fix: TLA in web worker (#8809)

Implementors of `deno_core::JsRuntime` might want to do additional actions
during each turn of event loop, eg. `deno_runtime::Worker` polls inspector,
`deno_runtime::WebWorker` receives/dispatches messages from/to worker host.
Previously `JsRuntime::mod_evaluate` was implemented in such fashion that it 
only polled `JsRuntime`'s event loop. This behavior turned out to be wrong 
in the example of `WebWorker` which couldn't receive/dispatch messages because
its implementation of event loop was never called. 

This commit rewrites "mod_evaluate" to return a handle to receiver that resolves
when module's promise resolves. It is now implementors responsibility to poll
event loop after calling `mod_evaluate`.

cli/tests/worker_with_top_level_await.ts

@@ -0,0 +1,15 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+import { serve } from "../../std/http/server.ts";
+
+const server = serve({ port: 8080 });
+
+self.onmessage = (e: MessageEvent) => {
+  console.log("TLA worker received message", e.data);
+};
+
+self.postMessage("hello");
+
+for await (const _r of server) {
+  // pass
+}

cli/tests/workers_test.ts

@@ -357,3 +357,22 @@ Deno.test({
     w.terminate();
   },
 });
+
+Deno.test({
+  name: "Worker with top-level-await",
+  fn: async function (): Promise<void> {
+    const promise = deferred();
+    const worker = new Worker(
+      new URL("./worker_with_top_level_await.ts", import.meta.url).href,
+      { deno: true, type: "module" },
+    );
+    worker.onmessage = (e): void => {
+      console.log("received from worker", e.data);
+      worker.postMessage("from main");
+      promise.resolve();
+    };
+
+    await promise;
+    worker.terminate();
+  },
+});

core/modules.rs

@@ -718,8 +718,8 @@ mod tests {
     let spec = ModuleSpecifier::resolve_url("file:///a.js").unwrap();
     let a_id_fut = runtime.load_module(&spec, None);
     let a_id = futures::executor::block_on(a_id_fut).expect("Failed to load");
-
-    futures::executor::block_on(runtime.mod_evaluate(a_id)).unwrap();
+    runtime.mod_evaluate(a_id);
+    futures::executor::block_on(runtime.run_event_loop()).unwrap();
     let l = loads.lock().unwrap();
     assert_eq!(
       l.to_vec(),
@@ -786,7 +786,8 @@ mod tests {
       let result = runtime.load_module(&spec, None).await;
       assert!(result.is_ok());
       let circular1_id = result.unwrap();
-      runtime.mod_evaluate(circular1_id).await.unwrap();
+      runtime.mod_evaluate(circular1_id);
+      runtime.run_event_loop().await.unwrap();
 
       let l = loads.lock().unwrap();
       assert_eq!(
@@ -863,7 +864,8 @@ mod tests {
       println!(">> result {:?}", result);
       assert!(result.is_ok());
       let redirect1_id = result.unwrap();
-      runtime.mod_evaluate(redirect1_id).await.unwrap();
+      runtime.mod_evaluate(redirect1_id);
+      runtime.run_event_loop().await.unwrap();
       let l = loads.lock().unwrap();
       assert_eq!(
         l.to_vec(),
@@ -1012,8 +1014,8 @@ mod tests {
       .boxed_local();
     let main_id =
       futures::executor::block_on(main_id_fut).expect("Failed to load");
-
-    futures::executor::block_on(runtime.mod_evaluate(main_id)).unwrap();
+    runtime.mod_evaluate(main_id);
+    futures::executor::block_on(runtime.run_event_loop()).unwrap();
 
     let l = loads.lock().unwrap();
     assert_eq!(

core/runtime.rs

@@ -825,12 +825,17 @@ impl JsRuntime {
     Ok(())
   }
 
+  // TODO(bartlomieju): make it return `ModuleEvaluationFuture`?
   /// Evaluates an already instantiated ES module.
   ///
+  /// Returns a receiver handle that resolves when module promise resolves.
+  /// Implementors must manually call `run_event_loop()` to drive module
+  /// evaluation future.
+  ///
   /// `AnyError` can be downcast to a type that exposes additional information
   /// about the V8 exception. By default this type is `JsError`, however it may
   /// be a different type if `RuntimeOptions::js_error_create_fn` has been set.
-  fn mod_evaluate_inner(
+  pub fn mod_evaluate(
     &mut self,
     id: ModuleId,
   ) -> mpsc::Receiver<Result<(), AnyError>> {
@@ -902,24 +907,6 @@ impl JsRuntime {
     receiver
   }
 
