Tidy up event loop handling code

.github/workflows/cli-features.yml

@@ -0,0 +1,23 @@
+# Tests extra CLI features and their dependency with core features.
+name: Test CLI Features
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  cli:
+    name: Test CLI Features
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ./.github/actions/ci-shared-setup
+        with:
+          os: linux
+
+      - name: Test `experimental_event_loop`
+        run: |
+          cargo build --package=javy-core --target=wasm32-wasi --release --features=experimental_event_loop
+          cargo test --package=javy-cli --features=experimental_event_loop --release -- --nocapture

crates/cli/tests/integration_test.rs

@@ -147,6 +147,20 @@ fn test_promises() -> Result<()> {
     Ok(())
 }
 
+#[cfg(feature = "experimental_event_loop")]
+#[test]
+fn test_promise_top_level_await() -> Result<()> {
+    let mut runner = Builder::default()
+        .bin(BIN)
+        .root(sample_scripts())
+        .input("top-level-await.js")
+        .build()?;
+    let (out, _, _) = run(&mut runner, &[]);
+
+    assert_eq!("bar", String::from_utf8(out)?);
+    Ok(())
+}
+
 #[test]
 fn test_exported_functions() -> Result<()> {
     let mut runner = Builder::default()

crates/cli/tests/sample-scripts/top-level-await.js

@@ -0,0 +1,6 @@
+async function foo() {
+  return Promise.resolve("bar");
+}
+
+const output = new TextEncoder().encode(await foo());
+Javy.IO.writeSync(1, output);

crates/core/src/execution.rs

@@ -3,7 +3,7 @@ use std::process;
 use anyhow::{anyhow, bail, Error, Result};
 use javy::{
     from_js_error,
-    quickjs::{context::EvalOptions, Module, Value},
+    quickjs::{context::EvalOptions, Ctx, Error as JSError, Module, Value},
     to_js_error, Runtime,
 };
 
@@ -24,21 +24,10 @@ pub fn run_bytecode(runtime: &Runtime, bytecode: &[u8]) {
             let module = unsafe { Module::load(this.clone(), bytecode)? };
             let (_, promise) = module.eval()?;
 
-            if cfg!(feature = "experimental_event_loop") {
-                // If the experimental event loop is enabled, trigger it.
-                promise.finish::<Value>().map(|_| ())
-            } else {
-                // Else we simply expect the promise to resolve immediately.
-                match promise.result() {
-                    None => Err(to_js_error(this, anyhow!(EVENT_LOOP_ERR))),
-                    Some(r) => r,
-                }
-            }
+            handle_maybe_promise(this.clone(), promise.into())
         })
         .map_err(|e| runtime.context().with(|cx| from_js_error(cx.clone(), e)))
-        // Prefer calling `process_event_loop` *outside* of the `with` callback,
-        // to avoid errors regarding multiple mutable borrows.
-        .and_then(|_| process_event_loop(runtime))
+        .and_then(|_: ()| ensure_pending_jobs(runtime))
         .unwrap_or_else(handle_error)
 }
 
@@ -63,34 +52,47 @@ pub fn invoke_function(runtime: &Runtime, fn_module: &str, fn_name: &str) {
             opts.global = false;
             let value = this.eval_with_options::<Value<'_>, _>(js, opts)?;
 
-            if let Some(promise) = value.as_promise() {
-                if cfg!(feature = "experimental_event_loop") {
-                    // If the experimental event loop is enabled, trigger it.
-                    promise.finish::<Value>().map(|_| ())
+            handle_maybe_promise(this.clone(), value)
+        })
+        .map_err(|e| runtime.context().with(|cx| from_js_error(cx.clone(), e)))
+        .and_then(|_: ()| ensure_pending_jobs(runtime))
+        .unwrap_or_else(handle_error)
+}
+
+/// Handles the promise returned by evaluating the JS bytecode.
+fn handle_maybe_promise(this: Ctx, value: Value) -> javy::quickjs::Result<()> {
+    match value.as_promise() {
+        Some(promise) => {
+            if cfg!(feature = "experimental_event_loop") {
+                // If the experimental event loop is enabled, trigger it.
+                let resolved = promise.finish::<Value>();
+                // `Promise::finish` returns Err(Wouldblock) when the all
+                // pending jobs have been handled.
+                if let Err(JSError::WouldBlock) = resolved {
+                    Ok(())
                 } else {
-                    // Else we simply expect the promise to resolve immediately.
-                    match promise.result() {
-                        None => Err(to_js_error(this, anyhow!(EVENT_LOOP_ERR))),
-                        Some(r) => r,
-                    }
+                    resolved.map(|_| ())
                 }
             } else {
-                Ok(())
+                // Else we simply expect the promise to resolve immediately.
+                match promise.result() {
+                    None => Err(to_js_error(this, anyhow!(EVENT_LOOP_ERR))),
+                    Some(r) => r,
+                }
             }
-        })
-        .map_err(|e| runtime.context().with(|cx| from_js_error(cx.clone(), e)))
-        .and_then(|_: ()| process_event_loop(runtime))
-        .unwrap_or_else(handle_error)
+        }
+        None => Ok(()),
+    }
 }
 
-fn process_event_loop(rt: &Runtime) -> Result<()> {
+fn ensure_pending_jobs(rt: &Runtime) -> Result<()> {
     if cfg!(feature = "experimental_event_loop") {
-        rt.resolve_pending_jobs()?
+        rt.resolve_pending_jobs()
     } else if rt.has_pending_jobs() {
         bail!(EVENT_LOOP_ERR);
+    } else {
+        Ok(())
     }
-
-    Ok(())
 }
 
 fn handle_error(e: Error) {