Question

What is the best way to atomically lock multiple files? One possibility I’ve thought of is a simple .lock file in the directory (I’m only protected from other instances of my program; I’m not necessarily trying to thwart other programs from disrespecting the lock.) However, everything I’ve learned about thread-safety screams “bad idea!” Would this be a safe way to go? Is trying to create a file atomic, so if I succeed in creating it, I know I have the lock? How should I go about this?

I’m writing my own bug tracker (mostly as an exercise, I know I could find good solutions out there,) which stores bugs in files on the network. I envision it kind of like SVN – a single directory gets taken over by the program, and used to hold bugs, revisions, screenshots, etc, all managed by the tracker. I’m assuming SVN has a way to ensure multiple clients don’t make commits at the exact same time, etc.

Answer 1

Open each file exclusively, if you find one that can’t be opened, then unlock the ones you’ve already locked. Always lock in alphabetical order to avoid deadlocks.

    private List<FileStream> OpenExclusive(List<string> filenames)
    {
        var streams = new List<FileStream>(filenames.Count);

        try
        {
            filenames.Sort();
            foreach (var name in filenames)
            {
                streams.Add(File.Open(name, 
                  FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None));
            }
            return streams;
        }
        catch (Exception)
        {
            CloseAll(streams);
            return null;
        }
    }

    private void CloseAll(List<FileStream> streams)
    {
        if (streams == null) return;
        foreach (var stream in streams)
        {
            stream.Close();
        }
    }