"""Add superseded_at to musehub_background_jobs

musehub#108 (MWP9): dedup for self-coalescing job types (fetch.mpack.prebuild)
now treats an already-'running' row as covering a new trigger, on the
assumption that the running job will read fresh data before finishing. That
assumption breaks when the running job's data read already started before
the new trigger's write landed -- confirmed live: a bulk commit-signature
repair batch landed 20 fixes, but a concurrently-running prebuild job's read
only captured 1 of them, serving a stale cached mpack to the next clone
until a later, unrelated trigger happened to catch up.

superseded_at lets enqueue_job flag a running row when a new trigger arrives
for it, and lets the job's own handler (process_fetch_mpack_prebuild_job)
check that flag after building and redo the build once more before marking
done, instead of finishing with stale coverage.

Additive and nullable -- existing/historical rows get NULL (never superseded).

Revision ID: 0078
Revises: 0077
"""
from __future__ import annotations

from typing import Sequence, Union

import sqlalchemy as sa
from alembic import op


# revision identifiers, used by Alembic.
revision: str = '0078'
down_revision: Union[str, None] = '0077'
branch_labels: Union[str, Sequence[str], None] = None
depends_on: Union[str, Sequence[str], None] = None


def upgrade() -> None:
    op.add_column(
        'musehub_background_jobs',
        sa.Column('superseded_at', sa.DateTime(timezone=True), nullable=True),
    )


def downgrade() -> None:
    op.drop_column('musehub_background_jobs', 'superseded_at')