Source Code for Mozilla Jetpack Features

Mozilla Labs have released Jetpack, an environment for writing Firefox extensions using JavaScript. It looks pretty neat and I'm interested in trying writing one myself.

The best way to learn a new environment is to look at working examples and you can find a growing number of these in the Jetpack Gallery.

You just install Jetpack and then install the 'Features' that interest you and try them out.

But where is the source code? It's not under the Tools menu and you can't do something simple like right-click the Feature icon in your status bar.

1: Enter 'about:jetpack' in the URL box
2: This brings up a page with links to various things including a tutorial and the API reference.
3: Click on 'Installed Features' to see a list of everything you have installed.
4: Click on 'view source' next to each Feature to bring up the JavaScript source in a new window.

I'm not a fan of too many browser extensions but some of them like Firebug, YSlow and S3Fox are invaluable. Jetpack will make it easier for folks like me to contribute new extensions.

Of course this only applies to the Firefox browser...

Remote Desktop Software

In the past I've used VNC as a remote desktop solution for viewing, say, a Linux desktop on a remote Mac, but I've now switched to NoMachine NX.

VNC is widespread and I think most Linuxes come with it already installed. But when I tried to use it recently with a Fedora VNC server and Mac OS X VNC client (Chicken of the VNC) I was seeing performance that made it unusable.

I think the problem stems from a variant of the VNC protocol or software called TightVNC and I believe the default Linux implementation uses this. Running a Tight VNC client on a Windows machine gave reasonable performance and doing the same on Mac OSX might solve the problem.

Instead I decided to check out NoMachine NX, which I had heard about in the past. This is commercial software but there are free versions of the server and client available for all the main platforms.

Installing the server (on Fedora) involves downloading RPMS for the client, node and server (you need all 3) and installing them:

# sudo rpm -i nxclient-3.4.0-5.x86_64.rpm
# sudo rpm -i nxnode-3.4.0-6.x86_64.rpm
# sudo rpm -i nxserver-3.4.0-8.x86_64.rpm

It installs into /usr/NX by default and you start/stop the server with
# /usr/NX/bin/nxserver --status|--start|--stop|--restart

You don't need to bother with any other NX options right now. At least, not if you are accessing machines in a private network. You may need to tweak your Fedora to allow the remote client to access your desktop (I didn't have to).

On the client end (Mac OS X in my case), download and install then fire up the application. It will give you a 'Connection Wizard' which is self explanatory.

The session will open up an X window on your Mac and I find the performance (on a local network) to be more than adequate. With remote desktops there can be issues with the 'scope' of keystrokes - for example you can't Cmd-C some text in the Mac world and expect to Cmd-V it into a Linux app - that sort of thing.

So if you want an open source solution then play around with VNC, otherwise go with NoMachine NX.

Fixing Broken Macs

If and when you need to resuscitate a Mac that won't boot up all the way, there are some commands that will save you a lot of time.

If your problem is not a true hardware issue then chances are there are disk errors and so the first task is to check and repair the disks. You can boot off an install CD/DVD or one with repair tools on it, but if you are comfortable with booting UNIX machines then skip that and...

Boot in Single User Mode

Hold down Command-S when you turn the machine on. You should get a gray screen which will quickly turn into a classic UNIX console with all sorts of cryptic text. At the bottom of which should be some instructions to either check the disks or to reboot in 'full' Single User Mode.

Disk repair with fsck

Your first step should be to run fsck, the UNIX disk checking program, with the options -fy. The 'f' forces fsck to check all available filesystems and 'y' answers Yes whenever it would ask you to approve a fix. If you skip the 'y' then you'll be sat there for hours responding to prompts.

# fsck -fy

That will likely produce lots of scary messages about different files. Don't sweat the details, just let fsck do its thing. Any files that it breaks were already broken by the underlying disk issue. fsck won't compound the damage, only repair most or all of it.

When it finishes, run it again! There can be dependency issues with disk fixes so to play it safe just run it multiple times until it reports no new errors. Typically the first run will get them all.

If you want to poke around on the filesystem then you can mount it with:
# mount -uw /

Reboot the System

If you are feeling lucky then just type 'reboot' at the prompt to see if your system comes back all the way. Fingers crossed...

If it doesn't then just restart the machine with Command-S as before and get back to Single User Mode.

'Full' Single User Mode

At this point you are in Single User Mode but with only the basic services. You can start up a bunch more stuff with

# sh /etc/rc.d

You'll see a bunch more verbage on the console. Keep an eye on it to see if any services are not able to start up - important clues to your problem. You'll either get a system prompt or an error. Hit return to get the prompt if you don't see one.

Now you have access to pretty much all the command line programs, man pages etc. an you are free to do untold damage to your system...

diskutil

One useful program to know about is diskutil, which is the Mac tool for messing with disks. Look at the man page for details.

To see what disks the system knows about:
# diskutil list

To eject a CD/DVD in the drive (your device name may be different)
# diskutil eject /dev/disk1

Zapping the PRAM

You'll see mention about zapping the PRAM memory in the machine by starting it up while holding down Option + Command + P + R (don't worry about Shift for the P and R). Hold them until your hear two chimes.

