[LayoutNG] Ensure ShapingLineBreaker moves offset forward

After the fix for crbug.com/915567, there are reports of
cases where ShapingLineBreaker does not move forward.

While I could not find a reproducible case for this, there
is a case it may occur from the code review. This patch fixes
the case, along with adding CHECK to ShapingLineBreaker.

This is a speculative fix.

Bug: 928178
Change-Id: I51cf733fdad9abf2feee529bfaf90a7e1bdbb09d
Reviewed-on: https://chromium-review.googlesource.com/c/1453777
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629151}

third_party/blink/renderer/platform/fonts/shaping/shaping_line_breaker.cc

@@ -255,7 +255,8 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
       return ShapeToEnd(start, first_safe, range_start, range_end);
     }
   }
-  DCHECK_GT(break_opportunity.offset, start);
+  // It is critical to move forward, or callers may end up in an infinite loop.
+  CHECK_GT(break_opportunity.offset, start);
 
   // If the start offset is not at a safe-to-break boundary the content between
   // the start and the next safe-to-break boundary needs to be reshaped and the
@@ -281,50 +282,58 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
 
   scoped_refptr<const ShapeResult> line_end_result;
   unsigned last_safe = break_opportunity.offset;
-  if (break_opportunity.offset > start) {
-    // If the previous valid break opportunity is not at a safe-to-break
-    // boundary reshape between the safe-to-break offset and the valid break
-    // offset. If the resulting width exceeds the available space the
-    // preceding boundary is tried until the available space is sufficient.
-    while (true) {
-      DCHECK_LE(first_safe, break_opportunity.offset);
-      last_safe = std::max(
-          result_->CachedPreviousSafeToBreakOffset(break_opportunity.offset),
-          first_safe);
-      DCHECK_LE(last_safe, break_opportunity.offset);
-      DCHECK_GE(last_safe, first_safe);
-      if (last_safe == break_opportunity.offset)
-        break;
-      DCHECK_LE(break_opportunity.offset, range_end);
-      if (is_overflow) {
-        line_end_result = Shape(last_safe, break_opportunity.offset);
-        break;
-      }
-      LayoutUnit safe_position = SnapStart(
-          result_->CachedPositionForOffset(last_safe - range_start), direction);
+  // If the previous valid break opportunity is not at a safe-to-break
+  // boundary reshape between the safe-to-break offset and the valid break
+  // offset. If the resulting width exceeds the available space the
+  // preceding boundary is tried until the available space is sufficient.
+  while (true) {
+    DCHECK_LE(first_safe, break_opportunity.offset);
+    last_safe = std::max(
+        result_->CachedPreviousSafeToBreakOffset(break_opportunity.offset),
+        first_safe);
+    DCHECK_LE(last_safe, break_opportunity.offset);
+    DCHECK_GE(last_safe, first_safe);
+    if (last_safe == break_opportunity.offset)
+      break;
+    DCHECK_LE(break_opportunity.offset, range_end);
+    if (is_overflow) {
       line_end_result = Shape(last_safe, break_opportunity.offset);
-      if (line_end_result->SnappedWidth() <=
-          FlipRtl(end_position - safe_position, direction))
-        break;
-
-      // Doesn't fit after the reshape. Try the previous break opportunity.
-      line_end_result = nullptr;
-      break_opportunity =
-          PreviousBreakOpportunity(break_opportunity.offset - 1, start);
-      if (break_opportunity.offset > start)
-        continue;
-
-      // No suitable break opportunity, not exceeding the available space,
-      // found. Any break opportunities beyond candidate_break won't fit
-      // either because the ShapeResult has the full context.
-      // This line will overflow, but there are multiple choices to break,
-      // because none can fit. The one after candidate_break is better for
-      // ligatures, but the one before is better for kernings.
-      break_opportunity = PreviousBreakOpportunity(candidate_break, start);
-      // Loop once more to compute last_safe for the new break opportunity.
-      is_overflow = true;
+      break;
+    }
+    LayoutUnit safe_position = SnapStart(
+        result_->CachedPositionForOffset(last_safe - range_start), direction);
+    line_end_result = Shape(last_safe, break_opportunity.offset);
+    if (line_end_result->SnappedWidth() <=
+        FlipRtl(end_position - safe_position, direction))
+      break;
+
+    // Doesn't fit after the reshape. Try the previous break opportunity.
+    line_end_result = nullptr;
+    break_opportunity =
+        PreviousBreakOpportunity(break_opportunity.offset - 1, start);
+    if (break_opportunity.offset > start)
+      continue;
+
+    // No suitable break opportunity, not exceeding the available space,
+    // found. Any break opportunities beyond candidate_break won't fit
+    // either because the ShapeResult has the full context.
+    // This line will overflow, but there are multiple choices to break,
+    // because none can fit. The one after candidate_break is better for
+    // ligatures, but the one before is better for kernings.
+    break_opportunity = PreviousBreakOpportunity(candidate_break, start);
+    if (break_opportunity.offset <= start) {
+      break_opportunity = NextBreakOpportunity(
+          std::max(candidate_break, start + 1), start, range_end);
+      if (break_opportunity.offset >= range_end) {
+        result_out->break_offset = range_end;
+        return ShapeToEnd(start, first_safe, range_start, range_end);
+      }
     }
+    // Loop once more to compute last_safe for the new break opportunity.
+    is_overflow = true;
   }
+  // It is critical to move forward, or callers may end up in an infinite loop.
+  CHECK_GT(break_opportunity.offset, start);
   DCHECK_GE(break_opportunity.offset, last_safe);
   DCHECK_EQ(break_opportunity.offset - start,
             (line_start_result ? line_start_result->NumCharacters() : 0) +
@@ -344,7 +353,6 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
     segments[count++] = {line_end_result.get(), last_safe, max_length};
   auto line_result = ShapeResultView::Create(&segments[0], count);
 
-  DCHECK_GT(break_opportunity.offset, start);
   DCHECK_LE(break_opportunity.offset, range_end);
   DCHECK_EQ(break_opportunity.offset - start, line_result->NumCharacters());