package main

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"syscall"
)

// errInUse means another process holds the database lock.
var errInUse = errors.New("the database is in use by a running stewardd or by 'stewardd restore'")

// lockDatabase takes the exclusive lock (on DATABASE.lock) that marks the
// database as in use by a running stewardd or by 'stewardd restore', so a
// restore can never replace the database under a running stewardd, and two
// stewardds can't serve the same database. Other commands (backup, user, ...)
// don't take it: they work alongside a running stewardd.
func lockDatabase(db string) (release func(), err error) {
	if err := os.MkdirAll(filepath.Dir(db), 0o750); err != nil {
		return nil, err
	}
	path := db + ".lock"
	// Read-only is enough for flock, and works whoever created the file.
	f, err := os.OpenFile(path, os.O_RDONLY|os.O_CREATE, 0o640)
	if err != nil {
		return nil, err
	}
	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
		f.Close()
		if errors.Is(err, syscall.EWOULDBLOCK) {
			return nil, errInUse
		}
		return nil, fmt.Errorf("locking %s: %w", path, err)
	}
	return func() {
		syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
		f.Close()
	}, nil
}