[SPLIT_MODULE] Make getWasmImports return Proxy (emscripten-core#25559)

When `SPLIT_MODULE` is set, this makes the `imports` object returned by
`getWasmImports` itself a Proxy, which redirects property requests
starting with `placeholder` to an inner handler that processes the
second module loading, and process all other property requests to the
original `imports` object itself.

This is a prepration to add multi-split loading functionality to the JS
runtime. In the multi-split mode, we don't have a single seconday module
`[modulename].deferred.wasm`, but instead multiple secondary modules. We
plan to have multiple placeholder namespace (e.g., `placeholder2`,
`placeholder3`, ...) that match those multiple files, and this can't be
hardcoded with one `placeholder` string as we currently do, so having
the `imports` object itself as a `Proxy` provides us flexibility to
handle multiple placeholder namespaces and load correct modules later.

Thanks @tlively for the idea!

Author: aheejin

src/preamble.js

@@ -488,26 +488,38 @@ async function getWasmBinary(binaryFile) {
 #if SPLIT_MODULE
 {{{ makeModuleReceiveWithVar('loadSplitModule', undefined, 'instantiateSync') }}}
 var splitModuleProxyHandler = {
-  get(target, prop, receiver) {
-    return (...args) => {
+  get(target, moduleName, receiver) {
+    if (moduleName.startsWith('placeholder')) {
+      return new Proxy({}, {
+        get(target, base, receiver) {
+          return (...args) => {
 #if ASYNCIFY == 2
-      throw new Error('Placeholder function "' + prop + '" should not be called when using JSPI.');
+            throw new Error('Placeholder function "' + base + '" should not be called when using JSPI.');
 #else
-      err(`placeholder function called: ${prop}`);
-      var imports = {'primary': wasmExports};
-      // Replace '.wasm' suffix with '.deferred.wasm'.
-      var deferred = wasmBinaryFile.slice(0, -5) + '.deferred.wasm'
-      loadSplitModule(deferred, imports, prop);
-      err('instantiated deferred module, continuing');
+            // TODO: Implement multi-split module loading
+#if RUNTIME_DEBUG
+            dbg(`placeholder function called: ${base}`);
+#endif
+            var imports = {'primary': wasmExports};
+            // Replace '.wasm' suffix with '.deferred.wasm'.
+            var deferred = wasmBinaryFile.slice(0, -5) + '.deferred.wasm'
+            loadSplitModule(deferred, imports, base);
+#if RUNTIME_DEBUG
+            dbg('instantiated deferred module, continuing');
+#endif
 #if RELOCATABLE
-      // When the table is dynamically laid out, the placeholder functions names
-      // are offsets from the table base. In the main module, the table base is
-      // always 1.
-      prop = 1 + parseInt(prop);
+            // When the table is dynamically laid out, the placeholder functions names
+            // are offsets from the table base. In the main module, the table base is
+            // always 1.
+            base = 1 + parseInt(base);
 #endif
-      return wasmTable.get({{{ toIndexType('prop') }}})(...args);
+            return wasmTable.get({{{ toIndexType('base') }}})(...args);
 #endif
+          }
+        }
+      });
     }
+    return target[moduleName];
   }
 };
 #endif
@@ -653,21 +665,22 @@ function getWasmImports() {
 #endif
 #endif
   // prepare imports
-  return {
+  var imports = {
 #if MINIFY_WASM_IMPORTED_MODULES
     'a': wasmImports,
 #else // MINIFY_WASM_IMPORTED_MODULES
     'env': wasmImports,
     '{{{ WASI_MODULE_NAME }}}': wasmImports,
 #endif // MINIFY_WASM_IMPORTED_MODULES
-#if SPLIT_MODULE
-    'placeholder': new Proxy({}, splitModuleProxyHandler),
-#endif
 #if RELOCATABLE
     'GOT.mem': new Proxy(wasmImports, GOTHandler),
     'GOT.func': new Proxy(wasmImports, GOTHandler),
 #endif
-  }
+  };
+#if SPLIT_MODULE
+  imports = new Proxy(imports, splitModuleProxyHandler);
+#endif
+  return imports;
 }
 
 // Create the wasm instance.

test/codesize/test_codesize_file_preload.expected.js

@@ -447,9 +447,10 @@ async function instantiateAsync(binary, binaryFile, imports) {
 
 function getWasmImports() {
   // prepare imports
-  return {
+  var imports = {
     "a": wasmImports
   };
+  return imports;
 }
 
 // Create the wasm instance.

test/codesize/test_codesize_minimal_O0.expected.js

@@ -658,10 +658,11 @@ async function instantiateAsync(binary, binaryFile, imports) {
 
 function getWasmImports() {
   // prepare imports
-  return {
+  var imports = {
     'env': wasmImports,
     'wasi_snapshot_preview1': wasmImports,
-  }
+  };
+  return imports;
 }
 
 // Create the wasm instance.

test/codesize/test_unoptimized_code_size.json

@@ -1,16 +1,16 @@
 {
-  "hello_world.js": 56566,
-  "hello_world.js.gz": 17601,
+  "hello_world.js": 56592,
+  "hello_world.js.gz": 17608,
   "hello_world.wasm": 15119,
   "hello_world.wasm.gz": 7444,
-  "no_asserts.js": 26608,
-  "no_asserts.js.gz": 8874,
+  "no_asserts.js": 26634,
+  "no_asserts.js.gz": 8883,
   "no_asserts.wasm": 12219,
   "no_asserts.wasm.gz": 6005,
-  "strict.js": 54581,
-  "strict.js.gz": 16947,
+  "strict.js": 54607,
+  "strict.js.gz": 16955,
   "strict.wasm": 15119,
   "strict.wasm.gz": 7442,
-  "total": 180212,
-  "total_gz": 64313
+  "total": 180290,
+  "total_gz": 64337
 }