save some database calls when getting top block hash and height

src/blockchain_db/blockchain_db.h

@@ -1052,9 +1052,11 @@ public:
    *
    * The subclass should return the hash of the most recent block
    *
+   * @param block_height if non NULL, returns the height of that block (ie, the blockchain height minus 1)
+   *
    * @return the top block's hash
    */
-  virtual crypto::hash top_block_hash() const = 0;
+  virtual crypto::hash top_block_hash(uint64_t *block_height = NULL) const = 0;
 
   /**
    * @brief fetch the top block

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2582,11 +2582,13 @@ std::vector<crypto::hash> BlockchainLMDB::get_hashes_range(const uint64_t& h1, c
   return v;
 }
 
-crypto::hash BlockchainLMDB::top_block_hash() const
+crypto::hash BlockchainLMDB::top_block_hash(uint64_t *block_height) const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
   uint64_t m_height = height();
+  if (block_height)
+    *block_height = m_height - 1;
   if (m_height != 0)
   {
     return get_block_hash_from_height(m_height - 1);

src/blockchain_db/lmdb/db_lmdb.h

@@ -233,7 +233,7 @@ public:
 
   virtual std::vector<crypto::hash> get_hashes_range(const uint64_t& h1, const uint64_t& h2) const;
 
-  virtual crypto::hash top_block_hash() const;
+  virtual crypto::hash top_block_hash(uint64_t *block_height = NULL) const;
 
   virtual block get_top_block() const;
 

src/blockchain_db/testdb.h

@@ -79,7 +79,7 @@ public:
   virtual crypto::hash get_block_hash_from_height(const uint64_t& height) const { return crypto::hash(); }
   virtual std::vector<cryptonote::block> get_blocks_range(const uint64_t& h1, const uint64_t& h2) const { return std::vector<cryptonote::block>(); }
   virtual std::vector<crypto::hash> get_hashes_range(const uint64_t& h1, const uint64_t& h2) const { return std::vector<crypto::hash>(); }
-  virtual crypto::hash top_block_hash() const { return crypto::hash(); }
+  virtual crypto::hash top_block_hash(uint64_t *block_height = NULL) const { if (block_height) *block_height = 0; return crypto::hash(); }
   virtual cryptonote::block get_top_block() const { return cryptonote::block(); }
   virtual uint64_t height() const { return 1; }
   virtual bool tx_exists(const crypto::hash& h) const { return false; }