libservo: Allow running more than one Servo test in a run (#36532)

A `Servo` instance can only be constructed once per program execution
and cannot be passed between threads. This change adds a special thread
to run `Servo` unit tests. This will allow creating suites of `WebView`
unit tests.

Testing: This change includes a new test.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

4 files changed, 90 insertions, 12 deletions

diff --git a/Cargo.lock b/Cargo.lock
index fefafec532a..7efa4ea8b72 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4306,6 +4306,7 @@ dependencies = [
 name = "libservo"
 version = "0.0.1"
 dependencies = [
+ "anyhow",
  "arboard",
  "background_hang_monitor",
  "base",
diff --git a/components/servo/Cargo.toml b/components/servo/Cargo.toml
index 158dae236b8..e4347d76af0 100644
--- a/components/servo/Cargo.toml
+++ b/components/servo/Cargo.toml
@@ -124,6 +124,7 @@ gaol = "0.2.1"
 webxr = { path = "../webxr", features = ["ipc", "glwindow", "headless", "openxr-api"] }
 
 [dev-dependencies]
+anyhow = "1.0.97"
 http = { workspace = true }
 libservo = { path = ".", features = ["tracing"] }
 rustls = { version = "0.23", default-features = false, features = ["aws-lc-rs"] }
diff --git a/components/servo/tests/common/mod.rs b/components/servo/tests/common/mod.rs
index d147ff75722..9f6b3f03449 100644
--- a/components/servo/tests/common/mod.rs
+++ b/components/servo/tests/common/mod.rs
@@ -3,23 +3,35 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::rc::Rc;
-use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, OnceLock};
 use std::time::Duration;
 
+use anyhow::Error;
 use compositing::windowing::EmbedderMethods;
 use compositing_traits::rendering_context::{RenderingContext, SoftwareRenderingContext};
+use crossbeam_channel::{Receiver, Sender, unbounded};
 use dpi::PhysicalSize;
 use embedder_traits::EventLoopWaker;
-use euclid::Scale;
+use parking_lot::Mutex;
 use servo::Servo;
 
 pub struct ServoTest {
     servo: Servo,
 }
 
+impl Drop for ServoTest {
+    fn drop(&mut self) {
+        self.servo.start_shutting_down();
+        while self.servo.spin_event_loop() {
+            std::thread::sleep(Duration::from_millis(1));
+        }
+        self.servo.deinit();
+    }
+}
+
 impl ServoTest {
-    pub fn new() -> Self {
+    fn new() -> Self {
         let rendering_context = Rc::new(
             SoftwareRenderingContext::new(PhysicalSize {
                 width: 500,
@@ -63,14 +75,59 @@ impl ServoTest {
     pub fn servo(&self) -> &Servo {
         &self.servo
     }
+
+    /// Run a Servo test. All tests are run in a `ServoTestThread` and serially. Currently
+    /// Servo does not support launching concurrent instances, in order to ensure
+    /// isolation and allow for more than a single test per instance.
+    pub fn run(
+        test_function: impl FnOnce(&ServoTest) -> Result<(), anyhow::Error> + Send + Sync + 'static,
+    ) {
+        static SERVO_TEST_THREAD: Mutex<OnceLock<ServoTestThread>> = Mutex::new(OnceLock::new());
+        let test_thread = SERVO_TEST_THREAD.lock();
+        test_thread
+            .get_or_init(ServoTestThread::new)
+            .run_test(Box::new(test_function));
+    }
 }
 
-impl Drop for ServoTest {
-    fn drop(&mut self) {
-        self.servo.start_shutting_down();
-        while self.servo.spin_event_loop() {
-            std::thread::sleep(Duration::from_millis(1));
+type TestFunction =
+    Box<dyn FnOnce(&ServoTest) -> Result<(), anyhow::Error> + Send + Sync + 'static>;
+
+struct ServoTestThread {
+    test_function_sender: Sender<TestFunction>,
+    result_receiver: Receiver<Result<(), Error>>,
+}
+
+impl ServoTestThread {
+    fn new() -> Self {
+        let (result_sender, result_receiver) = unbounded();
+        let (test_function_sender, test_function_receiver) = unbounded();
+
+        // Defined here rather than at the end of this method in order to take advantage
+        // of Rust type inference.
+        let thread = Self {
+            test_function_sender,
+            result_receiver,
+        };
+
+        let _ = std::thread::spawn(move || {
+            let servo_test = ServoTest::new();
+            while let Ok(incoming_test_function) = test_function_receiver.recv() {
+                let _ = result_sender.send(incoming_test_function(&servo_test));
+            }
+        });
+
+        thread
+    }
+
+    fn run_test(&self, test_function: TestFunction) {
+        let _ = self.test_function_sender.send(Box::new(test_function));
+        let result = self
+            .result_receiver
+            .recv()
+            .expect("Servo test thread should always return a result.");
+        if let Err(result) = result {
+            unreachable!("{result}");
         }
-        self.servo.deinit();
     }
 }
diff --git a/components/servo/tests/servo.rs b/components/servo/tests/servo.rs
index 5c77a57194f..2f71e909bee 100644
--- a/components/servo/tests/servo.rs
+++ b/components/servo/tests/servo.rs
@@ -2,12 +2,31 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+//! Servo API unit tests.
+//!
+//! Since all Servo tests must rust serially on the same thread, it is important
+//! that tests never panic. In order to ensure this, use `anyhow::ensure!` instead
+//! of `assert!` for test assertions. `ensure!` will produce a `Result::Err` in
+//! place of panicking.
+
 mod common;
 
+use anyhow::ensure;
 use common::*;
+use servo::WebViewBuilder;
+
+#[test]
+fn test_simple_servo_is_not_animating_by_default() {
+    ServoTest::run(|servo_test| {
+        ensure!(!servo_test.servo().animating());
+        Ok(())
+    });
+}
 
 #[test]
-fn test_simple_servo_start_and_stop() {
-    let shared_test = ServoTest::new();
-    assert!(!shared_test.servo().animating());
+fn test_simple_servo_construct_webview() {
+    ServoTest::run(|servo_test| {
+        WebViewBuilder::new(servo_test.servo()).build();
+        Ok(())
+    });
 }