package postgres

import (
	"context"
	"errors"
	"time"
)

var ErrLibraryExecutionBusy = errors.New("target already has an active executor")

func (s *Store) WithLibraryExecution(ctx context.Context, instance string, run func(context.Context, func(context.Context) error) error) error {
	if instance == "" {
		return errors.New("execution instance identity required")
	}
	// A lease expiry is not proof that the old writer has stopped. Keep a
	// shared PG advisory lock until the synchronous callback (including cleanup)
	// exits. No deferred unlock may run merely because ctx was cancelled.
	tx, err := s.db.Begin(ctx)
	if err != nil {
		return err
	}
	defer func() {
		cleanup, cancel := context.WithTimeout(context.WithoutCancel(ctx), 5*time.Second)
		defer cancel()
		_ = tx.Rollback(cleanup)
	}()
	var held bool
	if err = tx.QueryRow(ctx, `SELECT pg_try_advisory_xact_lock(hashtextextended($1,0))`, "library-execution:"+instance).Scan(&held); err != nil {
		return err
	}
	if !held {
		return ErrLibraryExecutionBusy
	}
	ping := func(probe context.Context) error { var one int; return tx.QueryRow(probe, `SELECT 1`).Scan(&one) }
	return run(ctx, ping)
}