Selection API: Add diagnostic code for extend() crash

third_party/WebKit/Source/core/editing/DOMSelection.cpp

 /*
  * Copyright (C) 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 441 matching lines @@
         IndexSizeError, String::number(offset) + " is not a valid offset.");
     return;
   }
   Range::checkNodeWOffset(node, offset, exceptionState);
   if (exceptionState.hadException())
     return;
 
   // 3. Let oldAnchor and oldFocus be the context object's anchor and focus, and
   // let newFocus be the boundary point (node, offset).
   const Position& oldAnchor = anchorPosition();
+  // TODO(tkent): Diagnostic checks for crbug.com/693578.  They should be
+  // removed before M58 branch.
+  if (oldAnchor.isNull()) {
+    if (Range* range = documentCachedRange()) {
+      LOG(FATAL)
+          << "Selection has a cached Range, but anchorPosition is null. start="
+          << range->startContainer() << " end=" << range->endContainer();
+    } else if (frame() && !frame()->selection().isNone()) {
+      LOG(FATAL) << "FrameSelection is not none, but anchorPosition is null.";
+    }
+  }
   DCHECK(!oldAnchor.isNull());
   const Position newFocus(node, offset);
 
   clearCachedRangeIfSelectionOfDocument();
 
   // 4. Let newRange be a new range.
   Range* newRange = Range::create(*frame()->document());
 
   // 5. If node's root is not the same as the context object's range's root, set
   // newRange's start and end to newFocus.
@@ skipping 339 matching lines @@
     m_treeScope->document().addConsoleMessage(
         ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, message));
 }
 
 DEFINE_TRACE(DOMSelection) {
   visitor->trace(m_treeScope);
   ContextClient::trace(visitor);
 }
 
 }  // namespace blink