[LayoutNG] Add DCHECK for when line breaker goes infinite loop

This patch adds a DCHECK to detect when |NGLineBreaker| goes
infinite loop by |HandleOverflow()|, |Rewind()|, but failed
to find appropriate secondary break point and |Rewind()| to
the same point again and again.

Change-Id: I9963fb5252f61fcfd0e5d768da8a4968f1f7c9e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1801474Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696740}

third_party/blink/renderer/core/layout/ng/inline/ng_line_breaker.cc

@@ -324,6 +324,10 @@ void NGLineBreaker::PrepareNextLine(NGLineInfo* line_info) {
   position_ = line_info->TextIndent();
 
   overflow_item_index_ = 0;
+
+#if DCHECK_IS_ON()
+  last_rewind_from_item_index_ = last_rewind_to_item_index_ = 0;
+#endif
 }
 
 void NGLineBreaker::NextLine(
@@ -1687,6 +1691,9 @@ void NGLineBreaker::HandleOverflow(NGLineInfo* line_info) {
       Rewind(0, line_info);
     state_ = LineBreakState::kContinue;
     overflow_item_index_ = 0;
+#if DCHECK_IS_ON()
+    last_rewind_from_item_index_ = last_rewind_to_item_index_ = 0;
+#endif
     return;
   }
 
@@ -1709,6 +1716,15 @@ void NGLineBreaker::HandleOverflow(NGLineInfo* line_info) {
 }
 
 void NGLineBreaker::Rewind(unsigned new_end, NGLineInfo* line_info) {
+#if DCHECK_IS_ON()
+  // Detect rewind-loop. If we're trying to rewind to the same index twice,
+  // we're in the infinite loop.
+  DCHECK(item_index_ != last_rewind_from_item_index_ ||
+         new_end != last_rewind_to_item_index_);
+  last_rewind_from_item_index_ = item_index_;
+  last_rewind_to_item_index_ = new_end;
+#endif
+
   NGInlineItemResults& item_results = *line_info->MutableResults();
   DCHECK_LT(new_end, item_results.size());
 

third_party/blink/renderer/core/layout/ng/inline/ng_line_breaker.h

@@ -289,6 +289,12 @@ class CORE_EXPORT NGLineBreaker {
   // This is copied from NGInlineNode, then updated after each forced line break
   // if 'unicode-bidi: plaintext'.
   TextDirection base_direction_;
+
+#if DCHECK_IS_ON()
+  // These fields are to detect rewind-loop.
+  unsigned last_rewind_from_item_index_ = 0;
+  unsigned last_rewind_to_item_index_ = 0;
+#endif
 };
 
 }  // namespace blink