Move away from the debounced watcher: we have our own debouncing via …

…the Graph, and we want to avoid things like notify-rs/notify#205.

# Delete this line to force CI to run the JVM tests.
[ci skip-jvm-tests]

src/rust/engine/watch/Cargo.toml

@@ -13,11 +13,7 @@ graph = { path = "../graph" }
 hashing = { path = "../hashing" }
 log = "0.4"
 logging = { path = "../logging" }
-# notify is currently an experimental API, we are pinning to https://docs.rs/notify/5.0.0-pre.1/notify/
-# because the latest prerelease at time of writing has removed the debounced watcher which we would like to use.
-# The author suggests they will add the debounced watcher back into the stable 5.0.0 release. When that happens
-# we can move to it.
-notify = { git = "https://github.com/notify-rs/notify", rev = "fba00891d9105e2f581c69fbe415a58cb7966fdd" }
+notify = "5.0.0-pre.2"
 parking_lot = "0.6"
 task_executor = { path = "../task_executor" }
 

src/rust/engine/watch/src/lib.rs

@@ -81,8 +81,14 @@ impl InvalidationWatcher {
     let canonical_build_root =
       std::fs::canonicalize(build_root.as_path()).map_err(|e| format!("{:?}", e))?;
     let (watch_sender, watch_receiver) = crossbeam_channel::unbounded();
-    let mut watcher: RecommendedWatcher = Watcher::new(watch_sender, Duration::from_millis(50))
-      .map_err(|e| format!("Failed to begin watching the filesystem: {}", e))?;
+    let mut watcher: RecommendedWatcher = Watcher::new_immediate(move |ev| {
+      if watch_sender.send(ev).is_err() {
+        // The watch thread shutting down first is ok, because it can exit when the Invalidatable
+        // is dropped.
+        debug!("Watch thread has shutdown, but Watcher is still running.");
+      }
+    })
+    .map_err(|e| format!("Failed to begin watching the filesystem: {}", e))?;
 
     let (liveness_sender, liveness_receiver) = crossbeam_channel::unbounded();