package main

import (
	"context"
	"fmt"
	"log"
	"math/rand"
	"os"
	"time"

	"github.com/jackc/pgx/v5/pgxpool"
	"golang.org/x/sync/errgroup"
)

const (
	workerCount = 10
	lockID      = 42
)

var (
	connPool *pgxpool.Pool
	logger   *log.Logger
)

func main() {
	ctx := context.Background()
	logger = log.New(os.Stdout, "", log.LstdFlags)
	var err error
	connPool, err = pgxpool.New(ctx, os.Getenv("POSTGRES_DSN"))
	must(err)

	grp, ctx := errgroup.WithContext(ctx)

	for i := 0; i < workerCount; i++ {
		i := i // infamous loop-closure workaround
		grp.Go(func() error {
			return worker(ctx, i)
		})
	}

	must(grp.Wait())
}

func worker(ctx context.Context, id int) error {
	logger.Printf("worker %d: starting\n", id)
	txn, err := connPool.Begin(ctx)
	must(err)

	const lockStatement = `SELECT pg_try_advisory_xact_lock($1)`

	jitter := time.Millisecond * time.Duration(rand.Intn(1000)-500)
	tick := time.NewTicker((time.Second * 4) + jitter)
	defer tick.Stop()
	for {
		select {
		case <-tick.C:
			row := txn.QueryRow(ctx, lockStatement, lockID)
			var hasLock bool
			if err := row.Scan(&hasLock); err != nil {
				return err
			}
			if hasLock {
				logger.Printf("worker %d: I have the lock\n", id)
			} else {
				logger.Printf("worker %d: I do not have the lock\n", id)
			}

			randomShouldYield := rand.Float64() > 0.7
			if hasLock && randomShouldYield {
				log.Printf("worker %d: releasing lock and restarting", id)
				if err := txn.Commit(ctx); err != nil {
					return err
				}
				return worker(ctx, id)
			}
		case <-ctx.Done():
			return fmt.Errorf("stopping worker %d: %w", id, ctx.Err())
		}
	}
}

func must(err error) {
	if err != nil {
		panic(err)
	}
}