete, we can be sure that no lingering child tasks are
going to get interrupted during pdb use and tty lock acquisition.
Further, even if new tasks do queue up to get the lock, the root will
incrementally send cancel msgs to each sub-actor only once the tty is
not locked by a (set of) child request task(s). Add shielding around all
the critical sections where the child attempts to allocate the lock from
the root such that it won't be disrupted from cancel messages from the
root after the acquire lock transaction has started.

tractor/_debug.py

@@ -124,10 +124,11 @@ class PdbwTeardown(pdbpp.Pdb):
 
 @asynccontextmanager
 async def _acquire_debug_lock(uid: Tuple[str, str]) -> AsyncIterator[None]:
-    """Acquire a actor local FIFO lock meant to mutex entry to a local
+    '''Acquire a actor local FIFO lock meant to mutex entry to a local
-    debugger entry point to avoid tty clobbering by multiple processes.
+    debugger entry point to avoid tty clobbering a global root process.
-    """
+
-    global _debug_lock, _global_actor_in_debug
+    '''
+    global _debug_lock, _global_actor_in_debug, _no_remote_has_tty
 
     task_name = trio.lowlevel.current_task().name
 
@@ -135,14 +136,59 @@ async def _acquire_debug_lock(uid: Tuple[str, str]) -> AsyncIterator[None]:
         f"Attempting to acquire TTY lock, remote task: {task_name}:{uid}"
     )
 
-    async with _debug_lock:
+    we_acquired = False
+
+    if _no_remote_has_tty is None:
+        # mark the tty lock as being in use so that the runtime
+        # can try to avoid clobbering any connection from a child
+        # that's currently relying on it.
+        _no_remote_has_tty = trio.Event()
+
+    try:
+        log.debug(
+            f"entering lock checkpoint, remote task: {task_name}:{uid}"
+        )
+        we_acquired = True
+        await _debug_lock.acquire()
+
+        # we_acquired = True
 
         _global_actor_in_debug = uid
         log.debug(f"TTY lock acquired, remote task: {task_name}:{uid}")
 
-        yield
+        # NOTE: critical section!
+        # this yield is unshielded.
+        # IF we received a cancel during the shielded lock
+        # entry of some next-in-queue requesting task,
+        # then the resumption here will result in that
+        # Cancelled being raised to our caller below!
+
+        # in this case the finally below should trigger
+        # and the surrounding calle side context should cancel
+        # normally relaying back to the caller.
+
+        yield _debug_lock
+
+    finally:
+        # if _global_actor_in_debug == uid:
+        if we_acquired and _debug_lock.locked():
+            _debug_lock.release()
+
+        # IFF there are no more requesting tasks queued up fire, the
+        # "tty-unlocked" event thereby alerting any monitors of the lock that
+        # we are now back in the "tty unlocked" state. This is basically
+        # and edge triggered signal around an empty queue of sub-actor
+        # tasks that may have tried to acquire the lock.
+        stats = _debug_lock.statistics()
+        if (
+            not stats.owner
+        ):
+            log.pdb(f"No more tasks waiting on tty lock! says {uid}")
+            _no_remote_has_tty.set()
+            _no_remote_has_tty = None
 
         _global_actor_in_debug = None
+
         log.debug(f"TTY lock released, remote task: {task_name}:{uid}")
 
 
@@ -169,30 +215,22 @@ async def _hijack_stdin_relay_to_child(
     bossing.
 
