Some polish from feedback

include/rocksdb/utilities/optimistic_transaction_db.h

@@ -46,11 +46,15 @@ enum class OccValidationPolicy {
 
 class OccLockBuckets {
  public:
-  // Most details in internal derived class
-  // Users should not derive from this class
+  // Most details in internal derived class.
+  // Users should not derive from this class.
   virtual ~OccLockBuckets() {}
 
   virtual size_t ApproximateMemoryUsage() const = 0;
+
+ private:
+  friend class OccLockBucketsImplBase;
+  OccLockBuckets() {}
 };
 
 // An object for sharing a pool of locks across DB instances.

utilities/transactions/optimistic_transaction.cc

@@ -125,6 +125,8 @@ Status OptimisticTransaction::CommitWithParallelValidate() {
   // In this way, txns from different threads all obey this rule so that
   // deadlock can be avoided.
   for (auto v : lk_ptrs) {
+    // WART: if an exception is thrown during a Lock(), previously locked will
+    // not be Unlock()ed. But a vector of MutexLock is likely inefficient.
     v->Lock();
   }
   Defer unlocks([&]() {

utilities/transactions/optimistic_transaction_db_impl.h

@@ -26,7 +26,7 @@ class OccLockBucketsImplBase : public OccLockBuckets {
 template <bool cache_aligned>
 class OccLockBucketsImpl : public OccLockBucketsImplBase {
  public:
-  OccLockBucketsImpl(size_t bucket_count) : locks_(bucket_count) {}
+  explicit OccLockBucketsImpl(size_t bucket_count) : locks_(bucket_count) {}
   port::Mutex& GetLockBucket(const Slice& key, uint64_t seed) override {
     return locks_.Get(key, seed);
   }