LibJS: Implement a more general InitializeHostDefinedRealm AO

The existing implementation of this AO lives in Interpreter::create(),
which makes it impossible to use without also constructing an
Interpreter.

This patch adds a new Realm::initialize_host_defined_realm() and takes
the global object and global this customization steps as Function
callback objects. This will be used by LibWeb to create realms during
Document construction.

Userland/Libraries/LibJS/Runtime/Realm.cpp

@@ -1,13 +1,16 @@
 /*
  * Copyright (c) 2021-2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/TypeCasts.h>
 #include <LibJS/Runtime/GlobalEnvironment.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Realm.h>
 #include <LibJS/Runtime/VM.h>
+#include <LibJS/Heap/DeferGC.h>
 
 namespace JS {
 
@@ -17,25 +20,89 @@ Realm* Realm::create(VM& vm)
     return vm.heap().allocate_without_global_object<Realm>();
 }
 
+// 9.6 InitializeHostDefinedRealm ( ), https://tc39.es/ecma262/#sec-initializehostdefinedrealm
+ThrowCompletionOr<NonnullOwnPtr<ExecutionContext>> Realm::initialize_host_defined_realm(VM& vm, Function<Value(Realm&)> create_global_object, Function<Value(Realm&)> create_global_this_value)
+{
+    DeferGC defer_gc(vm.heap());
+
+    // 1. Let realm be CreateRealm().
+    auto* realm = Realm::create(vm);
+
+    // 2. Let newContext be a new execution context.
+    auto new_context = make<ExecutionContext>(vm.heap());
+
+    // 3. Set the Function of newContext to null.
+    new_context->function = nullptr;
+
+    // 4. Set the Realm of newContext to realm.
+    new_context->realm = realm;
+
+    // 5. Set the ScriptOrModule of newContext to null.
+    new_context->script_or_module = {};
+
+    // 6. Push newContext onto the execution context stack; newContext is now the running execution context.
+    vm.push_execution_context(*new_context);
+
+    // 7. If the host requires use of an exotic object to serve as realm's global object,
+    //    let global be such an object created in a host-defined manner.
+    //    Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
+    Value global;
+    if (create_global_object)
+        global = create_global_object(*realm);
+    else
+        global = js_undefined();
+
+    // 8. If the host requires that the this binding in realm's global scope return an object other than the global object,
+    //    let thisValue be such an object created in a host-defined manner.
+    //    Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
+    Value this_value;
+    if (create_global_this_value)
+        this_value = create_global_this_value(*realm);
+    else
+        this_value = js_undefined();
+
+    // 9. Perform SetRealmGlobalObject(realm, global, thisValue).
+    realm->set_global_object(global, this_value);
+
+    // NOTE: Steps 10 & 11 are somewhat ad-hoc, since we store intrinsics on the global object.
+
+    // 10. Let globalObj be ? SetDefaultGlobalBindings(realm).
+    // 11. Create any host-defined global object properties on globalObj.
+    realm->global_object().initialize_global_object();
+
+    // 12. Return unused.
+    return new_context;
+}
+
 // 9.3.3 SetRealmGlobalObject ( realmRec, globalObj, thisValue ), https://tc39.es/ecma262/#sec-setrealmglobalobject
-void Realm::set_global_object(GlobalObject& global_object, Object* this_value)
+void Realm::set_global_object(Value global_object, Value this_value)
 {
-    // NOTE: Step 1 is not supported, the global object must be allocated elsewhere.
+    // 1. If globalObj is undefined, then
+    if (global_object.is_undefined()) {
+        // NOTE: Step 1 is not supported, the global object must be allocated elsewhere.
+        VERIFY_NOT_REACHED();
+    }
+
     // 2. Assert: Type(globalObj) is Object.
+    VERIFY(global_object.is_object());
+    VERIFY(is<GlobalObject>(global_object.as_object()));
 
     // Non-standard
-    global_object.set_associated_realm(*this);
+    verify_cast<GlobalObject>(global_object.as_object()).set_associated_realm(*this);
 
     // 3. If thisValue is undefined, set thisValue to globalObj.
-    if (!this_value)
-        this_value = &global_object;
+    if (this_value.is_undefined())
+        this_value = global_object;
+
+    // Non-standard
+    VERIFY(this_value.is_object());
 
     // 4. Set realmRec.[[GlobalObject]] to globalObj.
-    m_global_object = &global_object;
+    m_global_object = &verify_cast<GlobalObject>(global_object.as_object());
 
     // 5. Let newGlobalEnv be NewGlobalEnvironment(globalObj, thisValue).
     // 6. Set realmRec.[[GlobalEnv]] to newGlobalEnv.
-    m_global_environment = global_object.heap().allocate_without_global_object<GlobalEnvironment>(global_object, *this_value);
+    m_global_environment = m_global_object->heap().allocate_without_global_object<GlobalEnvironment>(verify_cast<GlobalObject>(global_object.as_object()), this_value.as_object());
 
     // 7. Return unused.
 }

Userland/Libraries/LibJS/Runtime/Realm.h

@@ -26,8 +26,9 @@ public:
     Realm() = default;
 
     static Realm* create(VM&);
+    static ThrowCompletionOr<NonnullOwnPtr<ExecutionContext>> initialize_host_defined_realm(VM&, Function<Value(Realm&)> create_global_object, Function<Value(Realm&)> create_global_this_value);
 
-    void set_global_object(GlobalObject&, Object* this_value = nullptr);
+    void set_global_object(Value global_object, Value this_value);
 
     [[nodiscard]] GlobalObject& global_object() const { return *m_global_object; }
     [[nodiscard]] GlobalEnvironment& global_environment() const { return *m_global_environment; }

Userland/Libraries/LibJS/Runtime/ShadowRealmConstructor.cpp

@@ -64,7 +64,7 @@ ThrowCompletionOr<Object*> ShadowRealmConstructor::construct(FunctionObject& new
 
     // 10. Perform ? SetRealmGlobalObject(realmRec, undefined, undefined).
     auto* new_global_object = vm.heap().allocate_without_global_object<GlobalObject>(*realm);
-    realm->set_global_object(*new_global_object, nullptr);
+    realm->set_global_object(new_global_object, js_undefined());
     new_global_object->initialize_global_object();
 
     // TODO: I don't think we should have these exactly like this, that doesn't work well with how