Use async/await instead of promises for wasm loading. NFC

These get lowered away by babel when targetting older engines.

Author: sbc100

src/preamble.js

@@ -649,7 +649,7 @@ function getBinarySync(file) {
 #endif
 }
 
-function getBinaryPromise(binaryFile) {
+async function getWasmBinary(binaryFile) {
 #if !SINGLE_FILE
   // If we don't have the binary yet, load it asynchronously using readAsync.
   if (!wasmBinary
@@ -658,16 +658,18 @@ function getBinaryPromise(binaryFile) {
 #endif
       ) {
     // Fetch the binary using readAsync
-    return readAsync(binaryFile).then(
-      (response) => new Uint8Array(/** @type{!ArrayBuffer} */(response)),
-      // Fall back to getBinarySync if readAsync fails
-      () => getBinarySync(binaryFile)
-    );
+    try {
+      /** @type{!ArrayBuffer} */
+      var response = await readAsync(binaryFile);
+      return new Uint8Array(response);
+    } catch {
+      // Fall back to getBinarySync below;
+    }
   }
 #endif
 
   // Otherwise, getBinarySync should be able to get it synchronously
-  return Promise.resolve(getBinarySync(binaryFile));
+  return getBinarySync(binaryFile);
 }
 
 #if LOAD_SOURCE_MAP
@@ -775,56 +777,47 @@ function resetPrototype(constructor, attrs) {
 #endif
 
 #if WASM_ASYNC_COMPILATION
-function instantiateArrayBuffer(binaryFile, imports) {
+async function instantiateArrayBuffer(binaryFile, imports) {
+  try {
+    var binary = await getWasmBinary(binaryFile);
+    var instance = await WebAssembly.instantiate(binary, imports);
 #if USE_OFFSET_CONVERTER
-  var savedBinary;
+    // wasmOffsetConverter needs to be assigned before calling resolve.
+    // See comments below in instantiateAsync.
+    wasmOffsetConverter = new WasmOffsetConverter(binary, instance.module);
 #endif
-  return new Promise((resolve, reject) => {
-    getBinaryPromise(binaryFile).then((binary) => {
-#if USE_OFFSET_CONVERTER
-      savedBinary = binary;
-#endif
-      return WebAssembly.instantiate(binary, imports);
-#if USE_OFFSET_CONVERTER
-    }).then((instance) => {
-      // wasmOffsetConverter needs to be assigned before calling resolve.
-      // See comments below in instantiateAsync.
-      wasmOffsetConverter = new WasmOffsetConverter(savedBinary, instance.module);
-      return instance;
-#endif
-    }).then(resolve, (reason) => {
-      err(`failed to asynchronously prepare wasm: ${reason}`);
-
+    return instance;
+  } catch (reason) {
+    err(`failed to asynchronously prepare wasm: ${reason}`);
 #if WASM == 2
 #if ENVIRONMENT_MAY_BE_NODE || ENVIRONMENT_MAY_BE_SHELL
-      if (typeof location != 'undefined') {
-#endif
-        // WebAssembly compilation failed, try running the JS fallback instead.
-        var search = location.search;
-        if (search.indexOf('_rwasm=0') < 0) {
-          location.href += (search ? search + '&' : '?') + '_rwasm=0';
-          // Return here to avoid calling abort() below.  The application
-          // still has a chance to start successfully do we don't want to
-          // trigger onAbort or onExit handlers.
-          return;
-        }
-#if ENVIRONMENT_MAY_BE_NODE || ENVIRONMENT_MAY_BE_SHELL
+    if (typeof location != 'undefined') {
+#endif
+      // WebAssembly compilation failed, try running the JS fallback instead.
+      var search = location.search;
+      if (search.indexOf('_rwasm=0') < 0) {
+        location.href += (search ? search + '&' : '?') + '_rwasm=0';
+        // Return here to avoid calling abort() below.  The application
+        // still has a chance to start successfully do we don't want to
+        // trigger onAbort or onExit handlers.
+        return;
       }
+#if ENVIRONMENT_MAY_BE_NODE || ENVIRONMENT_MAY_BE_SHELL
+    }
 #endif
 #endif // WASM == 2
 
 #if ASSERTIONS
-      // Warn on some common problems.
-      if (isFileURI(wasmBinaryFile)) {
-        err(`warning: Loading from a file URI (${wasmBinaryFile}) is not supported in most browsers. See https://emscripten.org/docs/getting_started/FAQ.html#how-do-i-run-a-local-webserver-for-testing-why-does-my-program-stall-in-downloading-or-preparing`);
-      }
+    // Warn on some common problems.
+    if (isFileURI(wasmBinaryFile)) {
+      err(`warning: Loading from a file URI (${wasmBinaryFile}) is not supported in most browsers. See https://emscripten.org/docs/getting_started/FAQ.html#how-do-i-run-a-local-webserver-for-testing-why-does-my-program-stall-in-downloading-or-preparing`);
+    }
 #endif
-      abort(reason);
-    });
-  });
+    abort(reason);
+  }
 }
 
