Add new API CacheReservationManager::GetDummyEntrySize() (#9072)

Summary:
Note: it might conflict with another CRM related PR https://github.com/facebook/rocksdb/pull/9071 and so will merge after that's merged.

Context:
As `CacheReservationManager` being used by more memory users, it is convenient to retrieve the dummy entry size for `CacheReservationManager` instead of hard-coding `256 * 1024` in writing tests. Plus it allows more flexibility to change our implementation on dummy entry size.

A follow-up PR is needed to replace those hard-coded dummy entry size value in `db_test2.cc`, `db_write_buffer_manager_test.cc`, `write_buffer_manager_test.cc`, `table_test.cc` and the ones introduced in https://github.com/facebook/rocksdb/pull/9072#issue-1034326069.
- Exposed the private static constexpr `kDummyEntrySize` through public static `CacheReservationManager::GetDummyEntrySize()`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9072

Test Plan:
- Passing new tests
- Passing existing tests

Reviewed By: ajkr

Differential Revision: D32043684

Pulled By: hx235

fbshipit-source-id: ddefc6921c052adab6a2cda2394eb26da3076a50

cache/cache_reservation_manager.h

@@ -75,6 +75,8 @@ class CacheReservationManager {
   Status UpdateCacheReservation(std::size_t new_memory_used);
   std::size_t GetTotalReservedCacheSize();
 
+  static constexpr std::size_t GetDummyEntrySize() { return kSizeDummyEntry; }
+
  private:
   static constexpr std::size_t kSizeDummyEntry = 256 * 1024;
   // The key will be longer than keys for blocks in SST files so they won't

cache/cache_reservation_manager_test.cc

@@ -25,7 +25,8 @@ class CacheReservationManagerTest : public ::testing::Test {
   static constexpr std::size_t kOneGigabyte = 1024 * 1024 * 1024;
   static constexpr int kNumShardBits = 0;  // 2^0 shard
 
-  static constexpr std::size_t kSizeDummyEntry = 256 * 1024;
+  static constexpr std::size_t kSizeDummyEntry =
+      CacheReservationManager::GetDummyEntrySize();
   static const std::size_t kCacheKeyPrefixSize =
       BlockBasedTable::kMaxCacheKeyPrefixSize + kMaxVarint64Length;
   static constexpr std::size_t kMetaDataChargeOverhead = 10000;
@@ -143,7 +144,8 @@ TEST(CacheReservationManagerIncreaseReservcationOnFullCacheTest,
      IncreaseCacheReservationOnFullCache) {
   constexpr std::size_t kOneMegabyte = 1024 * 1024;
   constexpr std::size_t kOneGigabyte = 1024 * 1024 * 1024;
-  constexpr std::size_t kSizeDummyEntry = 256 * 1024;
+  constexpr std::size_t kSizeDummyEntry =
+      CacheReservationManager::GetDummyEntrySize();
   constexpr std::size_t kMetaDataChargeOverhead = 10000;
 
   LRUCacheOptions lo;
@@ -307,7 +309,8 @@ TEST_F(CacheReservationManagerTest,
 TEST(CacheReservationManagerWithDelayedDecreaseTest,
      DecreaseCacheReservationWithDelayedDecrease) {
   constexpr std::size_t kOneGigabyte = 1024 * 1024 * 1024;
-  constexpr std::size_t kSizeDummyEntry = 256 * 1024;
+  constexpr std::size_t kSizeDummyEntry =
+      CacheReservationManager::GetDummyEntrySize();
   constexpr std::size_t kMetaDataChargeOverhead = 10000;
 
   LRUCacheOptions lo;
@@ -379,7 +382,8 @@ TEST(CacheReservationManagerWithDelayedDecreaseTest,
 TEST(CacheReservationManagerDestructorTest,
      ReleaseRemainingDummyEntriesOnDestruction) {
   constexpr std::size_t kOneGigabyte = 1024 * 1024 * 1024;
-  constexpr std::size_t kSizeDummyEntry = 256 * 1024;
+  constexpr std::size_t kSizeDummyEntry =
+      CacheReservationManager::GetDummyEntrySize();
   constexpr std::size_t kMetaDataChargeOverhead = 10000;
 
   LRUCacheOptions lo;