fix flaky unit tests in NvmCacheTest

Summary:
This change fixes flaky unit tests in NvmCacheTest.

For NvmCacheTest.IsNewCacheInstanceStat, previously, we were checking if nvm cache is new
(GlobalCacheStats.isNewNvmCache) by comparing cache instance creation time and the nvm cache's
creation time https://fburl.com/code/9x4fcl34. However, since we are resetting the nvm cache's
creation time to current time when marking truncate (https://fburl.com/code/xzsvozpj), this is
timing depenedt and thus could get wrong. This was causing NvmCacheTest.IsNewCacheInstanceStat
(https://fburl.com/code/2sr8eonn) to fail often (e.g., https://fburl.com/testinfra/9045ppjx)

For NvmCacheTest.testCreateItemAsIOBufChained, the creation time of chained item is reset when it is
created as iobuf, so the verification check against the creation time of the item is wrong.

Reviewed By: therealgymmy

Differential Revision: D49324117

fbshipit-source-id: 4b85c6902fb663724be8d6e9eebb75d78109768b

Author: Jaesoo Lee

cachelib/allocator/CacheAllocator-inl.h

@@ -3412,8 +3412,10 @@ GlobalCacheStats CacheAllocator<CacheTrait>::getGlobalCacheStats() const {
   ret.numActiveHandles = getNumActiveHandles();
 
   ret.isNewRamCache = cacheCreationTime_ == cacheInstanceCreationTime_;
+  // NVM cache is new either if newly created or started fresh with truncate
   ret.isNewNvmCache =
-      nvmCacheState_.getCreationTime() == cacheInstanceCreationTime_;
+      (nvmCacheState_.getCreationTime() == cacheInstanceCreationTime_) ||
+      nvmCacheState_.shouldStartFresh();
 
   return ret;
 }

cachelib/allocator/nvmcache/tests/NvmCacheTests.cpp

@@ -2179,8 +2179,10 @@ TEST_F(NvmCacheTest, testEvictCB) {
 void verifyItem(const Item& item, const Item& iobufItem) {
   ASSERT_EQ(item.isChainedItem(), iobufItem.isChainedItem());
   ASSERT_EQ(item.hasChainedItem(), iobufItem.hasChainedItem());
-  ASSERT_EQ(item.getCreationTime(), iobufItem.getCreationTime());
-  ASSERT_EQ(item.getExpiryTime(), iobufItem.getExpiryTime());
+  if (!item.isChainedItem()) {
+    ASSERT_EQ(item.getCreationTime(), iobufItem.getCreationTime());
+    ASSERT_EQ(item.getExpiryTime(), iobufItem.getExpiryTime());
+  }
   ASSERT_EQ(item.getSize(), iobufItem.getSize());
   ASSERT_EQ(
       0, std::memcmp(item.getMemory(), iobufItem.getMemory(), item.getSize()));