fix(runtime/ops): Fix watchfs remove event (#27041)

Fix related to #26906. 
Currently, if a file is removed, no event is emitted because the file
path no longer exists. As a result, [this
check](12b377247b/runtime/ops/fs_events.rs (L149))
returns false.

With this PR, an additional check is introduced to verify if the file
exists. If the file does not exist, a custom "remove" event is emitted.
This change is necessary because, based on tests conducted on macOS and
Linux (Ubuntu 24.04.1 LTS), Linux emits a "rename" event instead of a
"remove" event when a file is deleted. Introducing a dedicated "remove"
event ensures consistent and clearer behavior across platforms.

runtime/ops/fs_events.rs

@@ -109,6 +109,14 @@ fn starts_with_canonicalized(path: &Path, prefix: &str) -> bool {
   }
 }
 
+fn is_file_removed(event_path: &PathBuf) -> bool {
+  let exists_path = std::fs::exists(event_path);
+  match exists_path {
+    Ok(res) => !res,
+    Err(_) => false,
+  }
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum FsEventsError {
   #[error(transparent)]
@@ -150,6 +158,13 @@ fn start_watcher(
             })
           }) {
             let _ = sender.try_send(Ok(event.clone()));
+          } else if event.paths.iter().any(is_file_removed) {
+            let remove_event = FsEvent {
+              kind: "remove",
+              paths: event.paths.clone(),
+              flag: None,
+            };
+            let _ = sender.try_send(Ok(remove_event));
           }
         }
       }

tests/unit/fs_events_test.ts

@@ -45,6 +45,14 @@ async function makeTempDir(): Promise<string> {
   return testDir;
 }
 
+async function makeTempFile(): Promise<string> {
+  const testFile = await Deno.makeTempFile();
+  // The watcher sometimes witnesses the creation of it's own root
+  // directory. Delay a bit.
+  await delay(100);
+  return testFile;
+}
+
 Deno.test(
   { permissions: { read: true, write: true } },
   async function watchFsBasic() {
@@ -155,3 +163,25 @@ Deno.test(
     assert(done);
   },
 );
+
+Deno.test(
+  { permissions: { read: true, write: true } },
+  async function watchFsRemove() {
+    const testFile = await makeTempFile();
+    using watcher = Deno.watchFs(testFile);
+    async function waitForRemove() {
+      for await (const event of watcher) {
+        if (event.kind === "remove") {
+          return event;
+        }
+      }
+    }
+    const eventPromise = waitForRemove();
+
+    await Deno.remove(testFile);
+
+    // Expect zero events.
+    const event = await eventPromise;
+    assertEquals(event!.kind, "remove");
+  },
+);