Make TextSearcherICU skip results with zero length

In some cases TextSearcherICU might find result with zero length.
Example case from ClusterFuzz:

<script>
function go(){
setup()
}
window.onload=go;
function setup(){
window.find('\u0080')
}
</script>
<hr id='id9' contenteditable='true'</noscript>
>

Bug: 929217
Change-Id: Ic13b3a3595ae2feb83a7d54a2e8e532f7717e33b
Reviewed-on: https://chromium-review.googlesource.com/c/1457798
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629894}

third_party/blink/renderer/core/editing/iterators/text_searcher_icu.cc

@@ -139,6 +139,7 @@ TextSearcherICU::~TextSearcherICU() {
 
 void TextSearcherICU::SetPattern(const StringView& pattern,
                                  FindOptions options) {
+  DCHECK_GT(pattern.length(), 0u);
   options_ = options;
   SetCaseSensitivity(!(options & kCaseInsensitive));
   SetPattern(pattern.Characters16(), pattern.length());
@@ -186,6 +187,12 @@ bool TextSearcherICU::NextMatchResultInternal(MatchResultICU& result) {
 
   result.start = static_cast<wtf_size_t>(match_start);
   result.length = usearch_getMatchedLength(searcher_);
+  // Might be possible to get zero-length result with some Unicode characters
+  // that shouldn't actually match but is matched by ICU such as \u0080.
+  if (result.length == 0u) {
+    result.start = 0;
+    return false;
+  }
   return true;
 }
 

third_party/blink/renderer/core/editing/iterators/text_searcher_icu_test.cc

@@ -99,4 +99,18 @@ TEST(TextSearcherICUTest, FindSubstringWithOffset) {
   EXPECT_EQ(offset_result.length, first_result.length);
 }
 
+TEST(TextSearcherICUTest, FindControlCharacter) {
+  TextSearcherICU searcher;
+  const String& pattern = MakeUTF16(u8"\u0080");
+  searcher.SetPattern(pattern, 0);
+
+  const String& text = MakeUTF16("some text");
+  searcher.SetText(text.Characters16(), text.length());
+
+  MatchResultICU result;
+  EXPECT_FALSE(searcher.NextMatchResult(result));
+  EXPECT_EQ(0u, result.start);
+  EXPECT_EQ(0u, result.length);
+}
+
 }  // namespace blink