Avoid watchers for one-shot builds and preserve watcher errors (AI-assisted)

tests/tests/build.rs

@@ -108,3 +108,58 @@ fn run_build_test(test_name: &str) {
         assert_snapshot!(test_name, contents);
     });
 }
+
+#[cfg(target_os = "linux")]
+#[test]
+fn build_without_watch_does_not_create_inotify_instance() -> Result<(), Box<dyn std::error::Error>>
+{
+    const CHILD_ENV: &str = "ROJO_TEST_BUILD_WITHOUT_WATCH_CHILD";
+
+    // Isolate descriptor inspection from other tests that create file watchers.
+    if std::env::var_os(CHILD_ENV).is_none() {
+        let output = Command::new(std::env::current_exe()?)
+            .args([
+                "--exact",
+                "tests::build::build_without_watch_does_not_create_inotify_instance",
+                "--nocapture",
+            ])
+            .env(CHILD_ENV, "1")
+            .output()?;
+        assert!(
+            output.status.success(),
+            "{}\n{}",
+            String::from_utf8_lossy(&output.stdout),
+            String::from_utf8_lossy(&output.stderr)
+        );
+        return Ok(());
+    }
+
+    let project_dir = tempdir()?;
+    fs::write(
+        project_dir.path().join("default.project.json"),
+        r#"{"name":"WithoutWatcher","tree":{"$className":"Folder","$path":"src"}}"#,
+    )?;
+    fs::create_dir(project_dir.path().join("src"))?;
+    fs::write(project_dir.path().join("src/main.luau"), "return 42")?;
+    let output_path = project_dir.path().join("output.rbxmx");
+
+    librojo::cli::BuildCommand {
+        project: project_dir.path().to_path_buf(),
+        output: Some(output_path.clone()),
+        plugin: None,
+        watch: false,
+    }
+    .run()?;
+
+    assert!(fs::read_to_string(output_path)?.contains("return 42"));
+    // BuildCommand keeps its session alive until process exit, so an eagerly
+    // allocated watcher remains observable after the build completes.
+    for entry in fs::read_dir("/proc/self/fd")? {
+        match fs::read_link(entry?.path()) {
+            Ok(target) => assert_ne!(target, Path::new("anon_inode:inotify")),
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {}
+            Err(err) => return Err(err.into()),
+        }
+    }
+    Ok(())
+}