Fedora 12

For the moment I'm evaluating Fedora 12. I was a bit disappointed on the Kubuntu 9.10 release, the distribution felt a little bit slow and tiered. The only thing I'm satisfied with is the boot time, it starts quite fast.

The KDE4 desktop in Fedora feels kind of snappier than KDE in Kubuntu. I'm not sure if it's because Fedora is compiled for i686 and not i386 or if the team have done any optimizations. I didn't enabled desktop effects in neither distribution. I installed the nvidia proprietary driver in Kubuntu but use Nouveau (open source driver) in Fedora. I might try the proprietary driver in Fedora to see if I gain any speed.

There's one bad thing I found in Fedora in comparision with Kubuntu, KPackageKit. It takes ages before starting compared to the Kubuntu version so I'm using yum from the shell prompt instead. Well, to be honest, I'm not a heavy user of UI front-ends to package management systems. In Kubuntu I prefer using aptitude instead of KPackageKit.

I think I'll give Fedora a chance and try it out for a while.

FUSE - Filesystem in Userspace part 2

This is the second post on implementing file systems using FUSE. In this post I'll show you how to create a read-only file system that is backed by an XML file. Each tag in the XML file is either a file or a directory. If the tag is representing a directory, it should have a is_dir="1" attribute added. When the tag represents a file it should have its content between the tags. The following XML snippet shows an example:

<root is_dir="1">

  <a is_dir="1">
    <file1.txt>File content for this file</file1.txt>
    <hello>Hello World</hello>
  </a>

  <empty_dir is_dir="1" />

</root>

The example models the following directory tree:

/
|-- a
|   |-- file1.txt
|   `-- hello
`-- empty_dir

There are some limitations in having an XML file representing a file system. You can't for example have filenames starting with a dot because tags are not allowed to start with a dot in XML.

On to the code, the following snippet implements the read-only file system which is backed by an XML file:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import errno
import fuse
import stat
import os
import time
import xml.etree.ElementTree as etree

fuse.fuse_python_api = (0, 2)

# Use same timestamp for all files
_file_timestamp = int(time.time())

class MyStat(fuse.Stat):
    """
    Convenient class for Stat objects.
    Set up the stat object with appropriate
    values depending on constructor args.
    """
    def __init__(self, is_dir, size):
        fuse.Stat.__init__(self)
        if is_dir:
            self.st_mode = stat.S_IFDIR | 0555
            self.st_nlink = 2
        else:
            self.st_mode = stat.S_IFREG | 0444
            self.st_nlink = 1
            self.st_size = size
        self.st_atime = _file_timestamp
        self.st_mtime = _file_timestamp
        self.st_ctime = _file_timestamp

class MyFS(fuse.Fuse):
    def __init__(self, xml_tree, *args, **kw):
        fuse.Fuse.__init__(self, *args, **kw)
        self.tree = xml_tree

    def getattr(self, path):
        # We do not support 'dot' files
        # since xml tags cannot start with a dot.
        if path.find('/.') != -1:
            return -errno.ENOENT

        entry = self.tree.find(path)

        if entry is None:
            return -errno.ENOENT
        else:
            is_dir = entry.get('is_dir', False)
            size = entry.text and len(entry.text.strip()) or 0
            return MyStat(is_dir, size)

    def readdir(self, path, offset):
        yield fuse.Direntry('.')
        yield fuse.Direntry('..')
        for e in self.tree.find(path).getchildren():
            yield fuse.Direntry(e.tag)

    def open(self, path, flags):
        # Only support for 'READ ONLY' flag
        access_flags = os.O_RDONLY | os.O_WRONLY | os.O_RDWR
        if flags & access_flags != os.O_RDONLY:
            return -errno.EACCES
        else:
            return 0

    def read(self, path, size, offset):
        entry = self.tree.find(path)
        content = entry.text and entry.text.strip() or ''
        file_size = len(content)
        if offset < file_size:
            if offset + size > file_size:
                size = file_size - offset
            return content[offset:offset+size]
        else:
            return ''

if __name__ == '__main__':
    tree = etree.parse('tree.xml')

    fs = MyFS(tree)
    fs.parse(errex=1)
    fs.main()

I'm using the ElementTree XML API to parse the XML file. The ElementTree API is very easy to use and supports finding elements by specifying a path which fits very well in this context. For example, the following snippet shows how to get the file1.txt element from the example XML file above and extract the content between the tags:

import xml.etree.ElementTree as etree
tree = etree.parse('tree.xml')
el = tree.find('/a/file1.txt')
print el.text.strip() # Remove any white-spaces

The code hasn't change that much since the part 1 post. I've added two new methods which adds support for opening and reading files. The MyStat class is only used as an convenient class to help creating appropriate stat objects. You might notice that I don't do a lot of checking in the code, this is because FUSE do a lot of them for me, this page list some of the assumptions you can make when implementing file system using FUSE.

