Fix transfers to support mixins

Implement BlockchainLMDB::get_output_global_index()
- returns global output index for a given amount and amount output
  index.

Add information to debug statement for failed ring signature check
within Blockchain::check_tx_inputs()

Fixes bitmonerod RPC call "/getrandom_outs.bin" to return correct
output keys, used in creating a transaction with mixins.

TODO: get_output_global_index() could be refactored with part of
get_output_tx_and_index() as the latter uses the former's
functionality.  Keep track of LMDB read transaction.

src/cryptonote_core/BlockchainDB_impl/db_lmdb.cpp

@@ -477,7 +477,44 @@ tx_out BlockchainLMDB::output_from_blob(const blobdata& blob) const
 uint64_t BlockchainLMDB::get_output_global_index(const uint64_t& amount, const uint64_t& index) const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
-  return 0;
+  check_open();
+
+  txn_safe txn;
+  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+    throw0(DB_ERROR("Failed to create a transaction for the db"));
+
+  lmdb_cur cur(txn, m_output_amounts);
+
+  MDB_val_copy<uint64_t> k(amount);
+  MDB_val v;
+
+  auto result = mdb_cursor_get(cur, &k, &v, MDB_SET);
+  if (result == MDB_NOTFOUND)
+    throw1(OUTPUT_DNE("Attempting to get an output index by amount and amount index, but amount not found"));
+  else if (result)
+    throw0(DB_ERROR("DB error attempting to get an output"));
+
+  size_t num_elems = 0;
+  mdb_cursor_count(cur, &num_elems);
+  if (num_elems <= index)
+    throw1(OUTPUT_DNE("Attempting to get an output index by amount and amount index, but output not found"));
+
+  mdb_cursor_get(cur, &k, &v, MDB_FIRST_DUP);
+
+  for (uint64_t i = 0; i < index; ++i)
+  {
+    mdb_cursor_get(cur, &k, &v, MDB_NEXT_DUP);
+  }
+
+  mdb_cursor_get(cur, &k, &v, MDB_GET_CURRENT);
+
+  uint64_t glob_index = *(const uint64_t*)v.mv_data;
+
+  cur.close();
+
+  txn.commit();
+
+  return glob_index;
 }
 
 void BlockchainLMDB::check_open() const
@@ -1123,11 +1160,13 @@ crypto::public_key BlockchainLMDB::get_output_key(const uint64_t& amount, const
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
 
+  uint64_t glob_index = get_output_global_index(amount, index);
+
   txn_safe txn;
   if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
     throw0(DB_ERROR("Failed to create a transaction for the db"));
 
-  MDB_val_copy<uint64_t> k(get_output_global_index(amount, index));
+  MDB_val_copy<uint64_t> k(glob_index);
   MDB_val v;
   auto get_result = mdb_get(txn, m_output_keys, &k, &v);
   if (get_result == MDB_NOTFOUND)

src/cryptonote_core/blockchain.cpp

@@ -1849,7 +1849,7 @@ bool Blockchain::check_tx_inputs(const transaction& tx, uint64_t* pmax_used_bloc
     // signature spending it.
     if(!check_tx_input(in_to_key, tx_prefix_hash, tx.signatures[sig_index], pmax_used_block_height))
     {
-      LOG_PRINT_L0("Failed to check ring signature for tx " << get_transaction_hash(tx));
+      LOG_PRINT_L0("Failed to check ring signature for tx " << get_transaction_hash(tx) << "  vin key with k_image: " << in_to_key.k_image << "  sig_index: " << sig_index << "  *pmax_used_block_height: " << *pmax_used_block_height);
       return false;
     }