From eca08732450d9fa2eccea7c4103a54d8e449d447 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Thu, 4 Aug 2022 20:16:40 +0200
Subject: [PATCH] LibWeb: Always put a dummy execution context on the main
 thread VM stack

A lot of code assumes that there's a current execution context. By
setting up a dummy context right after creating the main thread VM,
we ensure that such code can always run.
---
 Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp | 6 ++++++
 Userland/Libraries/LibWeb/Bindings/MainThreadVM.h   | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
index e8432f0d5e..57cbd7501e 100644
--- a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
+++ b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
@@ -286,6 +286,12 @@ JS::VM& main_thread_vm()
         vm->host_resolve_imported_module = [&](JS::ScriptOrModule, JS::ModuleRequest const&) -> JS::ThrowCompletionOr<NonnullRefPtr<JS::Module>> {
             return vm->throw_completion<JS::InternalError>(vm->current_realm()->global_object(), JS::ErrorType::NotImplemented, "Modules in the browser");
         };
+
+        // NOTE: We push a dummy execution context onto the JS execution context stack,
+        //       just to make sure that it's never empty.
+        auto& custom_data = *verify_cast<WebEngineCustomData>(vm->custom_data());
+        custom_data.root_execution_context = make<JS::ExecutionContext>(vm->heap());
+        vm->push_execution_context(*custom_data.root_execution_context);
     }
     return *vm;
 }
diff --git a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.h b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.h
index a085543d6e..686ce3ccb1 100644
--- a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.h
+++ b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.h
@@ -29,6 +29,8 @@ struct WebEngineCustomData final : public JS::VM::CustomData {
     // https://dom.spec.whatwg.org/#mutation-observer-list
     // FIXME: This should be a set.
     NonnullRefPtrVector<DOM::MutationObserver> mutation_observers;
+
+    OwnPtr<JS::ExecutionContext> root_execution_context;
 };
 
 struct WebEngineCustomJobCallbackData final : public JS::JobCallback::CustomData {