Did you believe it would be this easy to create a mountable file system that uses an XML file for the layout? I didn't.

In the next (and final) post about FUSE I think I'll create some kind of 'use a service on the Internet' file system which can be useful and not just another toy fs.

FUSE - Filesystem in Userspace part 1

Here's the first part in a series of posts on implementing file systems using FUSE.

Creating a file system can seem to be an intimidating task but with FUSE it's very easy and it gets even easier with python bindings. Any decent Linux distribution should include support for FUSE and be configured such that regular users (or a group) can mount file systems written with FUSE without having root access.

If you are running Ubuntu you need to install the python-fuse package which contains the Python bindings for FUSE.

- Enough, show me the code

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import errno
import fuse
import stat
import time

fuse.fuse_python_api = (0, 2)

class MyFS(fuse.Fuse):
    def __init__(self, *args, **kw):
        fuse.Fuse.__init__(self, *args, **kw)

    def getattr(self, path):
        st = fuse.Stat()
        st.st_mode = stat.S_IFDIR | 0755
        st.st_nlink = 2
        st.st_atime = int(time.time())
        st.st_mtime = st.st_atime
        st.st_ctime = st.st_atime

        if path == '/':
            pass
        else:
            return - errno.ENOENT
        return st

if __name__ == '__main__':
    fs = MyFS()
    fs.parse(errex=1)
    fs.main()

The example above shows how easy it is to create a mountable file system, it does nothing but is still a valid file system. The MyFS class implements a file system with no entries, it will always return ENOENT (No such file or directory) except for the root path. You should take a look at the FUSE Python Reference page which contains details about FusePython and, among other things, describes the stat object returned from the getattr method.

Paste the example to a file, MyFS.py, and make the file executable:

$ chmod 755 MyFS.py

Now create a directory which will be used as mount point and mount the file system:

$ mkdir myfs
$ ls -l
totalt 4
drwxr-xr-x 2 mario mario 4096 2009-11-15 15:59 myfs
-rwxr-xr-x 1 mario mario  648 2009-11-15 15:59 MyFS.py
$ # This will mount the file system, try ./MyFS.py --help
$ ./MyFS.py myfs
$ ls -l
totalt 0
drwxr-xr-x 2 root  root    0 2009-11-15 16:10 myfs
-rwxr-xr-x 1 mario mario 648 2009-11-15 15:59 MyFS.py

Here we can see that the owner, size and time stamp changed on myfs after we mounted the file system. You can cd into myfs, but if you try to list the directory (file system root) you'll get an error:

$ cd myfs
$ ls
ls: reading directory .: Function not implemented

You got this error because we haven't implemented the readdir method which is invoked when listing a directory. To make this work we need to add a readdir method which returns (at least) the '.' and '..' entries. Add the following snippet to the class:

    def readdir(self, path, offset):
        for e in '.', '..':
            yield fuse.Direntry(e) 

Before you continue you should unmount the file system:

$ fusermount -u myfs

This will unmount your FUSE file system as a regular user and doesn't require root access.

Retry to mount and cd into the myfs directory:

$ ./MyFS.py myfs
$ cd myfs
$ ls -a
.  ..

Alright, I think I'll stop for today. Oh, just one more thing... You won't get any print outputs nor tracebacks from your code if you don't start the file system in foreground mode. This is done by adding '-f' as argument when starting the file system.

$ ./MyFS.py -f myfs

It can be hard to tell why something doesn't work if you can't see tracebacks and print outputs.

GMonitor

Ah, I've finally managed to make my new fresh Kubuntu 9.10 installation usable. Now it's time to do some KDE stuff and I'll show you how easy it's to create a plasmoid (applet/widget) for KDE4.

