Only watch directories, not files (#1859)

If a `DirectoryMonitor` is pointed at a file (and not a directory), fail on invocation of `start()`.

Validated with a unit test and manual testing using the `dir-monitor` example.

src/agent/onefuzz/src/monitor.rs

@@ -40,6 +40,13 @@ impl DirectoryMonitor {
         // Canonicalize so we can compare the watched dir to paths in the events.
         self.dir = fs::canonicalize(&self.dir).await?;
 
+        // Make sure we are watching a directory.
+        //
+        // This check will pass for symlinks to directories.
+        if !fs::metadata(&self.dir).await?.is_dir() {
+            bail!("monitored path is not a directory: {}", self.dir.display());
+        }
+
         // Initialize the watcher.
         self.watcher.watch(&self.dir, RecursiveMode::NonRecursive)?;
 

src/agent/onefuzz/src/monitor/tests.rs

@@ -37,6 +37,20 @@ timed_test!(test_monitor_nonexistent_path, async move {
     Ok(())
 });
 
+timed_test!(test_monitor_file, async move {
+    let dir = tempdir()?;
+
+    // Create a file to erroneously watch.
+    let file_path = dir.path().join("some-file.txt");
+    tokio::fs::write(&file_path, "aaaaaa").await?;
+
+    let mut monitor = DirectoryMonitor::new(&file_path)?;
+
+    assert!(monitor.start().await.is_err());
+
+    Ok(())
+});
+
 timed_test!(test_monitor_dir, async move {
     let dir = tempdir()?;
     let mut monitor = DirectoryMonitor::new(dir.path())?;