Monday, August 10, 2015

How Installing a Random Tool Caused Ruby on Rails to Stop Working

At work I maintain a little Ruby on Rails site. Not a particularly interesting site, just a set of pages of statistics gathered from various databases around the organization. It runs on a RHEL6 virtual machine and I write most of the code on my Windows laptop using RubyMine from JetBrains - a product I wholeheartedly endorse, by the way.

It does take a little patience to run Rails on Windows - not due to any deficit in either product, just because the vast majority of people who work with Rails are running it on some other platform, so when you have problems, a lot of times you're on your own.

Take this problem I ran into recently, for example. I launched RubyMine to check out a bug, fired up the ol' testing server, browsed to the web page, and was greeted with this:

JSON::ParserError in Statistics#index
Showing /app/views/layouts/application.html.erb where line #5 raised:

757: unexpected token at 'Node Commands

    node {operator} [options] [arguments]

        /? or /help   - Display this help message.
        list          - List nodes or node history or the cluster
        listcores     - List cores on the cluster
        view          - View properties of a node
        online        - Set nodes or node to online state
        offline       - Set one or more nodes to the offline state

For more information about HPC command-line tools,

(in /app/assets/javascripts/

Now, of all possible error messages to get, one involving HPC command-line tools was not one that I had ever considered. What's going on here?

The line that gives the error is one that occurs in every Rails application:

<%= javascript_include_tag 'application' %>

What it really comes down to saying is, take all of the Javascript and Coffeescript code in the application, and include it here. A little wasteful, perhaps, but in a production environment all of the code will get glommed into a single file appropriate for caching in the browser, so it's not a bad system. Still, where's the error?

Obviously it must be in the Coffeescript file referenced at the end of the error message. But why is it suddenly complaining about HPC? I went and commented that whole file out - no joy. The same error got reported with a different file. I could find no information anywhere about what the cause might have been, and I didn't expect searching for the error message itself would do any good. I work in an HPC - which stands for High  Performance Computing, incidentally - environment so I was fairly sure the error was something very specific to my machine.

I tried, of course. Several different Google searches failed - but I finally hit on the right combination of keywords that led me to a Google Plus comment on a YouTube tutorial.

It turns out that Rails, in its infinite struggle for flexibility, relies on a gem called ExecJS to decide how it's going to convert Coffeescript to Javascript. ExecJS, in turn, is of the opinion that Node.js is a really good way to do the conversion. And it determines if Node.js is available by trying to run an application called Node.

Which is fine, unless you happen to run Windows; and you happen to work in an HPC environment; and you happen to have gone to a conference the other week where you decided to install Microsoft's HPC tools; which happen to include a nice command for managing compute nodes; called - you guessed it - Node. ExecJS was trying to use Microsoft's Node command to convert Coffeescript to Javascript, resulting in the error you see. Hats off to Jonathan McDonald for nailing the issue.

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