-function instantiateAsync(binary, binaryFile, imports) {
+async function instantiateAsync(binary, binaryFile, imports) {
 #if !SINGLE_FILE
   if (!binary &&
       typeof WebAssembly.instantiateStreaming == 'function' &&
@@ -843,56 +836,44 @@ function instantiateAsync(binary, binaryFile, imports) {
       !ENVIRONMENT_IS_NODE &&
 #endif
       typeof fetch == 'function') {
-    return new Promise((resolve) => {
-      fetch(binaryFile, {{{ makeModuleReceiveExpr('fetchSettings', "{ credentials: 'same-origin' }") }}}).then((response) => {
-        // Suppress closure warning here since the upstream definition for
-        // instantiateStreaming only allows Promise<Repsponse> rather than
-        // an actual Response.
-        // TODO(https://github.com/google/closure-compiler/pull/3913): Remove if/when upstream closure is fixed.
-        /** @suppress {checkTypes} */
-        var result = WebAssembly.instantiateStreaming(response, imports);
-
+    try {
+      var response = fetch(binaryFile, {{{ makeModuleReceiveExpr('fetchSettings', "{ credentials: 'same-origin' }") }}});
 #if USE_OFFSET_CONVERTER
-        // We need the wasm binary for the offset converter. Clone the response
-        // in order to get its arrayBuffer (cloning should be more efficient
-        // than doing another entire request).
-        // (We must clone the response now in order to use it later, as if we
-        // try to clone it asynchronously lower down then we will get a
-        // "response was already consumed" error.)
-        var clonedResponsePromise = response.clone().arrayBuffer();
-#endif
-
-        result.then(
+      // We need the wasm binary for the offset converter. Clone the response
+      // in order to get its arrayBuffer (cloning should be more efficient
+      // than doing another entire request).
+      // (We must clone the response now in order to use it later, as if we
+      // try to clone it asynchronously lower down then we will get a
+      // "response was already consumed" error.)
+      var clonedResponse = (await response).clone();
+#endif
+      var result = WebAssembly.instantiateStreaming(response, imports);
 #if USE_OFFSET_CONVERTER
-          (instantiationResult) => {
-            // When using the offset converter, we must interpose here. First,
-            // the instantiation result must arrive (if it fails, the error
-            // handling later down will handle it). Once it arrives, we can
-            // initialize the offset converter. And only then is it valid to
-            // call receiveInstantiationResult, as that function will use the
-            // offset converter (in the case of pthreads, it will create the
-            // pthreads and send them the offsets along with the wasm instance).
-
-            clonedResponsePromise.then((arrayBufferResult) => {
-                wasmOffsetConverter = new WasmOffsetConverter(new Uint8Array(arrayBufferResult), instantiationResult.module);
-                resolve(instantiationResult);
-              },
-              (reason) => err(`failed to initialize offset-converter: ${reason}`)
-            );
-          },
+      // When using the offset converter, we must interpose here. First,
+      // the instantiation result must arrive (if it fails, the error
+      // handling later down will handle it). Once it arrives, we can
+      // initialize the offset converter. And only then is it valid to
+      // call receiveInstantiationResult, as that function will use the
+      // offset converter (in the case of pthreads, it will create the
+      // pthreads and send them the offsets along with the wasm instance).
+      var instantiationResult = await result;
+      var arrayBufferResult = await clonedResponse.arrayBuffer();
+      try {
+        wasmOffsetConverter = new WasmOffsetConverter(new Uint8Array(arrayBufferResult), instantiationResult.module);
+      } catch (reason) {
+        err(`failed to initialize offset-converter: ${reason}`);
+      }
+      return instantiationResult;
 #else
-          resolve,
-#endif
-          (reason) => {
-            // We expect the most common failure cause to be a bad MIME type for the binary,
-            // in which case falling back to ArrayBuffer instantiation should work.
-            err(`wasm streaming compile failed: ${reason}`);
-            err('falling back to ArrayBuffer instantiation');
-            return resolve(instantiateArrayBuffer(binaryFile, imports));
-          }
-        );
-      });
-    });
+      return await result;
+#endif
+    } catch (reason) {
+      // We expect the most common failure cause to be a bad MIME type for the binary,
+      // in which case falling back to ArrayBuffer instantiation should work.
+      err(`wasm streaming compile failed: ${reason}`);
+      err('falling back to ArrayBuffer instantiation');
+      // fall back of instantiateArrayBuffer below
+    };
   }
 #endif
   return instantiateArrayBuffer(binaryFile, imports);
@@ -938,7 +919,13 @@ function getWasmImports() {
 
