Make get_output_key method const

get_output_key method is commonly used when working with txs and their key images. Because the method is not const, passing blockchain object though const& or pointers to const is not possible in this context. This is especially problematic in external projects (e.g., projects in moneroexamples) that use monero C++ api to operate on the blockchain and txs.

Thus, having get_output_key method will simplify moving blockchain object around through const references and pointers to const objects.

src/blockchain_db/berkeleydb/db_bdb.cpp

@@ -1632,7 +1632,7 @@ output_data_t BlockchainBDB::get_output_key(const uint64_t& global_index) const
     return v;
 }
 
-output_data_t BlockchainBDB::get_output_key(const uint64_t& amount, const uint64_t& index)
+output_data_t BlockchainBDB::get_output_key(const uint64_t& amount, const uint64_t& index) const
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);
     check_open();
@@ -1641,7 +1641,7 @@ output_data_t BlockchainBDB::get_output_key(const uint64_t& amount, const uint64
     return get_output_key(glob_index);
 }
 
-tx_out_index BlockchainBDB::get_output_tx_and_index(const uint64_t& amount, const uint64_t& index)
+tx_out_index BlockchainBDB::get_output_tx_and_index(const uint64_t& amount, const uint64_t& index) const
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);
     std::vector < uint64_t > offsets;

src/blockchain_db/blockchain_db.h

@@ -1258,7 +1258,7 @@ public:
    *
    * @return the requested output data
    */
-  virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index) = 0;
+  virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index) const = 0;
 
   /**
    * @brief gets an output's tx hash and index
@@ -1310,7 +1310,7 @@ public:
    * @param offsets a list of amount-specific output indices
    * @param outputs return-by-reference a list of outputs' metadata
    */
-  virtual void get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs, bool allow_partial = false) = 0;
+  virtual void get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs, bool allow_partial = false) const = 0;
   
   /*
    * FIXME: Need to check with git blame and ask what this does to

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2535,7 +2535,7 @@ uint64_t BlockchainLMDB::get_num_outputs(const uint64_t& amount) const
   return num_elems;
 }
 
-output_data_t BlockchainLMDB::get_output_key(const uint64_t& amount, const uint64_t& index)
+output_data_t BlockchainLMDB::get_output_key(const uint64_t& amount, const uint64_t& index) const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
@@ -3242,7 +3242,7 @@ void BlockchainLMDB::get_output_tx_and_index_from_global(const std::vector<uint6
   TXN_POSTFIX_RDONLY();
 }
 
-void BlockchainLMDB::get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs, bool allow_partial)
+void BlockchainLMDB::get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs, bool allow_partial) const
 {
   if (amounts.size() != 1 && amounts.size() != offsets.size())
     throw0(DB_ERROR("Invalid sizes of amounts and offets"));

src/blockchain_db/lmdb/db_lmdb.h

@@ -242,8 +242,8 @@ public:
 
   virtual uint64_t get_num_outputs(const uint64_t& amount) const;
 
-  virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index);
+  virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index) const;
-  virtual void get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs, bool allow_partial = false);
+  virtual void get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<output_data_t> &outputs, bool allow_partial = false) const;
 
   virtual tx_out_index get_output_tx_and_index_from_global(const uint64_t& index) const;
   virtual void get_output_tx_and_index_from_global(const std::vector<uint64_t> &global_indices,

tests/unit_tests/testdb.h

@@ -89,11 +89,11 @@ public:
   virtual uint64_t get_tx_block_height(const crypto::hash& h) const { return 0; }
   virtual uint64_t get_num_outputs(const uint64_t& amount) const { return 1; }
   virtual uint64_t get_indexing_base() const { return 0; }
-  virtual cryptonote::output_data_t get_output_key(const uint64_t& amount, const uint64_t& index) { return cryptonote::output_data_t(); }
+  virtual cryptonote::output_data_t get_output_key(const uint64_t& amount, const uint64_t& index) const { return cryptonote::output_data_t(); }
   virtual cryptonote::tx_out_index get_output_tx_and_index_from_global(const uint64_t& index) const { return cryptonote::tx_out_index(); }
   virtual cryptonote::tx_out_index get_output_tx_and_index(const uint64_t& amount, const uint64_t& index) const { return cryptonote::tx_out_index(); }
   virtual void get_output_tx_and_index(const uint64_t& amount, const std::vector<uint64_t> &offsets, std::vector<cryptonote::tx_out_index> &indices) const {}
-  virtual void get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<cryptonote::output_data_t> &outputs, bool allow_partial = false) {}
+  virtual void get_output_key(const epee::span<const uint64_t> &amounts, const std::vector<uint64_t> &offsets, std::vector<cryptonote::output_data_t> &outputs, bool allow_partial = false) const {}
   virtual bool can_thread_bulk_indices() const { return false; }
   virtual std::vector<uint64_t> get_tx_output_indices(const crypto::hash& h) const { return std::vector<uint64_t>(); }
   virtual std::vector<uint64_t> get_tx_amount_output_indices(const uint64_t tx_index) const { return std::vector<uint64_t>(); }