Fix inheritance of polyfilled WebAssembly.Module

The polyfilled WebAssembly.Module instance was not inheriting from the
polyfilled constructor's prototype when it's created by WebAssembly.compile
or WebAssembly.compileStreaming.

Author: kateinoigakukun

polyfill.js

@@ -60,11 +60,19 @@ export function polyfill(WebAssembly) {
         module[polyfilledImportsSymbol] = parseImports(sourceBytes);
     }
 
+    // Modify the module's prototype chain to make the following inheritance
+    // test pass:
+    // * `module instanceof WebAssembly.Module === true`
+    // * `module instanceof newWebAssembly.Module === true`
+    const prependInheritance = (module) => {
+        Object.setPrototypeOf(module, newModule.prototype);
+    }
+
     // Hook the Module constructor to store the source bytes.
     const newModule = newWebAssembly.Module = function (bytes) {
         const module = new WebAssembly.Module(bytes);
         assignImports(module, bytes);
-        Object.setPrototypeOf(module, newModule.prototype);
+        prependInheritance(module);
         return module;
     }
     Object.setPrototypeOf(newModule.prototype, WebAssembly.Module.prototype);
@@ -73,6 +81,7 @@ export function polyfill(WebAssembly) {
     newWebAssembly.compile = async (source) => {
         const module = await WebAssembly.compile(source);
         assignImports(module, source);
+        prependInheritance(module);
         return module;
     };
 
@@ -83,6 +92,7 @@ export function polyfill(WebAssembly) {
             const clone = response.clone();
             const module = await WebAssembly.compileStreaming(response);
             assignImports(module, new Uint8Array(await clone.arrayBuffer()));
+            prependInheritance(module);
             return module;
         };
     }

test/check.mjs

@@ -146,6 +146,18 @@ async function checkPolyfill() {
       } else {
         module = getModule.fn();
       }
+
+      if (module instanceof WebAssembly.Module === false) {
+        process.stdout.write("\x1b[31mF\x1b[0m\n");
+        console.error(`Expected module to be instance of the original WebAssembly.Module, but got something wrong by ${getModule.name}`);
+        return false;
+      }
+      if (module instanceof polyfilledWebAssembly.Module === false) {
+        process.stdout.write("\x1b[31mF\x1b[0m\n");
+        console.error(`Expected module to be instance of the polyfilled WebAssembly.Module, but got something wrong by ${getModule.name}`);
+        return false;
+      }
+
       imports = polyfilledWebAssembly.Module.imports(module);
     } catch (e) {
       process.stdout.write("\x1b[31mF\x1b[0m\n");