takahe/stator/models.py

import datetime
import traceback
from typing import ClassVar, List, Optional, Type, cast

from asgiref.sync import sync_to_async
from django.db import models, transaction
from django.utils import timezone
from django.utils.functional import classproperty

from stator.graph import State, StateGraph, Transition


class StateField(models.CharField):
    """
    A special field that automatically gets choices from a state graph
    """

    def __init__(self, graph: Type[StateGraph], **kwargs):
        # Sensible default for state length
        kwargs.setdefault("max_length", 100)
        # Add choices and initial
        self.graph = graph
        kwargs["choices"] = self.graph.choices
        kwargs["default"] = self.graph.initial_state.name
        super().__init__(**kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        kwargs["graph"] = self.graph
        return name, path, args, kwargs

    def from_db_value(self, value, expression, connection):
        if value is None:
            return value
        return self.graph.states[value]

    def to_python(self, value):
        if isinstance(value, State) or value is None:
            return value
        return self.graph.states[value]

    def get_prep_value(self, value):
        if isinstance(value, State):
            return value.name
        return value


class StatorModel(models.Model):
    """
    A model base class that has a state machine backing it, with tasks to work
    out when to move the state to the next one.

    You need to provide a "state" field as an instance of StateField on the
    concrete model yourself.
    """

    # If this row is up for transition attempts
    state_ready = models.BooleanField(default=False)

    # When the state last actually changed, or the date of instance creation
    state_changed = models.DateTimeField(auto_now_add=True)

    # When the last state change for the current state was attempted
    # (and not successful, as this is cleared on transition)
    state_attempted = models.DateTimeField(blank=True, null=True)

    # If a lock is out on this row, when it is locked until
    # (we don't identify the lock owner, as there's no heartbeats)
    state_locked_until = models.DateTimeField(null=True, blank=True)

    # Collection of subclasses of us
    subclasses: ClassVar[List[Type["StatorModel"]]] = []

    class Meta:
        abstract = True

    def __init_subclass__(cls) -> None:
        if cls is not StatorModel:
            cls.subclasses.append(cls)

    @classproperty
    def state_graph(cls) -> Type[StateGraph]:
        return cls._meta.get_field("state").graph

    @classmethod
    async def atransition_schedule_due(cls, now=None) -> models.QuerySet:
        """
        Finds instances of this model that need to run and schedule them.
        """
        q = models.Q()
        for state in cls.state_graph.states.values():
            state = cast(State, state)
            if not state.terminal:
                q = q | models.Q(
                    (
                        models.Q(
                            state_attempted__lte=timezone.now()
                            - datetime.timedelta(seconds=state.try_interval)
                        )
                        | models.Q(state_attempted__isnull=True)
                    ),
                    state=state.name,
                )
        await cls.objects.filter(q).aupdate(state_ready=True)

    @classmethod
    def transition_get_with_lock(
        cls, number: int, lock_expiry: datetime.datetime
    ) -> List["StatorModel"]:
        """
        Returns up to `number` tasks for execution, having locked them.
        """
        with transaction.atomic():
            selected = list(
                cls.objects.filter(state_locked_until__isnull=True, state_ready=True)[
                    :number
                ].select_for_update()
            )
            cls.objects.filter(pk__in=[i.pk for i in selected]).update(
                state_locked_until=timezone.now()
            )
        return selected

    @classmethod
    async def atransition_get_with_lock(
        cls, number: int, lock_expiry: datetime.datetime
    ) -> List["StatorModel"]:
        return await sync_to_async(cls.transition_get_with_lock)(number, lock_expiry)

    @classmethod
    async def atransition_clean_locks(cls):
        await cls.objects.filter(state_locked_until__lte=timezone.now()).aupdate(
            state_locked_until=None
        )

    def transition_schedule(self):
        """
        Adds this instance to the queue to get its state transition attempted.

        The scheduler will call this, but you can also call it directly if you
        know it'll be ready and want to lower latency.
        """
        self.state_ready = True
        self.save()

    async def atransition_attempt(self) -> bool:
        """
        Attempts to transition the current state by running its handler(s).
        """
        # Try each transition in priority order
        for transition in self.state.transitions(automatic_only=True):
            try:
                success = await transition.get_handler()(self)
            except BaseException as e:
                await StatorError.acreate_from_instance(self, transition, e)
                traceback.print_exc()
                continue
            if success:
                await self.atransition_perform(transition.to_state.name)
                return True
        await self.__class__.objects.filter(pk=self.pk).aupdate(
            state_attempted=timezone.now(),
            state_locked_until=None,
            state_ready=False,
        )
        return False

    def transition_perform(self, state_name):
        """
        Transitions the instance to the given state name, forcibly.
        """
        if state_name not in self.state_graph.states:
            raise ValueError(f"Invalid state {state_name}")
        self.__class__.objects.filter(pk=self.pk).update(
            state=state_name,
            state_changed=timezone.now(),
            state_attempted=None,
            state_locked_until=None,
            state_ready=False,
        )

    atransition_perform = sync_to_async(transition_perform)


class StatorError(models.Model):
    """
    Tracks any errors running the transitions.
    Meant to be cleaned out regularly. Should probably be a log.
    """

    # appname.modelname (lowercased) label for the model this represents
    model_label = models.CharField(max_length=200)

    # The primary key of that model (probably int or str)
    instance_pk = models.CharField(max_length=200)

    # The state we moved from
    from_state = models.CharField(max_length=200)

    # The state we moved to (or tried to)
    to_state = models.CharField(max_length=200)

    # When it happened
    date = models.DateTimeField(auto_now_add=True)

    # Error name
    error = models.TextField()

    # Error details
    error_details = models.TextField(blank=True, null=True)

    @classmethod
    async def acreate_from_instance(
        cls,
        instance: StatorModel,
        transition: Transition,
        exception: Optional[BaseException] = None,
    ):
        return await cls.objects.acreate(
            model_label=instance._meta.label_lower,
            instance_pk=str(instance.pk),
            from_state=transition.from_state,
            to_state=transition.to_state,
            error=str(exception),
            error_details=traceback.format_exc(),
        )