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November 22, 2013

Filed under: tech»coding

Plug In, Turn On, Drop Out

This is me, thinking about plugins for Caret, as I find myself doing these days. In theory, extensibility is my focus for the next major release, because I think it's a big deal and a natural next step for a code editor. In practice, it's not that simple.

Approach #1: Postal Services

Chrome has a pretty tight security model applied to packaged apps, not the least of which is a strict content security policy. You can't run code from outside your application. You can't construct code using eval (that's good!) or new Function (that's bad). You can't add new files to your application (mostly).

Chrome does expose an inter-app messaging system similar to postMessage, and I initially thought about using this to create a series of hooks that external applications could use. Caret would broadcast notifications to registered listeners when it did something, and those listeners could respond. They could also trigger Caret commands via message (I do still plan to add this, it's too handy not to have).

Writing plugins this way is neatly encapsulated and secure, but it's also going to be intensely frustrating. It would require auditing much of Caret's code to make sure that it's all okay with asynchronous operation, which is not usually the case right now. I'd have to make sure that Caret is sufficiently chatty, because we'd need hooks everywhere, which would clutter the code with broadcast/response blocks. And it would probably mean writing a helper app to serve as a patchboard between applications, and as a debugging tool.

I'm not wild about this one.

Approach #2: Repo, Man

I've been trying to think of a way around the whole inter-app messaging paradigm for about a month now. At the same time, I've been responding to requests for Git and remote filesystem support, which will not be a core Caret feature. For some reason, thinking about the two in close proximity started me thinking along a new track: what if there were a way to work around the security policy using the HTML5 file system? I decided to run some tests.

It turns out this is absolutely possible: Chrome apps can download a script from any server that's whitelisted in their manifest, write that out to the filesystem, and then get a special URL to load that file into a <script> tag. I assume this has survived security audits because it involves jumping through too many hoops to be anything other than deliberate.

The advantages of this approach are numerous. Plugin code would operate directly alongside of Caret's source, able to access the same functions and modules and call the same APIs that I use. It would be powerful, and would not require users to publish plugins to the Chrome store as if they were full applications. And it would scale well--all I would need to do is maintain the index and provide some helper functions for developers to use when downloading and caching their code.

Unfortunately, it is also apparently forbidden by the Chrome Web Store policies, which state:

Packaged apps should not ... Download or execute scripts dynamically outside a sandboxed environment such as a webview or a sandboxed iframe.
At that point, we're back to postMessage unless I want to be banned from the store. So much for the workaround.

Approach #3: Local hosting

So how can I make plugins work for end users? Well, honestly, maybe I don't. One of the nice things about writing developer tools, particularly oddball developer tools, is that the people using them and wanting to expand on them are expected to have some degree of technical knowledge. They can be trusted to figure out processes that wouldn't necessarily be acceptable for average computer users. In this case, that might mean running Caret as an unpacked app.

Loading Caret from source is not difficult--I do it all the time while I'm testing. Right now, if someone wants to fork Caret and add their own features, that's easy enough to do (and actually, a couple of people have done so already). What it lacks is a simple entry point for people who want to contribute functionality without digging into all the modules I've already written.

By setting up a plugins directory and a little bit of infrastructure, it's possible to reach a middle ground. Developers who really want extra packages can load Caret from source, dump their code into a designated location, and have their code bootstrapped automatically. It's not as friendly as having web store distribution, and it's not as elegant as allowing for a central repo, but it does deliver power without requiring major rewrites.

Working through all these different approaches has given me a new appreciation for insecurity, which sounds funny but it's true. Obviously I'm in favor of secure computing, but working with mobile operating systems and Chrome OS that strongly sandbox their code tends to make a person aware of how helpful a few security holes can be, and vice versa: the same points for easy extension and flexibility are also weak points that can be exploited by an attacker. At times like this, even though I should maybe know better, that tradeoff seems absolutely worth it.

Future - Present - Past