AX: Initialize the AX thread on first client request.

https://bugs.webkit.org/show_bug.cgi?id=259952
<rdar://problem/113592230>

Reviewed by Tyler Wilcock.

The AX thread was being initialize after building an empty isolated tree in AXObjectCache::getOrCreateIsolatedTree. However, there are multiple client requests that happen before the AX thread was initialized that come through the WKAccessibilityWebPageObject. If the main thread is busy parsing and rendering the page, those requests may not be served in a long time, causing clients like VoiceOver to report irresponsive application. That's true especially during the loading of large pages. This patch moves the initialization of the AX thread to the first request received in [WKAccessibilityWebPageObject accessibilityAttributeValue:], mitigating main thread contention in this scenario.

* Source/WebCore/accessibility/AXObjectCache.cpp:
(WebCore::AXObjectCache::getOrCreateIsolatedTree):
* Source/WebCore/accessibility/AXObjectCache.h:
* Source/WebCore/accessibility/mac/AXObjectCacheMac.mm:
(WebCore::AXObjectCache::initializeAXThreadIfNeeded):
(WebCore::AXObjectCache::initializeSecondaryAXThread): Renamed.
* Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectMac.mm:
(-[WKAccessibilityWebPageObject accessibilityAttributeValue:]):

Canonical link: https://commits.webkit.org/266735@main

Source/WebCore/accessibility/AXObjectCache.cpp

@@ -940,7 +940,7 @@ RefPtr<AXIsolatedTree> AXObjectCache::getOrCreateIsolatedTree()
         tree = AXIsolatedTree::create(*this);
     setIsolatedTreeRoot(tree->rootNode().get());
 
-    AXObjectCache::initializeSecondaryAXThread();
+    initializeAXThreadIfNeeded();
 
     return tree;
 }

Source/WebCore/accessibility/AXObjectCache.h

@@ -473,6 +473,7 @@ class AXObjectCache : public CanMakeWeakPtr<AXObjectCache>, public CanMakeChecke
     void scheduleObjectRegionsUpdate(bool scheduleImmediately = false) { m_geometryManager->scheduleObjectRegionsUpdate(scheduleImmediately); }
     void willUpdateObjectRegions() { m_geometryManager->willUpdateObjectRegions(); }
     WEBCORE_EXPORT static bool isIsolatedTreeEnabled();
+    WEBCORE_EXPORT static void initializeAXThreadIfNeeded();
 private:
     static bool clientSupportsIsolatedTree();
     static bool isTestClient();
@@ -485,7 +486,6 @@ class AXObjectCache : public CanMakeWeakPtr<AXObjectCache>, public CanMakeChecke
     void updateIsolatedTree(AccessibilityObject&, AXNotification);
     void updateIsolatedTree(AccessibilityObject*, AXNotification);
     void updateIsolatedTree(const Vector<std::pair<RefPtr<AccessibilityObject>, AXNotification>>&);
-    static void initializeSecondaryAXThread();
 #endif
 
 protected:

Source/WebCore/accessibility/mac/AXObjectCacheMac.mm

@@ -713,12 +713,16 @@ static bool isTestAXClientType(AXClientType client)
     return enabled;
 }
 
-void AXObjectCache::initializeSecondaryAXThread()
+void AXObjectCache::initializeAXThreadIfNeeded()
 {
-    // Now that we have created our tree, initialize the secondary thread,
-    // so future requests come in on the other thread.
-    if (_AXSIsolatedTreeModeFunctionIsAvailable() && _AXSIsolatedTreeMode_Soft() == AXSIsolatedTreeModeSecondaryThread)
+    static bool axThreadInitialized = false;
+    if (LIKELY(axThreadInitialized || !isMainThread()))
+        return;
+
+    if (_AXSIsolatedTreeModeFunctionIsAvailable() && _AXSIsolatedTreeMode_Soft() == AXSIsolatedTreeModeSecondaryThread) {
         _AXUIElementUseSecondaryAXThread(true);
+        axThreadInitialized = true;
+    }
 }
 #endif // ENABLE(ACCESSIBILITY_ISOLATED_TREE)
 

Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectMac.mm

@@ -161,7 +161,12 @@ - (id)accessibilityAttributeValue:(NSString *)attribute
 {
     if (!WebCore::AXObjectCache::accessibilityEnabled())
         WebCore::AXObjectCache::enableAccessibility();
-
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    if (WebCore::AXObjectCache::isIsolatedTreeEnabled())
+        WebCore::AXObjectCache::initializeAXThreadIfNeeded();
+#endif
+
     if ([attribute isEqualToString:NSAccessibilityParentAttribute])
         return m_parent.get();