webassembly/proxy_js: Promote Python thenable to a Promise.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/proxy_js.js

@@ -89,7 +89,22 @@ function proxy_call_python(target, argumentsList) {
     if (argumentsList.length > 0) {
         Module._free(args);
     }
-    return proxy_convert_mp_to_js_obj_jsside_with_free(value);
+    const ret = proxy_convert_mp_to_js_obj_jsside_with_free(value);
+    if (ret instanceof PyProxyThenable) {
+        // In Python when an async function is called it creates the
+        // corresponding "generator", which must then be executed at
+        // the top level by an asyncio-like scheduler.  In JavaScript
+        // the semantics for async functions is that they are started
+        // immediately (their non-async prefix code is executed immediately)
+        // and only if they await do they return a Promise to delay the
+        // execution of the remainder of the function.
+        //
+        // Emulate the JavaScript behaviour here by resolving the Python
+        // async function.  We assume that the caller who gets this
+        // return is JavaScript.
+        return Promise.resolve(ret);
+    }
+    return ret;
 }
 
 function proxy_convert_js_to_mp_obj_jsside(js_obj, out) {

tests/ports/webassembly/fun_py_callback_js.mjs

+// Test using Python functions as JS callbacks.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+globalThis.asyncTimeout = (ms) =>
+    new Promise((resolve) => setTimeout(resolve, ms));
+
+mp.runPython(`
+import js
+
+def f0():
+    print("f0 run")
+
+async def f1():
+    print("f1 run")
+
+async def f2():
+    print("f2 start")
+    await js.asyncTimeout(0)
+    print("f2 end")
+
+async def f3():
+    print("f3 start")
+    await f2()
+    print("f3 end")
+
+js.setTimeout(f0, 0)
+js.setTimeout(f1, 50)
+js.setTimeout(f2, 100)
+js.setTimeout(f3, 150)
+`);

tests/ports/webassembly/fun_py_callback_js.mjs.exp

+f0 run
+f1 run
+f2 start
+f2 end
+f3 start
+f2 start
+f2 end
+f3 end