Cherry-pick c09328b. rdar://problem/111271905

    Occasional crashes under WebWheelEventCoalescer::takeOldestEventBeingProcessed()
    https://bugs.webkit.org/show_bug.cgi?id=258653
    rdar://111271905

    Reviewed by Jer Noble.

    Crash data suggest that we can reach WebWheelEventCoalescer::takeOldestEventBeingProcessed() with
    m_eventsBeingProcessed being empty. We get here after one or more trips to the web process for wheel
    event handling, so it's possible there's some code path where we can get here with an empty m_eventsBeingProcessed,
    but I have not figured out how, so do a defensive fix of returning a std::optional<NativeWebWheelEvent>.

    * Source/WebKit/Shared/WebWheelEventCoalescer.cpp:
    (WebKit::WebWheelEventCoalescer::takeOldestEventBeingProcessed):
    * Source/WebKit/Shared/WebWheelEventCoalescer.h:
    * Source/WebKit/UIProcess/WebPageProxy.cpp:
    (WebKit::WebPageProxy::wheelEventHandlingCompleted):

    Canonical link: https://commits.webkit.org/265625@main

Identifier: 263823.1687@safari-7616.1.21-branch

Source/WebKit/Shared/WebWheelEventCoalescer.cpp

@@ -155,9 +155,11 @@ bool WebWheelEventCoalescer::shouldDispatchEvent(const NativeWebWheelEvent& even
     return true;
 }
 
-NativeWebWheelEvent WebWheelEventCoalescer::takeOldestEventBeingProcessed()
+std::optional<NativeWebWheelEvent> WebWheelEventCoalescer::takeOldestEventBeingProcessed()
 {
-    ASSERT(hasEventsBeingProcessed());
+    if (m_eventsBeingProcessed.isEmpty())
+        return { };
+
     auto oldestSequence = m_eventsBeingProcessed.takeFirst();
     return oldestSequence->last();
 }

Source/WebKit/Shared/WebWheelEventCoalescer.h

@@ -38,7 +38,7 @@ class WebWheelEventCoalescer {
     bool shouldDispatchEvent(const NativeWebWheelEvent&);
     std::optional<WebWheelEvent> nextEventToDispatch();
 
-    NativeWebWheelEvent takeOldestEventBeingProcessed();
+    std::optional<NativeWebWheelEvent> takeOldestEventBeingProcessed();
 
     bool hasEventsBeingProcessed() const { return !m_eventsBeingProcessed.isEmpty(); }
 

Source/WebKit/UIProcess/WebPageProxy.cpp

@@ -3373,11 +3373,14 @@ void WebPageProxy::wheelEventHandlingCompleted(bool wasHandled)
 {
     auto oldestProcessedEvent = wheelEventCoalescer().takeOldestEventBeingProcessed();
 
-    LOG_WITH_STREAM(WheelEvents, stream << "WebPageProxy::wheelEventHandlingCompleted - finished handling " << platform(oldestProcessedEvent) << " handled " << wasHandled);
+    if (oldestProcessedEvent)
+        LOG_WITH_STREAM(WheelEvents, stream << "WebPageProxy::wheelEventHandlingCompleted - finished handling " << platform(*oldestProcessedEvent) << " handled " << wasHandled);
+    else
+        LOG_WITH_STREAM(WheelEvents, stream << "WebPageProxy::wheelEventHandlingCompleted - no event, handled " << wasHandled);
 
-    if (!wasHandled) {
-        m_uiClient->didNotHandleWheelEvent(this, oldestProcessedEvent);
-        pageClient().wheelEventWasNotHandledByWebCore(oldestProcessedEvent);
+    if (oldestProcessedEvent && !wasHandled) {
+        m_uiClient->didNotHandleWheelEvent(this, *oldestProcessedEvent);
+        pageClient().wheelEventWasNotHandledByWebCore(*oldestProcessedEvent);
     }
 
     if (auto eventToSend = wheelEventCoalescer().nextEventToDispatch()) {