     '''
-    global _no_remote_has_tty
-
-    # mark the tty lock as being in use so that the runtime
-    # can try to avoid clobbering any connection from a child
-    # that's currently relying on it.
-    _no_remote_has_tty = trio.Event()
-
     task_name = trio.lowlevel.current_task().name
 
     # TODO: when we get to true remote debugging
     # this will deliver stdin data?
 
     log.debug(
-        "Attempting to acquire TTY lock, "
+        "Attempting to acquire TTY lock\n"
         f"remote task: {task_name}:{subactor_uid}"
     )
 
     log.debug(f"Actor {subactor_uid} is WAITING on stdin hijack lock")
 
-    async with _acquire_debug_lock(subactor_uid):
-
-        # XXX: only shield the context sync step!
     with trio.CancelScope(shield=True):
 
+        async with _acquire_debug_lock(subactor_uid):
+
             # indicate to child that we've locked stdio
             await ctx.started('Locked')
             log.pdb(  # type: ignore
@@ -214,8 +252,6 @@ async def _hijack_stdin_relay_to_child(
     log.debug(
         f"TTY lock released, remote task: {task_name}:{subactor_uid}")
 
-    log.debug(f"Actor {subactor_uid} RELEASED stdin hijack lock")
-    _no_remote_has_tty.set()
     return "pdb_unlock_complete"
 
 
@@ -230,17 +266,21 @@ async def _breakpoint(debug_func) -> None:
     global _local_pdb_complete, _pdb_release_hook
     global _local_task_in_debug, _global_actor_in_debug
 
+    await trio.lowlevel.checkpoint()
+
     async def wait_for_parent_stdin_hijack(
         task_status=trio.TASK_STATUS_IGNORED
     ):
         global _debugger_request_cs
 
-        with trio.CancelScope() as cs:
+        with trio.CancelScope(shield=True) as cs:
             _debugger_request_cs = cs
 
             try:
                 async with get_root() as portal:
 
+                    log.error('got portal')
+
                     # this syncs to child's ``Context.started()`` call.
                     async with portal.open_context(
 
@@ -249,17 +289,21 @@ async def _breakpoint(debug_func) -> None:
 
                     ) as (ctx, val):
 
+                        log.error('locked context')
                         assert val == 'Locked'
 
                         async with ctx.open_stream() as stream:
 
+                            log.error('opened stream')
                             # unblock local caller
                             task_status.started()
 
-                            # TODO: shielding currently can cause hangs...
+                            try:
-                            # with trio.CancelScope(shield=True):
-
                                 await _local_pdb_complete.wait()
+
+                            finally:
+                                # TODO: shielding currently can cause hangs...
+                                with trio.CancelScope(shield=True):
                                     await stream.send('pdb_unlock')
 
                             # sync with callee termination
@@ -279,6 +323,7 @@ async def _breakpoint(debug_func) -> None:
 
     # TODO: need a more robust check for the "root" actor
     if actor._parent_chan and not is_root_process():
+
         if _local_task_in_debug:
             if _local_task_in_debug == task_name:
                 # this task already has the lock and is
@@ -303,6 +348,12 @@ async def _breakpoint(debug_func) -> None:
         # this **must** be awaited by the caller and is done using the
         # root nursery so that the debugger can continue to run without
         # being restricted by the scope of a new task nursery.
+
+        # NOTE: if we want to debug a trio.Cancelled triggered exception
+        # we have to figure out how to avoid having the service nursery
+        # cancel on this task start? I *think* this works below?
+        # actor._service_n.cancel_scope.shield = shield
+        with trio.CancelScope(shield=True):
             await actor._service_n.start(wait_for_parent_stdin_hijack)
 
     elif is_root_process():
@@ -320,6 +371,11 @@ async def _breakpoint(debug_func) -> None:
         # XXX: since we need to enter pdb synchronously below,
         # we have to release the lock manually from pdb completion
         # callbacks. Can't think of a nicer way then this atm.
+        if _debug_lock.locked():
+            log.warning(
+                'Root actor attempting to acquire active tty lock'
+                f' owned by {_global_actor_in_debug}')
+
         await _debug_lock.acquire()
 
