FabledCurator/backend/app/services/scheduler_service.py

"""FC-3d: pure logic for deciding which sources are due for a check.

The Celery tick task wraps `select_due_sources` and fires
`download_source.delay()` per result; this module knows nothing about
Celery, gallery-dl, or any side effect.
"""
from __future__ import annotations

from datetime import UTC, datetime, timedelta

from sqlalchemy import select
from sqlalchemy.dialects.postgresql import insert as pg_insert
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.orm import selectinload

from ..models import AppSetting, Artist, ImportSettings, Source

MIN_INTERVAL_SECONDS = 60
MAX_INTERVAL_SECONDS = 86400
MAX_BACKOFF_EXPONENT = 6

# AppSetting key stamped every time the Beat tick fires (see scan.py). The
# tick runs every 60s; the UI flags the scheduler as stalled if the last
# stamp is older than a few minutes.
SCHEDULER_LAST_TICK_KEY = "scheduler_last_tick_at"

# AppSetting key prefix for per-platform rate-limit cooldowns. When a
# download surfaces ErrorType.RATE_LIMITED, every other source on the same
# platform is deferred for PLATFORM_RATE_LIMIT_COOLDOWN_SECONDS so the next
# scan tick doesn't fire a burst of due same-platform sources back into the
# same limit. Per-source consecutive_failures backoff still applies on top
# of this — but this is PREVENTIVE (kills the same-tick burst from N due
# sources hammering the platform at once), while consecutive_failures is
# REACTIVE (slows the offender down over many cycles). Operator-confirmed
# 2026-05-30.
PLATFORM_COOLDOWN_KEY_PREFIX = "platform_cooldown:"
PLATFORM_RATE_LIMIT_COOLDOWN_SECONDS = 900  # 15 min


def compute_effective_interval(
    source: Source, artist: Artist, settings: ImportSettings,
) -> int:
    """Return the seconds between scheduled checks for one source.

    Precedence: source.check_interval_override > artist.check_interval_seconds
    > settings.download_schedule_default_seconds. Multiplied by
    2 ** min(consecutive_failures, MAX_BACKOFF_EXPONENT) and clamped to
    [MIN_INTERVAL_SECONDS, MAX_INTERVAL_SECONDS].
    """
    base = (
        source.check_interval_override
        or artist.check_interval_seconds
        or settings.download_schedule_default_seconds
    )
    exponent = min(max(0, source.consecutive_failures or 0), MAX_BACKOFF_EXPONENT)
    factor = 2 ** exponent
    raw = base * factor
    return max(MIN_INTERVAL_SECONDS, min(MAX_INTERVAL_SECONDS, raw))


async def set_platform_cooldown(
    session: AsyncSession, platform: str,
    seconds: int = PLATFORM_RATE_LIMIT_COOLDOWN_SECONDS,
) -> None:
    """Stamp a cooldown expiry on the given platform so select_due_sources
    skips every source on that platform until it expires.

    Called when a download surfaces ErrorType.RATE_LIMITED so the other
    sources on the same platform don't all retry into the same rate limit.
    Caller is responsible for committing the session.

    Uses INSERT...ON CONFLICT DO UPDATE so two concurrent workers hitting
    the same platform's rate limit don't race: a SELECT-then-INSERT pattern
    would let the loser's whole transaction (including the source-health
    update + event finalize) roll back on a unique-violation, stranding
    that event. Atomic upsert avoids that.
    """
    now = datetime.now(UTC)
    expires_at = (now + timedelta(seconds=seconds)).isoformat()
    key = f"{PLATFORM_COOLDOWN_KEY_PREFIX}{platform}"
    stmt = pg_insert(AppSetting.__table__).values(
        key=key, value=expires_at, updated_at=now,
    ).on_conflict_do_update(
        index_elements=["key"],
        set_={"value": expires_at, "updated_at": now},
    )
    await session.execute(stmt)


async def _platforms_in_cooldown(session: AsyncSession) -> dict[str, datetime]:
    """Return {platform: expires_at} for platforms whose cooldown is still
    in the future. Expired rows are ignored (a future maintenance sweep can
    delete them; they don't affect routing decisions on their own)."""
    rows = (await session.execute(
        select(AppSetting.key, AppSetting.value)
        .where(AppSetting.key.startswith(PLATFORM_COOLDOWN_KEY_PREFIX))
    )).all()
    if not rows:
        return {}
    now = datetime.now(UTC)
    active: dict[str, datetime] = {}
    for key, value in rows:
        try:
            expires_at = datetime.fromisoformat(value)
        except (ValueError, TypeError):
            continue
        if expires_at > now:
            active[key[len(PLATFORM_COOLDOWN_KEY_PREFIX):]] = expires_at
    return active


async def select_due_sources(session: AsyncSession) -> list[Source]:
    """Sources where (enabled, artist.auto_check) and now >= last_checked_at + effective_interval.

    Never-checked sources (last_checked_at IS NULL) are always due. Sources
    whose platform is currently in a rate-limit cooldown are excluded — the
    cooldown is the preventive half of the burst-prevention pair (per-source
    consecutive_failures backoff handles the offending source itself).

    Ordering: last_checked_at ASC NULLS FIRST, then id. Never-checked
    sources go first, then the longest-since-checked, so the most overdue
    sources hit Celery's FIFO download queue first. Anti-starvation: if
    queue throughput ever falls below the tick rate, a freshly-rerun source
    can't keep cutting in line ahead of one that hasn't been checked at all.
    Operator-confirmed 2026-05-30.
    """
    rows = (await session.execute(
        select(Source)
        .options(selectinload(Source.artist))
        .join(Artist, Source.artist_id == Artist.id)
        .where(Source.enabled.is_(True))
        .where(Artist.auto_check.is_(True))
        .order_by(Source.last_checked_at.asc().nulls_first(), Source.id)
    )).scalars().all()

    cooldowns = await _platforms_in_cooldown(session)
    settings = await ImportSettings.load(session)

    now = datetime.now(UTC)
    due: list[Source] = []
    for s in rows:
        if s.platform in cooldowns:
            continue
        interval = compute_effective_interval(s, s.artist, settings)
        if s.last_checked_at is None:
            due.append(s)
            continue
        elapsed = (now - s.last_checked_at).total_seconds()
        if elapsed >= interval:
            due.append(s)
    return due


def compute_next_check_at(
    source: Source, artist: Artist, settings: ImportSettings,
) -> datetime | None:
    """Return the projected datetime of the next check, or None if never checked."""
    if source.last_checked_at is None:
        return None
    interval = compute_effective_interval(source, artist, settings)
    return source.last_checked_at + timedelta(seconds=interval)


async def record_tick(session: AsyncSession) -> None:
    """Stamp the current time on the SCHEDULER_LAST_TICK_KEY AppSetting.

    Called once per Beat tick so the UI can prove the scheduler is alive.
    Commits its own write so the stamp survives even if the rest of the
    tick errors out.
    """
    now_iso = datetime.now(UTC).isoformat()
    row = (await session.execute(
        select(AppSetting).where(AppSetting.key == SCHEDULER_LAST_TICK_KEY)
    )).scalar_one_or_none()
    if row is None:
        session.add(AppSetting(key=SCHEDULER_LAST_TICK_KEY, value=now_iso))
    else:
        row.value = now_iso
    await session.commit()


async def scheduler_status(session: AsyncSession) -> dict:
    """Summarise scheduler health for the dashboard.

    Returns last_tick_at (when Beat last fired), next_due_at (earliest
    upcoming scheduled check across enabled auto-check sources), due_now
    (how many are due right now), and auto_sources (total under schedule).
    """
    last_tick_at = (await session.execute(
        select(AppSetting.value).where(AppSetting.key == SCHEDULER_LAST_TICK_KEY)
    )).scalar_one_or_none()

    rows = (await session.execute(
        select(Source)
        .options(selectinload(Source.artist))
        .join(Artist, Source.artist_id == Artist.id)
        .where(Source.enabled.is_(True))
        .where(Artist.auto_check.is_(True))
    )).scalars().all()
    settings = await ImportSettings.load(session)

    now = datetime.now(UTC)
    due_now = 0
    next_due_at: datetime | None = None
    for s in rows:
        if s.last_checked_at is None:
            due_now += 1
            continue
        nca = compute_next_check_at(s, s.artist, settings)
        if nca is None or nca <= now:
            due_now += 1
        elif next_due_at is None or nca < next_due_at:
            next_due_at = nca

    cooldowns = await _platforms_in_cooldown(session)

    return {
        "last_tick_at": last_tick_at,
        "next_due_at": next_due_at.isoformat() if next_due_at else None,
        "due_now": due_now,
        "auto_sources": len(rows),
        "platform_cooldowns": {p: dt.isoformat() for p, dt in cooldowns.items()},
    }