Fix overflow issue in token cache. (#6190)

* Fix overflow issue in token cache.

* Add test

sdk/identity/azure-identity/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bugs Fixed
 
+- Fix overflow issue in token cache.
+
 ### Other Changes
 
 - [[#6086]](https://github.com/Azure/azure-sdk-for-cpp/pull/6086) Correct minimum version specification for the Azure Core dependency. (A community contribution, courtesy of _[jdblischak](https://github.com/jdblischak)_)

sdk/identity/azure-identity/src/token_cache.cpp

@@ -18,7 +18,9 @@ bool TokenCache::IsFresh(
     DateTime::duration minimumExpiration,
     std::chrono::system_clock::time_point now)
 {
-  return item->AccessToken.ExpiresOn > (DateTime(now) + minimumExpiration);
+  // Even if ExpiresOn is unset, the default and lowest value of DateTime is time_point of 0
+  // Therefore, there is no risk of underflow with subtracting duration::max().
+  return (item->AccessToken.ExpiresOn - minimumExpiration) > DateTime(now);
 }
 
 namespace {

sdk/identity/azure-identity/test/ut/token_cache_test.cpp

@@ -42,6 +42,23 @@ public:
 
 using namespace std::chrono_literals;
 
+TEST(TokenCache, IsFreshOverflow)
+{
+  TestableTokenCache tokenCache;
+
+  EXPECT_EQ(tokenCache.m_cache.size(), 0UL);
+
+  DateTime const Tomorrow = std::chrono::system_clock::now() + 24h;
+
+  auto const token1 = tokenCache.GetToken("A", {}, DateTime::duration::max(), [=]() {
+    AccessToken result;
+    result.Token = "T1";
+    result.ExpiresOn = Tomorrow;
+    return result;
+  });
+  EXPECT_EQ(token1.Token, "T1");
+}
+
 TEST(TokenCache, GetReuseRefresh)
 {
   TestableTokenCache tokenCache;