         _global_actor_in_debug = actor.uid

tractor/_trionics.py

@@ -170,16 +170,25 @@ class ActorNursery:
 
         log.warning(f"Cancelling nursery in {self._actor.uid}")
         with trio.move_on_after(3) as cs:
+
             async with trio.open_nursery() as nursery:
+
                 for subactor, proc, portal in self._children.values():
+
+                    # TODO: are we ever even going to use this or
+                    # is the spawning backend responsible for such
+                    # things? I'm thinking latter.
                     if hard_kill:
                         proc.terminate()
+
                     else:
                         if portal is None:  # actor hasn't fully spawned yet
                             event = self._actor._peer_connected[subactor.uid]
                             log.warning(
                                 f"{subactor.uid} wasn't finished spawning?")
+
                             await event.wait()
+
                             # channel/portal should now be up
                             _, _, portal = self._children[subactor.uid]
 
@@ -239,6 +248,7 @@ async def _open_and_supervise_one_cancels_all_nursery(
             # As such if the strategy propagates any error(s) upwards
             # the above "daemon actor" nursery will be notified.
             async with trio.open_nursery() as ria_nursery:
+
                 anursery = ActorNursery(
                     actor,
                     ria_nursery,
@@ -249,41 +259,53 @@ async def _open_and_supervise_one_cancels_all_nursery(
                     # spawning of actors happens in the caller's scope
                     # after we yield upwards
                     yield anursery
-                    log.debug(
+
+                    log.runtime(
                         f"Waiting on subactors {anursery._children} "
                         "to complete"
                     )
 
                     # Last bit before first nursery block ends in the case
                     # where we didn't error in the caller's scope
-                    log.debug("Waiting on all subactors to complete")
+
+                    # signal all process monitor tasks to conduct
+                    # hard join phase.
                     anursery._join_procs.set()
 
                 except BaseException as err:
 
-                    if is_root_process() and (
+                    # If we error in the root but the debugger is
-                        type(err) in {
-                            Exception, trio.MultiError, trio.Cancelled
-                        }
-                    ):
-                        # if we error in the root but the debugger is
                     # engaged we don't want to prematurely kill (and
-                        # thus clobber access to) the local tty streams.
+                    # thus clobber access to) the local tty since it
-                        # instead try to wait for pdb to be released before
+                    # will make the pdb repl unusable.
+                    # Instead try to wait for pdb to be released before
                     # tearing down.
-                        debug_complete = _debug._pdb_complete
+                    if is_root_process():
-                        if debug_complete and not debug_complete.is_set():
+                        log.exception(f"we're root with {err}")
-                            log.warning(
-                                "Root has errored but pdb is active..waiting "
-                                "on debug lock")
-                            await _debug._pdb_complete.wait()
 
-                        # raise
+                        # wait to see if a sub-actor task
+                        # will be scheduled and grab the tty
+                        # lock on the next tick
+                        # await trio.testing.wait_all_tasks_blocked()
+
+                        debug_complete = _debug._no_remote_has_tty
+                        if (
+                            debug_complete and
+                            not debug_complete.is_set()
+                        ):
+                            log.warning(
+                                'Root has errored but pdb is in use by '
+                                f'child {_debug._global_actor_in_debug}\n'
+                                'Waiting on tty lock to release..')
+
+                            with trio.CancelScope(shield=True):
+                                await debug_complete.wait()
 
                     # if the caller's scope errored then we activate our
                     # one-cancels-all supervisor strategy (don't
                     # worry more are coming).
                     anursery._join_procs.set()
+
                     try:
                         # XXX: hypothetically an error could be
                         # raised and then a cancel signal shows up
@@ -322,7 +344,15 @@ async def _open_and_supervise_one_cancels_all_nursery(
                 # ria_nursery scope end
 
         # XXX: do we need a `trio.Cancelled` catch here as well?
-        except (Exception, trio.MultiError, trio.Cancelled) as err:
+        # this is the catch around the ``.run_in_actor()`` nursery
+        except (
+
+            Exception,
+            trio.MultiError,
+            trio.Cancelled
+
+        ) as err:
+
             # If actor-local error was raised while waiting on
             # ".run_in_actor()" actors then we also want to cancel all
             # remaining sub-actors (due to our lone strategy: