Issue #1691 - Part 5b: Stop modules from entraining the top-level JSScript. https://bugzilla.mozilla.org/show_bug.cgi?id=1489477

(cherry picked from commit 6ce0af856186e65fb4987daf4e06dc68fb3f7483)

js/src/builtin/ModuleObject.cpp

@@ -835,18 +835,22 @@ ModuleObject::fixEnvironmentsAfterCompartmentMerge()
     AssertModuleScopesMatch(this);
 }
 
-bool
+JSScript*
-ModuleObject::hasScript() const
+ModuleObject::maybeScript() const
 {
-    // When modules are parsed via the Reflect.parse() API, the module object
+    Value value = getReservedSlot(ScriptSlot);
-    // doesn't have a script.
+    if (value.isUndefined())
-    return !getReservedSlot(ScriptSlot).isUndefined();
+        return nullptr;
+
+    return value.toGCThing()->as<JSScript>();
 }
 
 JSScript*
 ModuleObject::script() const
 {
-    return static_cast<JSScript*>(getReservedSlot(ScriptSlot).toPrivate());
+    JSScript* ptr = maybeScript();
+    MOZ_RELEASE_ASSERT(ptr);
+    return ptr;
 }
 
 static inline void
@@ -906,8 +910,8 @@ ModuleObject::enclosingScope() const
 ModuleObject::trace(JSTracer* trc, JSObject* obj)
 {
     ModuleObject& module = obj->as<ModuleObject>();
-    if (module.hasScript()) {
+    JSScript* script = module.maybeScript();
-        JSScript* script = module.script();
+    if (script) {
         TraceManuallyBarrieredEdge(trc, &script, "Module script");
         module.setReservedSlot(ScriptSlot, PrivateValue(script));
     }
@@ -976,6 +980,11 @@ ModuleObject::execute(JSContext* cx, HandleModuleObject self, MutableHandleValue
 #endif
 
     RootedScript script(cx, self->script());
+
+    // The top-level script if a module is only ever executed once. Clear the
+    // reference to prevent us keeping this alive unnecessarily.
+    self->setReservedSlot(ScriptSlot, UndefinedValue());
+
     RootedModuleEnvironmentObject scope(cx, self->environment());
     if (!scope) {
         JS_ReportErrorASCII(cx, "Module declarations have not yet been instantiated");

js/src/builtin/ModuleObject.h

@@ -266,6 +266,7 @@ class ModuleObject : public NativeObject
 #endif
     void fixEnvironmentsAfterCompartmentMerge();
 
+    JSScript* maybeScript() const;
     JSScript* script() const;
     Scope* enclosingScope() const;
     ModuleEnvironmentObject& initialEnvironment() const;
@@ -312,7 +313,6 @@ class ModuleObject : public NativeObject
     static void trace(JSTracer* trc, JSObject* obj);
     static void finalize(js::FreeOp* fop, JSObject* obj);
 
-    bool hasScript() const;
     bool hasImportBindings() const;
     FunctionDeclarationVector* functionDeclarations();
 };

js/src/shell/ModuleLoader.js

@@ -38,6 +38,18 @@ Reflect.Loader = new class {
         module.declarationInstantiation();
         return module.evaluation();
     }
+
+    populateImportMeta(module, metaObject) {
+        // For the shell, use the script's filename as the base URL.
+
+        let path;
+        if (ReflectApply(MapPrototypeHas, this.modulePaths, [module])) {
+            path = ReflectApply(MapPrototypeGet, this.modulePaths, [module]);
+        } else {
+            path = "(unknown)";
+        }
+        metaObject.url = path;
+    }
 };
 
 setModuleResolveHook((referencingInfo, requestName) => {

js/src/shell/js.cpp

@@ -2830,7 +2830,7 @@ DisassembleToSprinter(JSContext* cx, unsigned argc, Value* vp, Sprinter* sprinte
             RootedScript script(cx);
             RootedValue value(cx, p.argv[i]);
             if (value.isObject() && value.toObject().is<ModuleObject>())
-                script = value.toObject().as<ModuleObject>().script();
+                script = value.toObject().as<ModuleObject>().maybeScript();
             else
                 script = ValueToScript(cx, value, fun.address());
             if (!script)
@@ -4132,23 +4132,48 @@ ShellGetModulePrivate(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
-ShellModuleMetadataHook(JSContext* cx, HandleObject module, HandleObject metaObject)
+static bool
+SetModuleMetadataHook(JSContext* cx, unsigned argc, Value* vp)
 {
-    // For the shell, just use the script's filename as the base URL.
+    CallArgs args = CallArgsFromVp(argc, vp);
-    RootedScript script(cx, module->as<ModuleObject>().script());
+    if (args.length() != 1) {
-    const char* filename = script->scriptSource()->filename();
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_MORE_ARGS_NEEDED,
-    MOZ_ASSERT(filename);
+                                  "setModuleMetadataHook", "0", "s");
-
-    RootedString url(cx, NewStringCopyZ<CanGC>(cx, filename));
-    if (!url)
         return false;
+    }
 
-    if (!JS_DefineProperty(cx, metaObject, "url", url, JSPROP_ENUMERATE))
+    if (!args[0].isObject() || !args[0].toObject().is<JSFunction>()) {
+        const char* typeName = InformalValueTypeName(args[0]);
+        JS_ReportErrorASCII(cx, "expected hook function, got %s", typeName);
         return false;
+    }
 
+    Handle<GlobalObject*> global = cx->global();
+    global->setReservedSlot(GlobalAppSlotModuleMetadataHook, args[0]);
+
+    args.rval().setUndefined();
     return true;
 }
 
+static bool
+CallModuleMetadataHook(JSContext* cx, HandleObject module, HandleObject metaObject)
+{
+    Handle<GlobalObject*> global = cx->global();
+    RootedValue hookValue(cx, global->getReservedSlot(GlobalAppSlotModuleMetadataHook));
+    if (hookValue.isUndefined()) {
+        JS_ReportErrorASCII(cx, "Module metadata hook not set");
+        return false;
+    }
+    MOZ_ASSERT(hookValue.toObject().is<JSFunction>());
+
+    JS::AutoValueArray<2> args(cx);
+    args[0].setObject(*module);
+    args[1].setObject(*metaObject);
+
+    RootedValue dummy(cx);
+    return JS_CallFunctionValue(cx, nullptr, hookValue, args, &dummy);
+}
+
 static bool
 SetModuleDynamicImportHook(JSContext* cx, unsigned argc, Value* vp)
 {
@@ -5616,7 +5641,11 @@ DumpScopeChain(JSContext* cx, unsigned argc, Value* vp)
         }
         script = JSFunction::getOrCreateScript(cx, fun);
     } else {
-        script = obj->as<ModuleObject>().script();
+        script = obj->as<ModuleObject>().maybeScript();
+        if (!script) {
+            JS_ReportErrorASCII(cx, "module does not have an associated script");
+            return false;
+        }
     }
 
     script->bodyScope()->dump();
@@ -7451,7 +7480,7 @@ ProcessArgs(JSContext* cx, OptionParser* op)
     if (const char* path = op->getStringOption("module-load-path"))
         moduleLoadPath = path;
 
-    if (!modulePaths.empty() && !InitModuleLoader(cx))
+    if (!InitModuleLoader(cx))
         return false;
 
     while (!filePaths.empty() || !codeChunks.empty() || !modulePaths.empty()) {