feat(watcher): Use notify_debouncer_full

Based on code by @nyarly, this debounces watch events before filtering
file paths.

`notify_debouncer_full` will do the hard part of making sure spurious
events are sufficiently removed before we match on them. From the
docs:

* Only emits a single Rename event if the rename From and To events can be matched
* Merges multiple Rename events
* Takes Rename events into account and updates paths for events that occurred before the rename event, but which haven't been emitted, yet
* Optionally keeps track of the file system IDs all files and stiches rename events together (FSevents, Windows)
* Emits only one Remove event when deleting a directory (inotify)
* Doesn't emit duplicate create events
* Doesn't emit Modify events after a Create event

Cargo.toml

@@ -26,7 +26,11 @@ thiserror = "1.0"
 
 # TODO: update to 0.3
 structopt.version = "0.2"
+
+# TODO: update to 0.3
 structopt.default-features = false
+
+# TODO: update to 0.3
 structopt.features = [
   # "default",
   "suggestions",
@@ -55,11 +59,14 @@ lazy_static = "1.4.0"
 md5 = "0.7.0"
 vec1 = ">= 1.1.0, <1.7.0"
 human-panic = { path = "vendor/human-panic" }
+notify-debouncer-full = "0.3.1"
 
 [dev-dependencies]
 # 1.0.0 requires at least rust 1.50
 proptest.version = "0.10.1"
+# 1.0.0 requires at least rust 1.50
 proptest.default-features = false
+# 1.0.0 requires at least rust 1.50
 proptest.features = [
   "std",
   # reenable if proptest kills the test runner

src/build_loop.rs

@@ -170,9 +170,10 @@ impl<'a> BuildLoop<'a> {
         nix_gc_root_user_dir: project::NixGcRootUserDir,
         logger: slog::Logger,
     ) -> anyhow::Result<BuildLoop<'a>> {
-        let mut watch = Watch::new(logger.clone()).map_err(|err| anyhow!(err))?;
+        let watch = Watch::new(&logger).map_err(|err| anyhow!(err))?;
         watch
-            .extend(vec![WatchPathBuf::Normal(
+            .add_to_watch_tx
+            .send(vec![WatchPathBuf::Normal(
                 project.nix_file.as_absolute_path().to_owned(),
             )])
             .with_context(|| {
@@ -201,7 +202,7 @@ impl<'a> BuildLoop<'a> {
         rx_ping: chan::Receiver<()>,
     ) -> crate::Never {
         let mut current_build = BuildState::NotRunning;
-        let rx_watcher = self.watch.rx.clone();
+        let rx_watcher = self.watch.watch_events_rx.clone();
 
         loop {
             debug!(self.logger, "looping build_loop";
@@ -247,19 +248,13 @@ impl<'a> BuildLoop<'a> {
 
                 // watcher found file change
                 recv(rx_watcher) -> msg => match msg {
-                    Ok(msg) => {
-                        match self.watch.process_watch_events(msg) {
-                            Some(changed) => {
-                                // TODO: this is not a started, this is just a scheduled!
-                                send_event(Event::Started {
-                                    nix_file: self.project.nix_file.clone(),
-                                    reason: Reason::FilesChanged(changed)
-                                });
-                                self.start_or_schedule_build(&mut current_build)
-                            },
-                            // No relevant file events
-                            None => {}
-                        }
+                    Ok(changed) => {
+                            // TODO: this is not a started, this is just a scheduled!
+                            send_event(Event::Started {
+                                nix_file: self.project.nix_file.clone(),
+                                reason: Reason::FilesChanged(changed)
+                            });
+                            self.start_or_schedule_build(&mut current_build)
                     },
                     Err(chan::RecvError) =>
                         debug!(self.logger, "notify chan was disconnected"; "project" => &self.project.nix_file)
@@ -340,7 +335,10 @@ impl<'a> BuildLoop<'a> {
         debug!(self.logger, "paths reduced"; "from" => original_paths_len, "to" => paths.len());
 
         // add all new (reduced) nix sources to the input source watchlist
-        self.watch.extend(paths.into_iter().collect::<Vec<_>>())?;
+        self.watch
+            .add_to_watch_tx
+            .send(paths.into_iter().collect::<Vec<_>>())
+            .map_err(BuildError::io)?;
 
         // root the result
         self.project