This is easy to do - and has never had any effect on any system that I've played with...

Booting from a CD/DVD

To force a boot from a CD with recovery tools on it, turn the machine on, pop the disk in and hold down the C key. You should get to a gray screen with the spinning indicator. Let go of the C key and you should get an OS install menu, or whatever you have on your CD.

If a simple disk repair doesn't fix the issue then you may need to reinstall the OS from disk. Not a problem but be sure to select 'Archive and Install' in order to keep your existing files. Some might say just go for a reinstall regardless of the problem - but I would strongly suggest doing a disk repair first.

Good luck with your fix.

Rails Production Environments

Two things that I always forget when working with a Rails app in production mode - without them you get the development environment.

Migrations:
$ rake db:migrate RAILS_ENV=production

Console
$ script/console production

SHA1 Digest of an Empty String

The use of SHA1 digests of data as unique identifiers is the answer to everything.

I know that appears to be ridiculous hyperbole, but I'll go into why I think that is the case in a longer post at some point. For now you'll have to take my word for it that using them as identifiers has simplified the inner workings of a big project tremendously. It's not just me either, the Software revision control system Git uses them as IDs for all its commits.

Anyway... one issue that can arise with them is that you notice that one specific digest string occurs more frequently than others in this uniformly distributed hash space... yikes... what's going on?

That digest would be da39a3ee5e6b4b0d3255bfef95601890afd80709 - make a note of it!

It represents the SHA1 digest of an Empty String

You might want to consider adding an explicit test for that string somewhere in your code.

Ideally you want a direct test for an empty string being used as an ID, but checking for this digest is a useful catch-all test.

Once I realized that this digest was being generated, and realized what it represented, then I could focus in on the root cause very quickly.

Capistrano and Environment Variables

When it works well, Capistrano is a great way to deploy Rails applications and take care of any remote server operations that are needed for that app.

But if you run into problems it can be a bear to troubleshoot. And the documentation is not great.

One issue that can catch you (or me!) unawares is the 'run' command that runs a Unix command on the remote host. I use these to symlink various files and to start up a daemon on the new deployment once that is complete.

You run into a problem if any of your remote commands require access to your Unix Environment Variables. The 'run' command is executed in a minimal environment without any of these.

The solution is to specify the variables that you need, such as PATH, as key/value pairs in the 'default_environment' hash in your cap deploy.rb file.

For example, here are two variables defined near the top of my file:

default_environment['AMAZON_ACCESS_KEY_ID'] = "YourAwsKeyHere"
default_environment['AMAZON_SECRET_ACCESS_KEY'] = "YourSecretAccessKeyHere"

Easy enough, once you know... not at all easy if you don't know!


 

Lightbox2 in a Rails App

Lightbox2, by Lokesh Dhakar, is an excellent JavaScript script for displaying images and slide shows as overlays on a web page, triggered by clicking on a thumbnail.

I wanted something like this to display screenshots in the online help for a Rails application. Overall it was pretty simple to set up, but here are some of the tweaks I needed to make.

1: Lightbox2 uses prototype.js and Scriptaculous. I had those in my Rails app but I found I had to upgrade those scripts to the latest versions in order to get Lightbox2 to work.

2: I had included the prototype and scriptaculous libraries using the Rails helper 'javascript_include_tag :defaults', but that alone is not sufficient. Lightbox2 needs the builder and effects scripts loaded. Lightbox2 says to use this line:

<script type="text/javascript" src="/javascripts/scriptaculous.js?load=effects,builder"></script>

And I put that right after the javascript_include_tag. This is undoubtedly duplication... I should probably just include the supporting scripts directly and leave out the Rails helper.

3: As the Lightbox2 docs point out, make sure you have the four default gif image files in the right places and that they are defined correctly in lightbox.js and lightbox.css.

4: Add the calls to lightbox.js and lightbox.css in your Rails layout file. Note that you have to change the paths from the example to suit Rails:

<script type="text/javascript" src="/javascripts/lightbox.js"></script>
<link rel="stylesheet" href="/stylesheets/lightbox.css" type="text/css" media="screen" />

5: Now you're ready to add images and links that use Lightbox2. The idea is very simple. Take a thumbnail image img tag and place within a 'a' tag pair. Make the href of the 'a' tag point to the full size image. Importantly, in the 'a' tag add the attribute 'rel="lightbox"'. For example:

<a href="/images/fullsize.jpg" rel="lightbox" title="Picture of a cat"><img src="/images/thumbnail.jpg" style="border: 1px solid #808080;"></a>

The title attribute of the 'a' tag becomes the caption of the overlay image. In this example I have included a border on the thumbnail image. That should do it for basic operation.

6: To fine tune the script you can edit lightbox.js and lightbox.css. For me the animation is too slow. You can disable the animation entirely but I preferred to just speed it up with a maximum setting of 10. I also messed with the CSS for the 'imageData' set of CSS IDs to change the font and specified a different image for the 'close' icon in the .js file.

Lightbox2 is pretty slick. You can do slide shows with it as well. Hope this helps you get it integrated with Rails.