 // Create the wasm instance.
 // Receives the wasm imports, returns the exports.
+#if WASM_ASYNC_COMPILATION
+// Funnily enough in JS the `async` keyword has to be on the same line as the
+// function keyword.
+async function createWasm() {
+#else
 function createWasm() {
+#endif
   // Load the wasm module and create an instance of using native support in the JS engine.
   // handle a generated wasm instance, receiving its exports and
   // performing other necessary setup
@@ -1106,17 +1093,23 @@ function createWasm() {
 #if RUNTIME_DEBUG
   dbg('asynchronously preparing wasm');
 #endif
-  instantiateAsync(wasmBinary, wasmBinaryFile, info).then(receiveInstantiationResult)
 #if MODULARIZE
-  // If instantiation fails, reject the module ready promise.
-  .catch(readyPromiseReject)
+  try {
 #endif
-  ;
+    var result = await instantiateAsync(wasmBinary, wasmBinaryFile, info);
+    receiveInstantiationResult(result);
 #if LOAD_SOURCE_MAP
-  getSourceMapPromise().then(receiveSourceMapJSON);
+    receiveSourceMapJSON(await getSourceMapPromise());
 #endif
-  return {}; // no exports yet; we'll fill them in later
-#else
+    return result;
+#if MODULARIZE
+  } catch (e) {
+    // If instantiation fails, reject the module ready promise.
+    readyPromiseReject(e);
+    throw e;
+  }
+#endif
+#else // WASM_ASYNC_COMPILATION
   var result = instantiateSync(wasmBinaryFile, info);
 #if PTHREADS || MAIN_MODULE
   return receiveInstance(result[0], result[1]);
@@ -1126,7 +1119,7 @@ function createWasm() {
   // When the regression is fixed, we can remove this if/else.
   return receiveInstance(result[0]);
 #endif
-#endif
+#endif // WASM_ASYNC_COMPILATION
 }
 
 #if !WASM_BIGINT

test/test_browser.py

@@ -5519,7 +5519,7 @@ def test(args, expect_fail):
       if expect_fail:
         js = read_file('a.out.js')
         create_file('a.out.js', 'let origFetch = fetch; fetch = undefined;\n' + js)
-        return self.run_browser('a.out.html', '/report_result?exception:fetch is not a function')
+        return self.run_browser('a.out.html', '/report_result?abort:both async and sync fetching of the wasm failed')
       else:
         return self.run_browser('a.out.html', '/report_result?exit:42')
 

test/test_other.py

@@ -2955,20 +2955,25 @@ def test_m_mm(self):
       self.assertNotContained('error', proc.stderr)
 
   @uses_canonical_tmp
-  def test_emcc_debug_files(self):
-    for opts in (0, 1, 2, 3):
-      for debug in (None, '1', '2'):
-        print(opts, debug)
-        if os.path.exists(self.canonical_temp_dir):
-          shutil.rmtree(self.canonical_temp_dir)
-
-        with env_modify({'EMCC_DEBUG': debug}):
-          self.run_process([EMCC, test_file('hello_world.c'), '-O' + str(opts)], stderr=PIPE)
-          if debug is None:
-            self.assertFalse(os.path.exists(self.canonical_temp_dir))
-          else:
-            print(sorted(os.listdir(self.canonical_temp_dir)))
-            self.assertExists(os.path.join(self.canonical_temp_dir, 'emcc-03-original.js'))
+  @parameterized({
+    'O0': ('-O0',),
+    'O1': ('-O1',),
+    'O2': ('-O2',),
+    'O3': ('-O3',),
+  })
+  def test_emcc_debug_files(self, opt):
+    for debug in (None, '1', '2'):
+      print('debug =', debug)
+      if os.path.exists(self.canonical_temp_dir):
+        shutil.rmtree(self.canonical_temp_dir)
+
+      with env_modify({'EMCC_DEBUG': debug}):
+        self.run_process([EMCC, test_file('hello_world.c'), opt], stderr=PIPE)
+        if debug is None:
+          self.assertFalse(os.path.exists(self.canonical_temp_dir))
+        else:
+          print(sorted(os.listdir(self.canonical_temp_dir)))
+          self.assertExists(os.path.join(self.canonical_temp_dir, 'emcc-03-original.js'))
 
   def test_debuginfo_line_tables_only(self):
     def test(do_compile):

tools/emscripten.py

@@ -989,7 +989,10 @@ def create_module(receiving, metadata, global_exports, library_symbols):
     module.append('var wasmImports = %s;\n' % sending)
 
   if not settings.MINIMAL_RUNTIME:
-    module.append("var wasmExports = createWasm();\n")
+    if settings.WASM_ASYNC_COMPILATION:
+      module.append("var wasmExports;\ncreateWasm();\n")
+    else:
+      module.append("var wasmExports = createWasm();\n")
 
   module.append(receiving)
   if settings.SUPPORT_LONGJMP == 'emscripten' or not settings.DISABLE_EXCEPTION_CATCHING: