Handle overflow, underflow in XPath substring position, length.

third_party/WebKit/Source/core/xml/XPathFunctionsTest.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/xml/XPathFunctions.h"
+
+#include "core/dom/Document.h"
+#include "core/xml/XPathExpressionNode.h"  // EvaluationContext
+#include "core/xml/XPathPredicate.h"       // Number, StringExpression
+#include "core/xml/XPathValue.h"
+#include "platform/heap/Handle.h"  // HeapVector, Member, etc.
+#include "testing/gtest/include/gtest/gtest.h"
+#include "wtf/Allocator.h"
+
+namespace blink {
+
+class XPathContext {

[yosin_UTC9, 2016/10/17 03:56:01]

Too avoid name crash, it is better to enclose XPathContext in anonymous
namespace.

+  STACK_ALLOCATED();
+
+ public:
+  XPathContext() : m_document(Document::create()), m_context(*m_document) {}
+
+  XPath::EvaluationContext& context() { return m_context; }
+  Document& document() { return *m_document; }
+
+ private:
+  Member<Document> m_document;

[yosin_UTC9, 2016/10/17 03:56:01]

nit: s/Member<Document>/const Member<Document>/

+  XPath::EvaluationContext m_context;
+};
+
+static String substring(const char* string, double pos, double len) {
+  XPathContext xpath;
+  HeapVector<Member<XPath::Expression>> args;
+  args.append(new XPath::StringExpression(string));
+  args.append(new XPath::Number(pos));
+  args.append(new XPath::Number(len));
+  XPath::Expression* call = XPath::createFunction("substring", args);
+  XPath::Value result = call->evaluate(xpath.context());
+  return result.toString();
+}
+
+TEST(XPathFunctionsTest, substring_emptyString) {
+  EXPECT_EQ("", substring("", 0., 1.))
+      << "substring of an empty string should be the empty string";
+}
+
+TEST(XPathFunctionsTest, substring) {
+  EXPECT_EQ("hello", substring("well hello there", 6., 5.));
+}
+
+TEST(XPathFunctionsTest, substring_negativePosition) {
+  EXPECT_EQ("hello", substring("hello, world!", -4., 10.))
+      << "negative start positions should impinge on the result length";
+  // Try to underflow the length adjustment for negative positions.
+  EXPECT_EQ("", substring("hello", std::numeric_limits<long>::min() + 1, 1.));
+}
+
+TEST(XPathFunctionsTest, substring_negativeLength) {
+  EXPECT_EQ("", substring("hello, world!", 1., -3.))
+      << "negative lengths should result in an empty string";
+
+  EXPECT_EQ("", substring("foo", std::numeric_limits<long>::min(), 1.))
+      << "large (but long representable) negative position should result in "
+      << "an empty string";
+}
+
+TEST(XPathFunctionsTest, substring_extremePositionLength) {

[yosin_UTC9, 2016/10/17 03:56:01]

We want to have test cases for NaN and +Inifity/-Ininity?
We may want to have -0.

+  EXPECT_EQ("", substring("no way", 1e100, 7.))
+      << "extremely large positions should result in the empty string";
+
+  EXPECT_EQ("no way", substring("no way", -1e200, 1e300))
+      << "although these indices are not representable as long, this should "
+      << "produce the string because indices are computed as doubles";
+}
+
+}  // namespace blink