//go:build unix

package datasets

import (
	"context"
	"errors"
	"fmt"
	"os"
	"syscall"
	"time"
)

// lockPollInterval is how often a contended lock is retried. The lock is held
// for the duration of a dataset download (minutes), so sub-second acquisition
// latency is irrelevant.
const lockPollInterval = 150 * time.Millisecond

// flockExclusive acquires an exclusive flock on path, creating the lock file
// if needed, and blocks until it is held or ctx is cancelled.
//
// It uses non-blocking LOCK_NB attempts in a poll loop rather than a blocking
// flock in a goroutine: the file descriptor is only ever touched by this
// goroutine, so there is no race between a pending syscall and Close on
// cancellation.
func flockExclusive(ctx context.Context, path string) (func(), error) {
	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o644)
	if err != nil {
		return nil, fmt.Errorf("open lock file: %w", err)
	}
	for {
		err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
		if err == nil {
			return func() {
				_ = syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
				_ = f.Close()
			}, nil
		}
		if !errors.Is(err, syscall.EWOULDBLOCK) {
			f.Close()
			return nil, fmt.Errorf("flock: %w", err)
		}
		select {
		case <-ctx.Done():
			f.Close()
			return nil, ctx.Err()
		case <-time.After(lockPollInterval):
		}
	}
}