LibJS+LibWeb: Set [[CanBlock]] false to Agent for window agent

similar-origin window agents have the [[CanBlock]] flag set to false.
Achieve this by hooking up JS's concept with an agent to HTML::Agent.
For now, this is only hooked up to the similar-origin window agent
case but should be extended to the other agent types in the future.

Libraries/LibJS/Forward.h

@@ -154,6 +154,7 @@ namespace JS {
 
 class ASTNode;
 class Accessor;
+class Agent;
 struct AsyncGeneratorRequest;
 class BigInt;
 class BoundFunction;

Libraries/LibJS/Runtime/Agent.cpp

@@ -1,19 +1,28 @@
 /*
  * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <LibJS/Runtime/Agent.h>
+#include <LibJS/Runtime/VM.h>
 
 namespace JS {
 
+Agent::~Agent() = default;
+
 // 9.7.2 AgentCanSuspend ( ), https://tc39.es/ecma262/#sec-agentcansuspend
-bool agent_can_suspend()
+bool agent_can_suspend(VM const& vm)
 {
-    // FIXME: 1. Let AR be the Agent Record of the surrounding agent.
-    // FIXME: 2. Return AR.[[CanBlock]].
-    return true;
+    // 1. Let AR be the Agent Record of the surrounding agent.
+    auto const* agent = vm.agent();
+
+    // 2. Return AR.[[CanBlock]].
+    // NOTE: We default to true if no agent has been provided (standalone LibJS with no embedder).
+    if (!agent)
+        return true;
+    return agent->can_block();
 }
 
 }

Libraries/LibJS/Runtime/Agent.h

@@ -1,13 +1,29 @@
 /*
  * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <LibGC/Function.h>
+#include <LibGC/Root.h>
+#include <LibJS/Forward.h>
+
 namespace JS {
 
-bool agent_can_suspend();
+// https://tc39.es/ecma262/#sec-agents
+class Agent {
+public:
+    virtual ~Agent();
+
+    // [[CanBlock]]
+    virtual bool can_block() const = 0;
+
+    virtual void spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition) = 0;
+};
+
+bool agent_can_suspend(VM const&);
 
 }

Libraries/LibJS/Runtime/AtomicsObject.cpp

@@ -186,7 +186,7 @@ static ThrowCompletionOr<Value> do_wait(VM& vm, WaitMode mode, TypedArrayBase& t
         timeout = max(timeout_number.as_double(), 0.0);
 
     // 10. If mode is sync and AgentCanSuspend() is false, throw a TypeError exception.
-    if (mode == WaitMode::Sync && !agent_can_suspend())
+    if (mode == WaitMode::Sync && !agent_can_suspend(vm))
         return vm.throw_completion<TypeError>(ErrorType::AgentCannotSuspend);
 
     // FIXME: Implement the remaining steps when we support SharedArrayBuffer.

Libraries/LibJS/Runtime/Completion.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <AK/TypeCasts.h>
+#include <LibJS/Runtime/Agent.h>
 #include <LibJS/Runtime/Completion.h>
 #include <LibJS/Runtime/NativeFunction.h>
 #include <LibJS/Runtime/Promise.h>
@@ -98,9 +99,9 @@ ThrowCompletionOr<Value> await(VM& vm, Value value)
 
     // FIXME: Since we don't support context suspension, we attempt to "wait" for the promise to resolve
     //        by syncronously running all queued promise jobs.
-    if (auto* custom_data = vm.custom_data()) {
+    if (auto* agent = vm.agent()) {
         // Embedder case (i.e. LibWeb). Runs all promise jobs by performing a microtask checkpoint.
-        custom_data->spin_event_loop_until(GC::create_function(vm.heap(), [success] {
+        agent->spin_event_loop_until(GC::create_function(vm.heap(), [success] {
             return success.has_value();
         }));
     } else {

Libraries/LibJS/Runtime/VM.cpp

@@ -37,12 +37,12 @@
 
 namespace JS {
 
-ErrorOr<NonnullRefPtr<VM>> VM::create(OwnPtr<CustomData> custom_data)
+ErrorOr<NonnullRefPtr<VM>> VM::create(OwnPtr<Agent> agent)
 {
     ErrorMessages error_messages {};
     error_messages[to_underlying(ErrorMessage::OutOfMemory)] = ErrorType::OutOfMemory.message();
 
-    auto vm = adopt_ref(*new VM(move(custom_data), move(error_messages)));
+    auto vm = adopt_ref(*new VM(move(agent), move(error_messages)));
 
     WellKnownSymbols well_known_symbols {
 #define __JS_ENUMERATE(SymbolName, snake_name) \
@@ -63,12 +63,12 @@ static constexpr auto make_single_ascii_character_strings(IndexSequence<code_poi
 
 static constexpr auto single_ascii_character_strings = make_single_ascii_character_strings(MakeIndexSequence<128>());
 
-VM::VM(OwnPtr<CustomData> custom_data, ErrorMessages error_messages)
+VM::VM(OwnPtr<Agent> agent, ErrorMessages error_messages)
     : m_heap(this, [this](HashMap<GC::Cell*, GC::HeapRoot>& roots) {
         gather_roots(roots);
     })
     , m_error_messages(move(error_messages))
-    , m_custom_data(move(custom_data))
+    , m_agent(move(agent))
 {
     m_bytecode_interpreter = make<Bytecode::Interpreter>(*this);
 

Libraries/LibJS/Runtime/VM.h

@@ -21,6 +21,7 @@
 #include <LibGC/RootVector.h>
 #include <LibJS/CyclicModule.h>
 #include <LibJS/ModuleLoading.h>
+#include <LibJS/Runtime/Agent.h>
 #include <LibJS/Runtime/CommonPropertyNames.h>
 #include <LibJS/Runtime/Completion.h>
 #include <LibJS/Runtime/Error.h>
@@ -46,13 +47,7 @@ enum class EvalMode {
 
 class VM : public RefCounted<VM> {
 public:
-    struct CustomData {
-        virtual ~CustomData() = default;
-
-        virtual void spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition) = 0;
-    };
-
-    static ErrorOr<NonnullRefPtr<VM>> create(OwnPtr<CustomData> = {});
+    static ErrorOr<NonnullRefPtr<VM>> create(OwnPtr<Agent> = {});
     ~VM();
 
     GC::Heap& heap() { return m_heap; }
@@ -245,7 +240,8 @@ public:
     Function<void(Promise&)> on_promise_rejection_handled;
     Function<void(Object const&, PropertyKey const&)> on_unimplemented_property_access;
 
-    CustomData* custom_data() { return m_custom_data; }
+    Agent* agent() { return m_agent; }
+    Agent const* agent() const { return m_agent; }
 
     void save_execution_context_stack();
     void clear_execution_context_stack();
@@ -294,7 +290,7 @@ private:
 #undef __JS_ENUMERATE
     };
 
-    VM(OwnPtr<CustomData>, ErrorMessages);
+    VM(OwnPtr<Agent>, ErrorMessages);
 
     void load_imported_module(ImportedModuleReferrer, ModuleRequest const&, GC::Ptr<GraphLoadingState::HostDefined>, ImportedModulePayload);
     ThrowCompletionOr<void> link_and_eval_module(CyclicModule&);
@@ -339,7 +335,7 @@ private:
 
     u32 m_execution_generation { 0 };
 
-    OwnPtr<CustomData> m_custom_data;
+    OwnPtr<Agent> m_agent;
 
     OwnPtr<Bytecode::Interpreter> m_bytecode_interpreter;