use instantiateWasm override and platform fetch

tfjs-backend-wasm/README.md

@@ -82,6 +82,15 @@ setWasmPath(yourCustomPath); // or tf.wasm.setWasmPath when using <script> tags.
 tf.setBackend('wasm').then(() => {...});
 ```
 
+If you are using platform that does not support fetch directly, please set the
+optional `usePlatformFetch` to true:
+
+```ts
+import {setWasmPath} from '@tensorflow/tfjs-backend-wasm';
+const usePlatformFetch = true;
+setWasmPath(yourCustomPath, usePlatformFetch); // or tf.wasm.setWasmPath when using <script> tags.
+tf.setBackend('wasm').then(() => {...});
+```
 ## Benchmarks
 
 The benchmarks below show inference times (ms) for two different edge-friendly

tfjs-backend-wasm/src/backend_wasm.ts

@@ -175,12 +175,29 @@ registerBackend('wasm', async () => {
   return new BackendWasm(wasm);
 }, WASM_PRIORITY);
 
+function createInstantiateWasmFunc(path: string) {
+  // tslint:disable-next-line:no-any
+  return (imports: any, callback: any) => {
+    util.fetch(path, {credentials: 'same-origin'}).then((response) => {
+      if (!response['ok']) {
+        imports.env.a(`failed to load wasm binary file at '${path}'`);
+      }
+      response.arrayBuffer().then(binary => {
+        WebAssembly.instantiate(binary, imports).then(output => {
+          callback(output.instance);
+        });
+      });
+    });
+    return {};
+  };
+}
+
 /**
  * Initializes the wasm module and creates the js <--> wasm bridge.
  *
  * NOTE: We wrap the wasm module in a object with property 'wasm' instead of
- * returning Promise<BackendWasmModule> to avoid freezing Chrome (last tested in
- * Chrome 76).
+ * returning Promise<BackendWasmModule> to avoid freezing Chrome (last tested
+ * in Chrome 76).
  */
 export async function init(): Promise<{wasm: BackendWasmModule}> {
   return new Promise((resolve, reject) => {
@@ -192,6 +209,12 @@ export async function init(): Promise<{wasm: BackendWasmModule}> {
         }
         return prefix + path;
       };
+      // use wasm instantiateWasm override when system fetch is not available.
+      // For detail references
+      // https://github.com/emscripten-core/emscripten/blob/2bca083cbbd5a4133db61fbd74d04f7feecfa907/tests/manual_wasm_instantiate.html#L170
+      if (customFetch) {
+        factoryConfig.instantiateWasm = createInstantiateWasmFunc(wasmPath);
+      }
     }
     const wasm = wasmFactory(factoryConfig);
     const voidReturnType: string = null;
@@ -220,7 +243,8 @@ export async function init(): Promise<{wasm: BackendWasmModule}> {
         return;
       }
       if (initAborted) {
-        // Emscripten calls `onAbort` twice, resulting in double error messages.
+        // Emscripten calls `onAbort` twice, resulting in double error
+        // messages.
         return;
       }
       initAborted = true;
@@ -248,24 +272,30 @@ function typedArrayFromBuffer(
 
 let wasmPath: string = null;
 let initAborted = false;
-
+let customFetch = false;
 /**
  * Sets the path to the `.wasm` file which will be fetched when the wasm
  * backend is initialized. See
  * https://github.com/tensorflow/tfjs/blob/master/tfjs-backend-wasm/README.md#using-bundlers
  * for more details.
+ * @param path wasm file path or url
+ * @param usePlatformFetch optional boolean to use platform fetch to download
+ *     the wasm file, default to false.
  */
 /** @doc {heading: 'Environment', namespace: 'wasm'} */
-export function setWasmPath(path: string): void {
+export function setWasmPath(path: string, usePlatformFetch = false): void {
   if (initAborted) {
     throw new Error(
         'The WASM backend was already initialized. Make sure you call ' +
         '`setWasmPath()` before you call `tf.setBackend()` or `tf.ready()`');
   }
   wasmPath = path;
+  customFetch = usePlatformFetch;
 }
 
 /** Used in unit tests. */
 export function resetWasmPath(): void {
   wasmPath = null;
+  customFetch = false;
+  initAborted = false;
 }

tfjs-backend-wasm/src/index.ts

@@ -16,5 +16,6 @@
  */
 
 import './kernels/all_kernels';
+
 export {BackendWasm, setWasmPath} from './backend_wasm';
 export {version as version_wasm} from './version';

tfjs-backend-wasm/src/index_test.ts

@@ -16,7 +16,7 @@
  */
 
 import * as tf from '@tensorflow/tfjs-core';
-import {registerBackend, removeBackend, test_util} from '@tensorflow/tfjs-core';
+import {registerBackend, removeBackend, test_util, util} from '@tensorflow/tfjs-core';
 // tslint:disable-next-line:no-imports-from-dist
 import {ALL_ENVS, BROWSER_ENVS, describeWithFlags} from '@tensorflow/tfjs-core/dist/jasmine_util';
 
@@ -79,6 +79,35 @@ describeWithFlags('wasm init', BROWSER_ENVS, () => {
     expect(wasmPath).toBe('invalid/path');
   });
 
+  it('backend init works when the path is valid and use platform fetch',
+     async () => {
+       const usePlatformFetch = true;
+       const validPath = '/base/wasm-out/tfjs-backend-wasm.wasm';
+       setWasmPath(validPath, usePlatformFetch);
+       let wasmPath: string;
+       const realFetch = util.fetch;
+       spyOn(util, 'fetch').and.callFake((path: string) => {
+         wasmPath = path;
+         return realFetch(path);
+       });
+       expect(await tf.setBackend('wasm-test')).toBe(true);
+       expect(wasmPath).toBe(validPath);
+     });
+
+  it('backend init fails when the path is invalid and use platform fetch',
+     async () => {
+       const usePlatformFetch = true;
+       setWasmPath('invalid/path', usePlatformFetch);
+       let wasmPath: string;
+       const realFetch = util.fetch;
+       spyOn(util, 'fetch').and.callFake((path: string) => {
+         wasmPath = path;
+         return realFetch(path);
+       });
+       expect(await tf.setBackend('wasm-test')).toBe(false);
+       expect(wasmPath).toBe('invalid/path');
+     });
+
   it('backend init succeeds with default path', async () => {
     expect(await tf.setBackend('wasm-test')).toBe(true);
   });

tfjs-backend-wasm/wasm-out/tfjs-backend-wasm.d.ts

@@ -31,6 +31,7 @@ export interface BackendWasmModule extends EmscriptenModule {
 
 export interface WasmFactoryConfig {
   locateFile?(path: string, prefix: string): string;
+  instantiateWasm?: Function;
 }
 
 declare var moduleFactory: (settings: WasmFactoryConfig) => BackendWasmModule;