Non-Exchangeable, Non-Refundable

I travelled on Eurostar today and learnt something about non-exchangeable, non-refundable tickets in the process so thought I'd share in case it can be useful to someone else. Eurostar sells several types of tickets in several classes (Standard, Standard Premier and Business Premier). The higher the class and the more flexible the ticket, the greater the price. So the cheapest tickets are non-exchangeable, non-refundable standard class tickets. Once bought, such tickets cannot be exchanged against another on a different train, cannot be refunded in case you don't want to travel anymore but they can most certainly be upgraded to the next travel class up, as I did today. Of course, it requires you paying an upgrade price, which may not be cheap. The fact that you can upgrade any ticket makes sense because:

• An upgrade doesn't fall under the non-refundable rule because you're not asking for a refund, and in fact you're paying extra for the upgrade;
• It doesn't fall under the non-exchangeable rule either because you're not asking for an exchange as you still want to travel on the same train at the same date: you just want to upgrade your existing ticket.

So if you ask general information staff in the station and are told that you cannot upgrade your ticket, don't take their word for it, go to the sales counter. The only reason why you would not be able to upgrade (and pay Eurostar more money) is if the travel class you want to upgrade to is already fully booked on your train.

Crash of the Day

Received today from a colleague:

Please note that if somebody opens Build log excel in Microsoft excel 2007 and updates it while a filter put on any column, the file crashes.

So please avoid updating the build log in Microsoft excel 2007.

So we're talking about a fairy simple file created in Excel 2003 that crashes Excel 2007 if you try to update it while a filter is set on any column... Sigh...

MS Works to MS Word: LibreOffice to the Rescue

I am at my mum's for Easter and one of the first things she asked me to look at had to do with her computer. She had this document that she wrote using Microsoft Works aeons ago that she wanted to open again. Of course, she's now using Microsoft Word and Word has no idea how to open Works files, even though both products are produced from the same software company.

What to do? The answer is very simple but rather counter-intuitive for people not used to open source software: LibreOffice. From the point of view of someone who lives in a world where closed source is the norm, how can a free office wannabe solve a problem that the mighty MS Office can't solve? Simple: as highlighted by Michael Meeks at FOSDEM earlier this year, LibreOffice wants to have the largest possible list of supported file formats so that they can support their users in reading their old documents stored in long forgotten format and hopefully migrate them to modern and preferably open document formats. As a result, LibreOffice supports MS Works and MS Word out of the box.

So back to my mum's document, retrieving the content was then very easy: copy the document to a USB key, open it on my laptop using LibreOffice, save it again and copy the new document back to her PC. In this case, it meant saving it back as an MS Word document. In an ideal world, I would have saved it as ODF and installed LibreOffice on her computer but I'll leave that for another day.

So if you have any old document lying around that you can't open anymore, try LibreOffice first, you'll be surprised how many weird and wonderful formats it supports. If it still doesn't work, consider contributing a filter to the project or at least reporting the issue and providing sample files so that the developers can build such a filter.

Outlook Error

In the meaningless error category, let me present today's effort by Microsoft Outlook:

D-Bus Experiments in Vala

Most modern Linux desktop distributions now include D-Bus. It enables different applications in the same user session to communicate with each other or with system services. So I thought I'd experiment with D-Bus using Vala and starting with the published example.

Example 1: Ping Loop

I started with a simple ping client and server based on the example above, the main difference being that I added a loop in the client. The server code takes a message and an integer, prints out the message and the received integer, then adds one to the integer before returning the result:

[DBus (name = "org.example.Demo")]
public class DemoServer : Object {

    public int ping (string msg, int i) {
        stdout.printf ("%s [%d]\n", msg, i);
        return i+1;
    }
}

void on_bus_aquired (DBusConnection conn) {
    try {
        conn.register_object ("/org/example/demo",
            new DemoServer ());
    } catch (IOError e) {
        stderr.printf ("Could not register service\n");
    }
}

void main () {
    Bus.own_name (
        BusType.SESSION, "org.example.Demo",
        BusNameOwnerFlags.NONE,
        on_bus_aquired,
        () => {},
        () => stderr.printf ("Could not aquire name\n"));

    new MainLoop ().run ();
}

server.vala

And the client simply queries the server in a loop every second and prints out the input and output values:

[DBus (name = "org.example.Demo")]
interface DemoClient : Object {
    public abstract int ping (string msg, int i)
        throws IOError;
}

void main () {
    int i = 1;
    int j;
    while(true) {
        try {
            DemoClient client = Bus.get_proxy_sync (
                BusType.SESSION, "org.example.Demo",
                "/org/example/demo");

            j = client.ping ("ping", i);
            stdout.printf ("%d => %d\n", i, j);
            i = j;

        } catch (IOError e) {
            stderr.printf ("%s\n", e.message);
        }
        Thread.usleep(1000000);
    }
}

client.vala

Compiling both programs requires the gio-2.0 package:

$ valac --pkg gio-2.0 server.vala
$ valac --pkg gio-2.0 client.vala

Start the server followed by the client in two separate terminal windows and you should see them exchange data.

Example 2: Graceful Termination

The problem with the code above is that if the server process terminates while the client is still running, an exception occurs and the client doesn't recover. It would be nice if we could get the client to terminate gracefully when the server stops. The Vala GDBus library provides the ability to detect when a service comes up or is brought down using watches. When you setup a watch, you need a callback method for the watch to call. That callback method needs to be called on a different thread than the main client thread. This is handled by the MainLoop class but it means that the core client loop needs to be run in its own thread, which complicates the client code a bit. In the code below, the client code has been encapsulated into a Demo class and the client loop thread is controlled using a simple boolean attribute.

[DBus (name = "org.example.Demo")]
interface DemoClient : Object {
    public abstract int ping (string msg, int i)
        throws IOError;
}

public class Demo : Object {
    private bool server_up = true;
    
    private DemoClient client;
    
    private uint watch;
    
    private MainLoop main_loop;
    
    public Demo() {
        try {
            watch = Bus.watch_name(
                BusType.SESSION,
                "org.example.Demo",
                BusNameWatcherFlags.AUTO_START,
                () => {},
                on_name_vanished
            );
            client = Bus.get_proxy_sync (
                BusType.SESSION,
                "org.example.Demo",
                "/org/example/demo");
            server_up = true;
        } catch (IOError e) {
            stderr.printf ("%s\n", e.message);
            server_up = false;
        }
    }
    
    public void start() {
        main_loop = new MainLoop();
        Thread.create(() => {
            run();
            return null;
        }, false);
        main_loop.run();
    }
    
    public void run() {
        int i = 1;
        int j;
        while(server_up) {
            try {
                j = client.ping ("ping", i);
                stdout.printf ("%d => %d\n", i, j);
                i = j;
            } catch (IOError e) {
                stderr.printf ("%s\n", e.message);
            }
            if (server_up)
                Thread.usleep(1000000);
        }
        main_loop.quit();
    }
    
    void on_name_vanished(DBusConnection conn, string name) {
        stdout.printf ("%s vanished, closing down.\n", name);
        server_up = false;
    }
}

void main () {
    Demo demo = new Demo();
    demo.start();
}

client.vala

The server code is unchanged.

With this version, once the server and client are started, stopping the server through CTRL-C will notify the client which will then stop gracefully. You can even have multiple clients, they will all stop in the same way. The advantage of doing this is that the client has a chance to clean up any resource it holds before stopping.

Example 3: Peers and Service Migration

All of the above is great and we can now design client programs that share a common service. But when dealing with software that is started by the user, the client/server model is not always the best option: you need to ensure that the server is started before any client is and that it is only stopped after the last client has stopped. And what happens is the server fails? What would be really nice is if we could have peers: no difference between client and server, just a single executable that can be run multiple times, the first process to start acts as a server for the other ones and when it stops responsibility for the service is taken over by one of the other processes if any are still running. The last process to terminate closes the service. D-Bus makes it easy to implement for the following reasons:

• Only one process can own a service;
• All service calls go via D-Bus so a client process will not notice if the server process actually changes, as long as the service is still available;
• The client and server processes can be the same process or different processes, it makes no difference to the client.