-  pub async fn mod_evaluate(&mut self, id: ModuleId) -> Result<(), AnyError> {
-    let mut receiver = self.mod_evaluate_inner(id);
-
-    poll_fn(|cx| {
-      if let Poll::Ready(maybe_result) = receiver.poll_next_unpin(cx) {
-        debug!("received module evaluate {:#?}", maybe_result);
-        // If `None` is returned it means that runtime was destroyed before
-        // evaluation was complete. This can happen in Web Worker when `self.close()`
-        // is called at top level.
-        let result = maybe_result.unwrap_or(Ok(()));
-        return Poll::Ready(result);
-      }
-      let _r = self.poll_event_loop(cx)?;
-      Poll::Pending
-    })
-    .await
-  }
-
   fn dyn_import_error(&mut self, id: ModuleLoadId, err: AnyError) {
     let state_rc = Self::state(self.v8_isolate());
     let context = self.global_context();
@@ -1110,7 +1097,8 @@ impl JsRuntime {
           v8::PromiseState::Fulfilled => {
             state.pending_mod_evaluate.take();
             scope.perform_microtask_checkpoint();
-            sender.try_send(Ok(())).unwrap();
+            // Receiver end might have been already dropped, ignore the result
+            let _ = sender.try_send(Ok(()));
           }
           v8::PromiseState::Rejected => {
             let exception = promise.result(scope);
@@ -1120,7 +1108,8 @@ impl JsRuntime {
             let err1 = exception_to_err_result::<()>(scope, exception, false)
               .map_err(|err| attach_handle_to_error(scope, err, exception))
               .unwrap_err();
-            sender.try_send(Err(err1)).unwrap();
+            // Receiver end might have been already dropped, ignore the result
+            let _ = sender.try_send(Err(err1));
           }
         }
       }
@@ -2259,7 +2248,7 @@ pub mod tests {
     runtime.mod_instantiate(mod_a).unwrap();
     assert_eq!(dispatch_count.load(Ordering::Relaxed), 0);
 
-    runtime.mod_evaluate_inner(mod_a);
+    runtime.mod_evaluate(mod_a);
     assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
   }
 
@@ -2502,7 +2491,8 @@ pub mod tests {
     )
     .unwrap();
 
-    futures::executor::block_on(runtime.mod_evaluate(module_id)).unwrap();
+    runtime.mod_evaluate(module_id);
+    futures::executor::block_on(runtime.run_event_loop()).unwrap();
 
     let _snapshot = runtime.snapshot();
   }

runtime/web_worker.rs

@@ -315,7 +315,28 @@ impl WebWorker {
     module_specifier: &ModuleSpecifier,
   ) -> Result<(), AnyError> {
     let id = self.js_runtime.load_module(module_specifier, None).await?;
-    self.js_runtime.mod_evaluate(id).await
+
+    let mut receiver = self.js_runtime.mod_evaluate(id);
+    tokio::select! {
+      maybe_result = receiver.next() => {
+        debug!("received worker module evaluate {:#?}", maybe_result);
+        // If `None` is returned it means that runtime was destroyed before
+        // evaluation was complete. This can happen in Web Worker when `self.close()`
+        // is called at top level.
+        let result = maybe_result.unwrap_or(Ok(()));
+        return result;
+      }
+
+      event_loop_result = self.run_event_loop() => {
+        if self.has_been_terminated() {
+          return Ok(());
+        }
+        event_loop_result?;
+        let maybe_result = receiver.next().await;
+        let result = maybe_result.unwrap_or(Ok(()));
+        return result;
+      }
+    }
   }
 
   /// Returns a way to communicate with the Worker from other threads.
@@ -374,6 +395,8 @@ impl WebWorker {
       let msg = String::from_utf8(msg.to_vec()).unwrap();
       let script = format!("workerMessageRecvCallback({})", msg);
 
+      // TODO(bartlomieju): set proper script name like "deno:runtime/web_worker.js"
+      // so it's dimmed in stack trace instead of using "__anonymous__"
       if let Err(e) = self.execute(&script) {
         // If execution was terminated during message callback then
         // just ignore it

runtime/worker.rs

@@ -10,6 +10,7 @@ use crate::permissions::Permissions;
 use deno_core::error::AnyError;
 use deno_core::futures::future::poll_fn;
 use deno_core::futures::future::FutureExt;
+use deno_core::futures::stream::StreamExt;
 use deno_core::serde_json;
 use deno_core::serde_json::json;
 use deno_core::url::Url;
@@ -211,7 +212,21 @@ impl MainWorker {
   ) -> Result<(), AnyError> {
     let id = self.preload_module(module_specifier).await?;
     self.wait_for_inspector_session();
-    self.js_runtime.mod_evaluate(id).await
+    let mut receiver = self.js_runtime.mod_evaluate(id);
+    tokio::select! {
+      maybe_result = receiver.next() => {
+        debug!("received module evaluate {:#?}", maybe_result);
+        let result = maybe_result.expect("Module evaluation result not provided.");
+        return result;
+      }
+
+      event_loop_result = self.run_event_loop() => {
+        event_loop_result?;
+        let maybe_result = receiver.next().await;
+        let result = maybe_result.expect("Module evaluation result not provided.");
+        return result;
+      }
+    }
   }
 
   fn wait_for_inspector_session(&mut self) {