I've created a simple applet which shows the number of unread mail in a Gmail inbox. We already know how to fetch the list with unread mail, which I have explained in a previous blog post. Now I'll create a nice applet showing the count. I'm going to keep it as simple as possible, writing hard-coded values, skip error handling and so on to avoid making things more complicated than necessary.

There are several good tutorials at techbase.kde.org which explains plasma programming. They cover the basics and explains things in detail so I'll only make references to the pages instead of repeating what's already been written.

The first tutorial you should read (you don't have to read it right now, you can read it after you tried doing the applet) is Python Plasma Getting Started. The tutorial covers setting up a simple plasmoid, packaging, installing and running.

I'll name the plasmoid GMonitor because it only monitors the mailbox and shows the number of unread mail, it will not actually try to do some kind of notification (yet?!).

I've put the code on github, you can clone it with:

$ git clone git://github.com/mariob/gmonitor.git

You'll find a Makefile in the git which can be useful when doing plasmoids. The Makefile supports installing, un-installing, viewing, packaging and updating the plasmoid.

The directory tree looks like this:

gmonitor/
|-- Makefile
|-- README
|-- contents
|   `-- code
|       `-- main.py
`-- metadata.desktop

The gmonitor directory is the project home and contents/code contains the python source. The metadata.desktop file will be explained below.

To be able to install and run your applet you need to provide a metadata.desktop file to plasma. The metadata.desktop file contains important information about the applet and you can read more about the file in the Plasma Getting Started tutorial.

Feel free to change the file. The Name specifies the applet name and the Icon field gives the name to the icon to associated with this applet. These two fields are typically shown when listing applets in the 'Add Widget' dialog. There are two important fields which must be present for plasma to run your applet, the X-Plasma-API field specifies which script-engine to be used and the X-Plasma-MainScript field which script to be executed. It's also good to know that the X-KDE-PluginInfo-Name fields is used as a plasmoid identification, so it should be unique.

Some notes about the implementation. There are two classes, GMonitor and MailFrame (I know, poor name). The MaiFrame class has an icon and a label. When the mail count is set to zero (by calling setCount(cnt)), the icon is disabled, grayed and the label is set to 'No new mail'. When the mail count is set to a value greater than zero, the icon is enabled, colored and the label is set to 'Count: x', where x is the number of mail in the inbox. See the images below.

Sample of what the applet looks like

When the icon is enabled and clicked, a 'clicked()' signal is emitted. The GMonitor class connects the openBrowser() method to the 'clicked()' signal. The openBrowser() method opens a default browser in KDE to load the google mail url. The GMonitor also sets up a timer to fetch the feed in 60 second interval. The fetchFeed() method uses the KIO framework in KDE to download the content from a URL and parseFeed() parses the downloaded feed and emits a 'mailcount' signal.

The implementation depends on feedparser which can be installed on Ubuntu by running:

$ sudo aptitude install python-feedparser

You can also download and put the feedparser.py file inside the code directory.

Not that bad. It's actually possible to create a very simple 'good looking' Gmail monitor in less than 100 lines of code (excluding comments) with Python and KDE.

To view this plasmoid without installing it run (while standing in the gmonitor folder):

$ make view

This might only work on KDE4.3, not sure if the plasmoidviewer in 4.2 supports running plasmoids without first installing them. If it fails you should install the plasmoid and run plasmoidviewer yourself (or modify the makefile):

$ make install
$ plasmoidviewer pysnippet-gmonitor

If you never heard of Qt signals and slots you should read this introduction on the topic. For example, a button can 'emit' a signal when it's clicked. A slot (function/method) can be connected to a signal and each time the signal is emitted the function/method will be called.

You can find the Python KDE4.3 API docs here. Unfortunately, I don't find them as good as the Qt docs.

When running the applet, KDE will ask for the username/password when it tries to connect to Google. You can check the 'remember the password' checkbox which will make KDE cache the password as long as the desktop session is alive. If you choose not to, you'll have to enter your username/password each time the code tries to fetch the feed. A better solution is to use KWallet.

Ok, hope you enjoyed it and start doing cool plasmoids!

Kubuntu 9.10

I've installed Kubuntu 9.10 and I'm busy configuring it to a usable state. For the moment I don't have too much spare time but I'm almost done.

I hope I'll have time to do a post about creating plasmoids (KDE4 desktop widgets) in the next couple of days. I think I'll do a gmonitor applet which will show the number of unread mail in your Gmail inbox.