Therefore, the implementation of the peer program is simple:

• Start the server, followed by the client and ignore any failure in starting the server: if another process has already started, the service will be available to the client;
• When the peer is notified of the loss of service, try to start the server but ignore any failure: if another process was notified first, it will have started the server and there will be no interruption in service for the client thread.

We only need a single Vala file to implement the peer:

[DBus (name = "org.example.Demo")]
interface DemoClient : Object {
    public abstract int ping (string msg, int i)
        throws IOError;
}

[DBus (name = "org.example.Demo")]
public class DemoServer : Object {

    public int ping (string msg, int i) {
        stdout.printf ("%s [%d]\n", msg, i);
        return i+1;
    }
}

public class Demo : Object {
    private DemoClient client;
    
    private uint watch;
    
    public Demo() {
        // nothing to initialise
    }
    
    public void start() {
        start_server();
        start_client();
        new MainLoop().run();
    }
    
    public void start_client() {
        try {
            watch = Bus.watch_name(
                BusType.SESSION,
                "org.example.Demo",
                BusNameWatcherFlags.AUTO_START,
                on_name_appeared,
                on_name_vanished
            );
            client = Bus.get_proxy_sync (
                BusType.SESSION,
                "org.example.Demo",
                "/org/example/demo");
            Thread.create(() => {
                run_client();
                return null;
            }, false);
        } catch (IOError e) {
            stderr.printf ("Could not create proxy\n");
        }
    }
    
    public void run_client() {
        int i = 1;
        int j;
        while(true) {
            try {
                j = client.ping ("ping", i);
                stdout.printf ("%d => %d\n", i, j);
                i = j;
            } catch (IOError e) {
                stderr.printf ("Could not send message\n");
            }
            Thread.usleep(1000000);
        }
    }
    
    public void start_server() {
        Bus.own_name (
            BusType.SESSION,
            "org.example.Demo",
            BusNameOwnerFlags.NONE,
            on_bus_aquired,
            () => stderr.printf ("Name aquired\n"),
            () => stderr.printf ("Could not aquire name\n"));
    }
    
    void on_name_appeared(DBusConnection conn, string name) {
        stdout.printf ("%s appeared.\n", name);
    }
    
    void on_name_vanished(DBusConnection conn, string name) {
        stdout.printf (
            "%s vanished, attempting to start server.\n",
            name);
        start_server();
    }

    void on_bus_aquired (DBusConnection conn) {
        try {
            conn.register_object (
                "/org/example/demo",
                new DemoServer ());
        } catch (IOError e) {
            stderr.printf ("Could not register service\n");
        }
    }
}

void main () {
    Demo demo = new Demo();
    demo.start();
}

peer.vala

Start several peers in different terminal windows, at least 3 or 4. You will notice that the first process starts a server thread that all client threads connect to. When the process that provides the service terminates, the next process in the chain takes over the responsibility for the service. And finally, when the last process terminates, the service is closed.

Note that if you want to check D-Bus activity while running the programs in this introduction, you can either install the D-Feet tool or run dbus-monitor from the command line.

The Devil's in the Details

A good piece of software should make boring, repetitive and error prone tasks easy. One of them that photographers face regularly is resizing a batch of images. The typical situation is the day after a party when everybody wants a copy of the photos you took, either by email or on a CD. Quite often, people don't want the full size but they'd quite like copies of all the photos. I found myself in such a situation on New Year's Day: 150-odd photos that needed resizing and burning to a CD. Luckily, I had my trusty Ubuntu laptop running Shotwell. Shotwell does exactly the right thing when you want to export photos: it allows you to resize them all to a size that you specify, as shown in the dialog below:

Shotwell's export dialog

My laptop took a few minutes to resize all 150 photos when I exported them. Next to me was a friend who uses a proprietary operating system that shall remain nameless and who commented: Wow, that's cool! When I do that, I have to resize each photo manually and it takes ages! Sometimes the best features are the simple ones.