After detach, there's no document to notify parsing is finished.

third_party/WebKit/Source/core/html/parser/HTMLViewSourceParserTest.cpp

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/html/parser/HTMLViewSourceParser.h"
+
+#include "core/dom/DocumentInit.h"
+#include "core/dom/DocumentParser.h"
+#include "core/html/HTMLViewSourceDocument.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+// This is a regression test for https://crbug.com/664915
+TEST(HTMLViewSourceParserTest, DetachThenFinish_ShouldNotCrash) {
+  String mimeType("text/html");
+  HTMLViewSourceDocument* document =
+      HTMLViewSourceDocument::create(DocumentInit(), mimeType);
+  HTMLViewSourceParser* parser =
+      HTMLViewSourceParser::create(*document, mimeType);
+  // A client may detach the parser from the document.
+  parser->detach();
+  // A DocumentWriter may call finish() after detach().
+  static_cast<DocumentParser*>(parser)->finish();
+  // The test passed if finish did not crash.
+}
+
+}  // namespace blink