235 lines
7.8 KiB
Python
235 lines
7.8 KiB
Python
import datetime
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import pprint
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import traceback
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from typing import ClassVar, List, Optional, Type, Union, cast
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from asgiref.sync import sync_to_async
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from django.db import models, transaction
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from django.utils import timezone
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from django.utils.functional import classproperty
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from stator.graph import State, StateGraph
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class StateField(models.CharField):
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"""
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A special field that automatically gets choices from a state graph
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"""
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def __init__(self, graph: Type[StateGraph], **kwargs):
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# Sensible default for state length
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kwargs.setdefault("max_length", 100)
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# Add choices and initial
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self.graph = graph
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kwargs["choices"] = self.graph.choices
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kwargs["default"] = self.graph.initial_state.name
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super().__init__(**kwargs)
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def deconstruct(self):
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name, path, args, kwargs = super().deconstruct()
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kwargs["graph"] = self.graph
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return name, path, args, kwargs
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def get_prep_value(self, value):
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if isinstance(value, State):
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return value.name
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return value
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class StatorModel(models.Model):
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"""
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A model base class that has a state machine backing it, with tasks to work
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out when to move the state to the next one.
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You need to provide a "state" field as an instance of StateField on the
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concrete model yourself.
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"""
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# If this row is up for transition attempts
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state_ready = models.BooleanField(default=False)
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# When the state last actually changed, or the date of instance creation
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state_changed = models.DateTimeField(auto_now_add=True)
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# When the last state change for the current state was attempted
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# (and not successful, as this is cleared on transition)
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state_attempted = models.DateTimeField(blank=True, null=True)
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# If a lock is out on this row, when it is locked until
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# (we don't identify the lock owner, as there's no heartbeats)
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state_locked_until = models.DateTimeField(null=True, blank=True)
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# Collection of subclasses of us
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subclasses: ClassVar[List[Type["StatorModel"]]] = []
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class Meta:
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abstract = True
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def __init_subclass__(cls) -> None:
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if cls is not StatorModel:
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cls.subclasses.append(cls)
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@classproperty
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def state_graph(cls) -> Type[StateGraph]:
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return cls._meta.get_field("state").graph
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@classmethod
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async def atransition_schedule_due(cls, now=None) -> models.QuerySet:
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"""
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Finds instances of this model that need to run and schedule them.
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"""
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q = models.Q()
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for state in cls.state_graph.states.values():
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state = cast(State, state)
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if not state.externally_progressed:
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q = q | models.Q(
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(
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models.Q(
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state_attempted__lte=timezone.now()
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- datetime.timedelta(
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seconds=cast(float, state.try_interval)
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)
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)
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| models.Q(state_attempted__isnull=True)
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),
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state=state.name,
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)
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await cls.objects.filter(q).aupdate(state_ready=True)
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@classmethod
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def transition_get_with_lock(
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cls, number: int, lock_expiry: datetime.datetime
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) -> List["StatorModel"]:
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"""
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Returns up to `number` tasks for execution, having locked them.
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"""
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with transaction.atomic():
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selected = list(
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cls.objects.filter(state_locked_until__isnull=True, state_ready=True)[
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:number
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].select_for_update()
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)
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cls.objects.filter(pk__in=[i.pk for i in selected]).update(
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state_locked_until=lock_expiry
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)
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return selected
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@classmethod
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async def atransition_get_with_lock(
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cls, number: int, lock_expiry: datetime.datetime
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) -> List["StatorModel"]:
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return await sync_to_async(cls.transition_get_with_lock)(number, lock_expiry)
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@classmethod
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async def atransition_clean_locks(cls):
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await cls.objects.filter(state_locked_until__lte=timezone.now()).aupdate(
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state_locked_until=None
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)
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def transition_schedule(self):
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"""
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Adds this instance to the queue to get its state transition attempted.
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The scheduler will call this, but you can also call it directly if you
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know it'll be ready and want to lower latency.
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"""
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self.state_ready = True
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self.save()
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async def atransition_attempt(self) -> Optional[State]:
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"""
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Attempts to transition the current state by running its handler(s).
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"""
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current_state = self.state_graph.states[self.state]
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# If it's a manual progression state don't even try
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# We shouldn't really be here in this case, but it could be a race condition
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if current_state.externally_progressed:
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print("Externally progressed state!")
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return None
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try:
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next_state = await current_state.handler(self)
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except BaseException as e:
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await StatorError.acreate_from_instance(self, e)
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traceback.print_exc()
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else:
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if next_state:
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# Ensure it's a State object
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if isinstance(next_state, str):
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next_state = self.state_graph.states[next_state]
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# Ensure it's a child
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if next_state not in current_state.children:
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raise ValueError(
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f"Cannot transition from {current_state} to {next_state} - not a declared transition"
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)
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await self.atransition_perform(next_state)
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return next_state
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await self.__class__.objects.filter(pk=self.pk).aupdate(
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state_attempted=timezone.now(),
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state_locked_until=None,
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state_ready=False,
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)
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return None
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def transition_perform(self, state: Union[State, str]):
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"""
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Transitions the instance to the given state name, forcibly.
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"""
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if isinstance(state, State):
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state = state.name
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if state not in self.state_graph.states:
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raise ValueError(f"Invalid state {state}")
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self.__class__.objects.filter(pk=self.pk).update(
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state=state,
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state_changed=timezone.now(),
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state_attempted=None,
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state_locked_until=None,
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state_ready=False,
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)
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atransition_perform = sync_to_async(transition_perform)
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class StatorError(models.Model):
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"""
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Tracks any errors running the transitions.
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Meant to be cleaned out regularly. Should probably be a log.
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"""
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# appname.modelname (lowercased) label for the model this represents
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model_label = models.CharField(max_length=200)
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# The primary key of that model (probably int or str)
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instance_pk = models.CharField(max_length=200)
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# The state we were on
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state = models.CharField(max_length=200)
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# When it happened
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date = models.DateTimeField(auto_now_add=True)
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# Error name
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error = models.TextField()
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# Error details
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error_details = models.TextField(blank=True, null=True)
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@classmethod
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async def acreate_from_instance(
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cls,
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instance: StatorModel,
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exception: Optional[BaseException] = None,
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):
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detail = traceback.format_exc()
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if exception and len(exception.args) > 1:
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detail += "\n\n" + "\n\n".join(
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pprint.pformat(arg) for arg in exception.args
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)
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return await cls.objects.acreate(
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model_label=instance._meta.label_lower,
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instance_pk=str(instance.pk),
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state=instance.state,
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error=str(exception),
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error_details=detail,
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)
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