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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.
ctx_debugger
Tyler Goodlet 2021-06-29 15:15:32 -04:00
parent be1fcb2a5b
commit 551816e80d
2 changed files with 144 additions and 58 deletions
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132
tractor/_debug.py
View File

@ -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
debugger entry point to avoid tty clobbering by multiple processes.
"""
global _debug_lock, _global_actor_in_debug
'''Acquire a actor local FIFO lock meant to mutex entry to a local
debugger entry point to avoid tty clobbering a global root process.
'''
global _debug_lock, _global_actor_in_debug, _no_remote_has_tty
task_name = trio.lowlevel.current_task().name
@ -135,15 +136,60 @@ 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!
_global_actor_in_debug = None
log.debug(f"TTY lock released, remote task: {task_name}:{uid}")
# 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}")
# @contextmanager
@ -169,53 +215,43 @@ 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):
with trio.CancelScope(shield=True):
# 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
f"Actor {subactor_uid} ACQUIRED stdin hijack lock")
# wait for unlock pdb by child
async with ctx.open_stream() as stream:
try:
assert await stream.receive() == 'pdb_unlock'
# wait for unlock pdb by child
async with ctx.open_stream() as stream:
try:
assert await stream.receive() == 'pdb_unlock'
except trio.BrokenResourceError:
# XXX: there may be a race with the portal teardown
# with the calling actor which we can safely ignore
# the alternative would be sending an ack message
# and allowing the client to wait for us to teardown
# first?
pass
except trio.BrokenResourceError:
# XXX: there may be a race with the portal teardown
# with the calling actor which we can safely ignore
# the alternative would be sending an ack message
# and allowing the client to wait for us to teardown
# first?
pass
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"
@ -243,17 +279,21 @@ async def _breakpoint(
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(
@ -262,18 +302,22 @@ async def _breakpoint(
) 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...
# with trio.CancelScope(shield=True):
try:
await _local_pdb_complete.wait()
await _local_pdb_complete.wait()
await stream.send('pdb_unlock')
finally:
# TODO: shielding currently can cause hangs...
with trio.CancelScope(shield=True):
await stream.send('pdb_unlock')
# sync with callee termination
assert await ctx.result() == "pdb_unlock_complete"
@ -292,6 +336,7 @@ async def _breakpoint(
# 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
@ -316,7 +361,13 @@ async def _breakpoint(
# 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.
await actor._service_n.start(wait_for_parent_stdin_hijack)
# 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():
@ -333,6 +384,11 @@ async def _breakpoint(
# 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

70
tractor/_trionics.py
View File

@ -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 (
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.
# instead try to wait for pdb to be released before
# tearing down.
debug_complete = _debug._pdb_complete
if debug_complete and not debug_complete.is_set():
log.warning(
"Root has errored but pdb is active..waiting "
"on debug lock")
await _debug._pdb_complete.wait()
# 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 since it
# will make the pdb repl unusable.
# Instead try to wait for pdb to be released before
# tearing down.
if is_root_process():
log.exception(f"we're root with {err}")
# 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: