bvandeusen
f1a664e5a7
CI lint flagged UP047 — use the native generic syntax def get_or_create[T](...) instead of typing.TypeVar on Python 3.14. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
53 lines
2.2 KiB
Python
53 lines
2.2 KiB
Python
"""Shared DB-access helpers for the async services.
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`get_or_create` centralizes the race-safe find-or-create dance — SELECT, then on
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a miss a savepoint INSERT that recovers (NOT a full rollback) when a concurrent
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worker inserted the same row first. It was hand-rolled identically in
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ArtistService, TagService and ExtensionService; divergent copies are exactly how
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the duplicate-row / race bugs in [[reference_scalar_one_or_none_duplicates]] crept
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in, so it lives in one place now (DRY pattern sweep 2026-06-09).
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Note: this is the ASYNC sibling of `Importer._get_or_create` (sync, used by the
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filesystem-import path). The two can't share an implementation across the
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sync/async boundary; the importer one stays as the lone sync consumer.
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"""
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from __future__ import annotations
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from collections.abc import Awaitable, Callable
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from sqlalchemy import Select
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from sqlalchemy.exc import IntegrityError
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from sqlalchemy.ext.asyncio import AsyncSession
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async def get_or_create[T](
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session: AsyncSession,
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select_stmt: Select,
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factory: Callable[[], Awaitable[T]],
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) -> tuple[T, bool]:
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"""Race-safe find-or-create. Returns ``(row, created)``.
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Run ``select_stmt`` (scalar_one_or_none); if a row exists, return it with
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``created=False``. Otherwise open a SAVEPOINT and ``await factory()`` — which
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must add its row(s), flush, and return the primary row. On ``IntegrityError``
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(a concurrent worker inserted the same row first) roll back the SAVEPOINT —
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NOT the outer transaction, which would lose the caller's surrounding work —
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and re-run ``select_stmt`` (scalar_one) to return the row the other worker
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created. The caller owns the outer commit.
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A UNIQUE/partial-unique constraint matching ``select_stmt``'s predicate is
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required for the recovery to trip; without it a duplicate slips through.
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"""
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existing = (await session.execute(select_stmt)).scalar_one_or_none()
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if existing is not None:
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return existing, False
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sp = await session.begin_nested()
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try:
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row = await factory()
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await sp.commit()
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return row, True
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except IntegrityError:
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await sp.rollback()
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return (await session.execute(select_stmt)).scalar_one(), False
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