Cherry-pick 8299d76. rdar://problem/109875865

    Change compositing policy in response to memory pressure status
    https://bugs.webkit.org/show_bug.cgi?id=259580
    rdar://109875865

    Reviewed by Simon Fraser.

    We switch to a more conservative compositing policy when the
    MemoryPressureHandler has a change in memory usage policy at most every
    2 seconds. In some cases we can enter memory warning and critical memory
    pressure situations and get a foreground jetsam before this 2s has
    expired. This change will respond to recent memory warning and memory
    pressure notifications in order to change the compositing policy. While we
    can't guarantee to not jetsam, this does improve our chances of
    reducing layer counts before the system gets critically low on
    memory.

    * Source/WTF/wtf/MemoryPressureHandler.h:
    (WTF::MemoryPressureHandler::isUnderMemoryWarning const):
    * Source/WebCore/rendering/RenderLayerCompositor.cpp:
    (WebCore::RenderLayerCompositor::updateCompositingPolicy):

    Canonical link: https://commits.webkit.org/266497@main

Identifier: 265870.289@safari-7616.1.27.10-branch

Source/WTF/wtf/MemoryPressureHandler.h

@@ -93,6 +93,16 @@ class MemoryPressureHandler {
         m_lowMemoryHandler = WTFMove(handler);
     }
 
+    bool isUnderMemoryWarning() const
+    {
+        auto memoryPressureStatus = m_memoryPressureStatus.load();
+        return memoryPressureStatus == MemoryPressureStatus::SystemWarning
+#if PLATFORM(MAC)
+            || m_memoryUsagePolicy == MemoryUsagePolicy::Conservative
+#endif
+            || memoryPressureStatus == MemoryPressureStatus::ProcessLimitWarning;
+    }
+
     bool isUnderMemoryPressure() const
     {
         auto memoryPressureStatus = m_memoryPressureStatus.load();

Source/WebCore/rendering/RenderLayerCompositor.cpp

@@ -593,14 +593,18 @@ bool RenderLayerCompositor::updateCompositingPolicy()
         m_compositingPolicy = page().compositingPolicyOverride().value();
         return m_compositingPolicy != currentPolicy;
     }
-
+
+    const auto isCurrentlyUnderMemoryPressureOrWarning = [] {
+        return MemoryPressureHandler::singleton().isUnderMemoryPressure() || MemoryPressureHandler::singleton().isUnderMemoryWarning();
+    };
+
     static auto cachedMemoryPolicy = WTF::MemoryUsagePolicy::Unrestricted;
-    static MonotonicTime cachedMemoryPolicyTime;
-    static constexpr auto memoryPolicyCachingDuration = 2_s;
-    auto now = MonotonicTime::now();
-    if (now - cachedMemoryPolicyTime > memoryPolicyCachingDuration) {
+    bool nowUnderMemoryPressure = isCurrentlyUnderMemoryPressureOrWarning();
+    static bool cachedIsUnderMemoryPressureOrWarning = nowUnderMemoryPressure;
+
+    if (cachedIsUnderMemoryPressureOrWarning != nowUnderMemoryPressure) {
         cachedMemoryPolicy = MemoryPressureHandler::singleton().currentMemoryUsagePolicy();
-        cachedMemoryPolicyTime = now;
+        cachedIsUnderMemoryPressureOrWarning = nowUnderMemoryPressure;
     }
 
     m_compositingPolicy = cachedMemoryPolicy == WTF::MemoryUsagePolicy::Unrestricted ? CompositingPolicy::Normal : CompositingPolicy::Conservative;