Prevent race condition when watching model

This temporarily leads to two initial loads of the model. I'm going to
fix that in subsequent commits.

fj-app/src/main.rs

@@ -80,15 +80,6 @@ fn main() -> anyhow::Result<()> {
         parameters.insert(key, value);
     }
 
-    // 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.
-    //
-    // This is being tracked in the following issue:
-    // https://github.com/hannobraun/fornjot/issues/32
     let shape = model.load(&parameters)?;
 
     let shape_processor = ShapeProcessor::new(args.tolerance)?;

fj-app/src/model.rs

@@ -5,6 +5,7 @@ use std::{
     path::PathBuf,
     process::Command,
     sync::mpsc,
+    thread,
 };
 
 use notify::Watcher as _;
@@ -100,6 +101,8 @@ impl Model {
         parameters: HashMap<String, String>,
     ) -> Result<Watcher, Error> {
         let (tx, rx) = mpsc::sync_channel(0);
+        let tx2 = tx.clone();
+
         let watch_path = self.src_path.clone();
 
         let mut watcher = notify::recommended_watcher(
@@ -155,6 +158,14 @@ impl Model {
 
         watcher.watch(&watch_path, notify::RecursiveMode::Recursive)?;
 
+        // To prevent a race condition between the initial load and the start of
+        // watching, we'll trigger the initial load here, after having started
+        // watching.
+        //
+        // Will panic, if the receiving end has panicked. Not much we can do
+        // about that, if it happened.
+        thread::spawn(move || tx2.send(()).unwrap());
+
         Ok(Watcher {
             _watcher: Box::new(watcher),
             channel: rx,