"""Fence approved stable serving transitions and rollback against ABA changes."""

import sqlalchemy as sa

from alembic import op

revision = "20260910_0034"
down_revision = "20260909_0033"
branch_labels = None
depends_on = None


def upgrade() -> None:
    op.add_column("capability_contracts", sa.Column(
        "serving_generation", sa.BigInteger(), nullable=False, server_default="0"))
    with op.batch_alter_table("capability_contracts") as batch:
        batch.create_check_constraint("ck_capability_serving_generation", "serving_generation >= 0")


def downgrade() -> None:
    contracts = sa.table("capability_contracts", sa.column("serving_generation"))
    if op.get_bind().scalar(sa.select(sa.func.count()).select_from(contracts).where(
        contracts.c.serving_generation != 0
    )):
        raise RuntimeError("Serving route history exists; restore the pre-migration backup instead.")
    with op.batch_alter_table("capability_contracts") as batch:
        batch.drop_constraint("ck_capability_serving_generation", type_="check")
        batch.drop_column("serving_generation")