tahoe plugins: facilitate extensions/applications written in Python

[zooko]

Suppose you want to write an application that uses Tahoe, or suppose you want to add an extension to Tahoe. You can run your code in a separate process and use the WAPI, but if your application or extension is written in Python it can have better control over Tahoe's behavior, be a bit more efficient, and be simpler by making Python method invocations to Tahoe instead of HTTP requests.

But how should you get your code loaded into the same Python interpreter as the Tahoe code? The answer is you either use import to load the Tahoe code and then you run it (precisely how, I'm not sure -- perhaps by calling startService() on it?), or you arrange for the Tahoe app to load your code at runtime when it is started by e.g. the command-line tahoe start.

The latter case is called a "plugin". There are three examples of plugin systems that we could use:

1. The setuptools plugin system, as exemplified by trac: ​http://trac.edgewall.org/wiki/TracPlugins

In this system, you put a ".egg" containing your code into a directory, and then you configure in a configuration file whether you want trac to load a plugin by that name.

2. The twisted plugin system, as documented here: ​http://twistedmatrix.com/projects/core/documentation/howto/plugin.html

I don't know if this is much used, but the way David Reid explained it to me it sounded nice and simple. :-)

3. The mercurial extension system, as documented here:

​http://www.selenic.com/mercurial/wiki/index.cgi/UsingExtensions

You just put the name of your extension into your Mercurial config file, and then when it runs it attempts to import ThatName, so you have to make sure a module (or directory-with-init.py-file) by that name is on your PYTHONPATH.

[exarkun]

So ... yea ... but this ticket is too general. Plugins *for what*? To have a plugin, you need a plugin interface. To have a plugin interface, you need an extension point. Tahoe-LAFS has many possible extension points. I implemented one recently to let plugins supply an alternate storage protocol.

We probably need a ticket per desired extension point.