Check DisplayItemCient aliveness in cached subsequences

third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp b/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
index f474b93a45cf01ec08a84531734f1bad0981aa85..f9e29676edbb5333a4de69c14bf904def1f2a31f 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
@@ -66,8 +66,20 @@ void PaintController::processNewItem(DisplayItem& displayItem)
     DCHECK(!skippingCache() || !displayItem.isCached());
 
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS
-    if (!skippingCache() && (displayItem.isCacheable() || displayItem.isCached()))
-        displayItem.client().beginShouldKeepAlive(this);
+    if (!skippingCache()) {
+        if (displayItem.isCacheable() || displayItem.isCached()) {
+            displayItem.client().beginShouldKeepAlive(this);
+            if (!m_subsequenceClients.isEmpty())
+                displayItem.client().beginShouldKeepAlive(m_subsequenceClients.last());

[chrishtr, 2016/06/18 08:49:57]

What does this do exactly? beginShouldKeepAlive for the begin-subsequence marker
when processing something within the subsequence?
I don't fully understand this yet.

[Xianzhu, 2016/06/18 17:41:30]

Yes, this is to mark the new display item shouldKeepAlive under the current
subsequence. The shouldKeepAlive status will end when the subsequence owner is
invalidated or deleted, which is handled in DisplayItemClient.cpp.

Added comments.

+        }
+
+        if (displayItem.getType() == DisplayItem::Subsequence) {
+            m_subsequenceClients.append(&displayItem.client());
+        } else if (displayItem.getType() == DisplayItem::EndSubsequence) {
+            CHECK(m_subsequenceClients.last() == &displayItem.client());
+            m_subsequenceClients.removeLast();
+        }
+    }
 #endif
 
     if (displayItem.isCached())
@@ -403,6 +415,7 @@ void PaintController::updateCacheGeneration()
         displayItem.client().setDisplayItemsCached(m_currentCacheGeneration);
     }
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS
+    CHECK(m_subsequenceClients.isEmpty());
     DisplayItemClient::endShouldKeepAliveAllClients(this);
 #endif
 }