Model modifications happening during fj-host initialization could be missed

[hannobraun]

The initialization of the host application roughly looks like this:

1. Load the model.
2. Do a bunch of other stuff.
3. Set up the mechanism that watches the model for changes.

If the model were to be modified during step 2., that change would be missed. I don't think this is very likely, and have certainly never seen it in real use, but none the less, there's a race condition here.

I haven't looked into this in a while, but last time I did, I got the impression that it isn't easy to fix, and probably a rework of that aspect of the architecture would be required.

In case anyone wants to take a look, here's where the model is loaded:

fornjot/src/main.rs

Lines 53 to 59 in 416df5f

	// Since we're loading the model before setting up the watcher below,
	// there's a race condition, and a modification could be missed between
	// those two events.
	//
	// This can't be addressed with the current structure, since the watcher
	// closure takes ownership of the model.
	let shape = model.load(&parameters)?;

And here's where the watcher is set up:

fornjot/src/main.rs

Lines 123 to 163 in 416df5f

	let watch_path = model.src_path();
	let mut watcher = notify::recommended_watcher(
	move |event: notify::Result<notify::Event>| {
	// TASK: Figure out when this error can happen, find a better way to
	// handle it.
	let event = event.expect("Error handling watch event");
	//Various acceptable ModifyKind kinds. Varies across platforms (e.g. MacOs vs. Windows10)
	if let notify::EventKind::Modify(notify::event::ModifyKind::Any)
	| notify::EventKind::Modify(notify::event::ModifyKind::Data(
	notify::event::DataChange::Any,
	))
	| notify::EventKind::Modify(notify::event::ModifyKind::Data(
	notify::event::DataChange::Content,
	)) = event.kind
	{
	let shape = match model.load(&parameters) {
	Ok(shape) => shape,
	Err(model::Error::Compile) => {
	// It would be better to display an error in the UI,
	// where the user can actually see it. Issue:
	// https://github.com/hannobraun/fornjot/issues/30
	println!("Error compiling model");
	return;
	}
	Err(err) => {
	panic!("Error reloading model: {:?}", err);
	}
	};
	// This will panic, if the other end is disconnected, which is
	// probably the result of a panic on that thread, or the
	// application is being shut down.
	//
	// Either way, not much we can do about it here, except maybe to
	// provide a better error message in the future.
	watcher_tx.send(shape).unwrap();
	}
	},
	)?;
	watcher.watch(&watch_path, notify::RecursiveMode::Recursive)?;

[hannobraun]

I don't plan to work on this, as I don't think this is likely to happen in practice. And since more changes to the model/plugin functionality are going to happen (see #7), I hope that a cleaner architecture that takes care of this problem will reveal itself during that process.