Meteor: downloading files during build process - build

Question

Does anyone have experience managing the build process for a smart package, so that external files can be downloaded and served?
For example, a meteor smart package the wraps a javascript library can curl the library when the app is being built, and serve it directly from the server. This is useful for things like jquery-ui, the Ace editor, etc.
Specifically, I'm asking about the proper way to use Package._transitional_registerBuildPlugin, which was introduced in 0.6.5. Any examples would be great.
Grepping Meteor code only turns up things that add source handlers, so it's not very helpful for deciding what to do for downloading a file.
./packages/templating/package.js:Package._transitional_registerBuildPlugin({
./packages/stylus/package.js:Package._transitional_registerBuildPlugin({
./packages/meteor/package.js:Package._transitional_registerBuildPlugin({
./packages/coffeescript/package.js:Package._transitional_registerBuildPlugin({
./packages/less/package.js:Package._transitional_registerBuildPlugin({
A related question: what is a smart package allowed to write to its own .build folder?

Answer 1

I am aware that this is not exactly an answer for your question, but I couldn't resist posting this idea :)
The easiest thing that comes to my mind is to implement a simple "proxy" server using curl as you suggested. It could be more or less something like:
var spawn = Npm.require('child_process').spawn;
WebApp.connectHandlers.stack.splice (0, 0, {
route: '/jquery.js',
handle: function(req, res, next) {
spawn('curl', [
'https://code.jquery.com/jquery-1.10.2.min.js'
]).stdout.pipe(res);
res.writeHead(200, {
'Content-Type': 'text/javascript'
});
},
});
Though, it does not solve - out of the box - the problem of controlling the load order.
EDIT
If you prefer not to call curl every single time the file is requested by the user, you can always cache it somewhere on the server, right? Also this technique allows you to load additional libraries on demand, so only when they're actually needed. Note, that things like ace editor can be pretty heavy.

Answer 2

I ended up implementing this as a build plugin as part of my ShareJS smart package.. It's not the prettiest thing, but does what it needs to do. My approach is leading toward a generalized smart package for processing CDN downloads from Meteor packages.
I expect that the path to doing things like this will become easier and clearer as the build API becomes better documented.