MessageEvent::m_data lock is not held when accessed

https://bugs.webkit.org/show_bug.cgi?id=258806
rdar://111681401

Reviewed by Yusuke Suzuki.

The unnamed Locker instance is discarded, releasing the lock.
Fix by naming the instance.
Add declarations to prevent similar bugs.

* Source/WTF/wtf/Locker.h:
(WTF::Locker::Locker): Deleted.
(WTF::Locker::~Locker): Deleted.
(WTF::Locker::tryLock): Deleted.
(WTF::Locker::lockable): Deleted.
(WTF::Locker::operator bool const): Deleted.
(WTF::Locker::unlockEarly): Deleted.
(WTF::Locker::operator=): Deleted.
(WTF::Locker::unlock): Deleted.
(WTF::Locker::lock): Deleted.
* Source/WebCore/dom/MessageEvent.cpp:
(WebCore::MessageEvent::initMessageEvent):
(WebCore::MessageEvent::memoryCost const):
* Source/WebCore/dom/MessageEvent.h:

Canonical link: https://commits.webkit.org/265732@main

Author: kkinnunen-apple

Source/WTF/wtf/Locker.h

@@ -59,7 +59,7 @@ using AdoptLockTag = std::adopt_lock_t;
 constexpr AdoptLockTag AdoptLock;
 
 template<typename T>
-class Locker : public AbstractLocker {
+class [[nodiscard]] Locker : public AbstractLocker { // NOLINT
 public:
     explicit Locker(T& lockable) : m_lockable(&lockable) { lock(); }
     explicit Locker(T* lockable) : m_lockable(lockable) { lock(); }

Source/WebCore/dom/MessageEvent.cpp

@@ -113,7 +113,7 @@ void MessageEvent::initMessageEvent(const AtomString& type, bool canBubble, bool
     initEvent(type, canBubble, cancelable);
 
     {
-        Locker { m_concurrentDataAccessLock };
+        Locker locker { m_concurrentDataAccessLock };
         m_data = JSValueTag { };
     }
     // FIXME: This code is wrong: we should emit a write-barrier. Otherwise, GC can collect it.
@@ -134,7 +134,7 @@ EventInterface MessageEvent::eventInterface() const
 
 size_t MessageEvent::memoryCost() const
 {
-    Locker { m_concurrentDataAccessLock };
+    Locker locker { m_concurrentDataAccessLock };
     return WTF::switchOn(m_data, [] (JSValueTag) -> size_t {
         return 0;
     }, [] (const Ref<SerializedScriptValue>& data) -> size_t {

Source/WebCore/dom/MessageEvent.h

@@ -94,7 +94,7 @@ class MessageEvent final : public Event {
 
     EventInterface eventInterface() const final;
 
-    DataType m_data;
+    DataType m_data WTF_GUARDED_BY_LOCK(m_concurrentDataAccessLock);
     String m_origin;
     String m_lastEventId;
     std::optional<MessageEventSource> m_source;