tart/Sources/tart/FileLock.swift

import Foundation
import System

enum FileLockError: Error, Equatable {
    case Failed(_ message: String)
    case AlreadyLocked
}

class FileLock {
    let url: URL
    let fd: Int32

    init(lockURL: URL) throws {
        url = lockURL
        fd = open(lockURL.path, 0)
    }

    deinit {
        close(fd)
    }

    func trylock() throws -> Bool {
        try flockWrapper(LOCK_EX | LOCK_NB)
    }

    func lock() throws {
        _ = try flockWrapper(LOCK_EX)
    }

    func unlock() throws {
        _ = try flockWrapper(LOCK_UN)
    }

    func flockWrapper(_ operation: Int32) throws -> Bool {
        let ret = flock(fd, operation)
        if ret != 0 {
            let details = Errno(rawValue: CInt(errno))

            if (operation & LOCK_NB) != 0 && details == .wouldBlock {
                return false
            }

            throw FileLockError.Failed("failed to lock \(url): \(details)")
        }

        return true
    }
}