"""Alembic environment — reads DATABASE_URL from app config."""

from logging.config import fileConfig

from sqlalchemy import engine_from_config, pool, text

from alembic import context
from backend.app.config import get_config
from backend.app.models import Base

# Arbitrary fixed 64-bit key for the session/transaction advisory lock that
# serializes concurrent `alembic upgrade head` runs. Every `web` replica runs
# migrations in its entrypoint, so under `docker stack deploy` two replicas can
# boot at once and race the same DDL — duplicate CREATE TABLE, then a crashed
# replica (operator-flagged 2026-06-07: 0040 raced; one backend died with
# AdminShutdown). The first replica to reach the lock migrates; the rest block,
# then find the version table already at head and apply nothing.
_MIGRATION_LOCK_KEY = 0xFCA1E35C

config = context.config

if config.config_file_name is not None:
    fileConfig(config.config_file_name)

config.set_main_option("sqlalchemy.url", get_config().database_url_sync)

target_metadata = Base.metadata


def run_migrations_offline() -> None:
    url = config.get_main_option("sqlalchemy.url")
    context.configure(
        url=url,
        target_metadata=target_metadata,
        literal_binds=True,
        dialect_opts={"paramstyle": "named"},
        compare_type=True,
    )
    with context.begin_transaction():
        context.run_migrations()


def run_migrations_online() -> None:
    connectable = engine_from_config(
        config.get_section(config.config_ini_section, {}),
        prefix="sqlalchemy.",
        poolclass=pool.NullPool,
    )
    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            target_metadata=target_metadata,
            compare_type=True,
        )
        with context.begin_transaction():
            # Serialize concurrent migrators (see _MIGRATION_LOCK_KEY). A
            # transaction-scoped advisory lock: the first replica to get here
            # holds it for the whole upgrade and is auto-released when this
            # transaction ends. A sibling replica blocks on this line, and only
            # once the leader commits does it proceed to read the version table
            # — now at head — so it runs zero migrations instead of re-applying
            # the same DDL. The lock is acquired BEFORE run_migrations() reads
            # the current revision, which is what makes the no-op correct.
            connection.execute(
                text("SELECT pg_advisory_xact_lock(:k)"),
                {"k": _MIGRATION_LOCK_KEY},
            )
            context.run_migrations()


if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()