wallet: check a key image isn't already present when adding one

If it is, it points to reuse of a tx key, which isn't meant to happen.
If it does, a key image collision means that only one of those
outputs is spendable, so the wallet selects the larger amount,
unless that output was spent already.

This causes a discrepancy betewen reported received inputs and
payment total.

Since tx keys are 256 bits, this should never happen except if
done on purpose, or if a sender uses a bad PRNG.

src/wallet/wallet2.cpp

@@ -323,23 +323,60 @@ void wallet2::process_new_transaction(const cryptonote::transaction& tx, uint64_
 	THROW_WALLET_EXCEPTION_IF(tx.vout.size() <= o, error::wallet_internal_error, "wrong out in transaction: internal index=" +
 				  std::to_string(o) + ", total_outs=" + std::to_string(tx.vout.size()));
 
+        crypto::key_image ki;
+	cryptonote::keypair in_ephemeral;
+	cryptonote::generate_key_image_helper(m_account.get_keys(), tx_pub_key, o, in_ephemeral, ki);
+	THROW_WALLET_EXCEPTION_IF(in_ephemeral.pub != boost::get<cryptonote::txout_to_key>(tx.vout[o].target).key,
+				  error::wallet_internal_error, "key_image generated ephemeral public key not matched with output_key");
+
+        auto kit = m_key_images.find(ki);
+	THROW_WALLET_EXCEPTION_IF(kit != m_key_images.end() && kit->second >= m_transfers.size(),
+            error::wallet_internal_error, std::string("Unexpected transfer index from key image: ")
+            + "got " + (kit == m_key_images.end() ? "<none>" : boost::lexical_cast<std::string>(kit->second))
+            + ", m_transfers.size() is " + boost::lexical_cast<std::string>(m_transfers.size()));
+        if (kit == m_key_images.end())
+        {
 	  m_transfers.push_back(boost::value_initialized<transfer_details>());
 	  transfer_details& td = m_transfers.back();
 	  td.m_block_height = height;
 	  td.m_internal_output_index = o;
 	  td.m_global_output_index = res.o_indexes[o];
 	  td.m_tx = tx;
+          td.m_key_image = ki;
 	  td.m_spent = false;
-	cryptonote::keypair in_ephemeral;
-	cryptonote::generate_key_image_helper(m_account.get_keys(), tx_pub_key, o, in_ephemeral, td.m_key_image);
-	THROW_WALLET_EXCEPTION_IF(in_ephemeral.pub != boost::get<cryptonote::txout_to_key>(tx.vout[o].target).key,
-				  error::wallet_internal_error, "key_image generated ephemeral public key not matched with output_key");
-
 	  m_key_images[td.m_key_image] = m_transfers.size()-1;
 	  LOG_PRINT_L0("Received money: " << print_money(td.amount()) << ", with tx: " << get_transaction_hash(tx));
 	  if (0 != m_callback)
 	    m_callback->on_money_received(height, td.m_tx, td.m_internal_output_index);
         }
+	else if (m_transfers[kit->second].m_spent || m_transfers[kit->second].amount() >= tx.vout[o].amount)
+        {
+	  LOG_ERROR("key image " << epee::string_tools::pod_to_hex(ki)
+              << " from received " << print_money(tx.vout[o].amount) << " output already exists with "
+              << (m_transfers[kit->second].m_spent ? "spent" : "unspent") << " "
+              << print_money(m_transfers[kit->second].amount()) << ", received output ignored");
+        }
+        else
+        {
+	  LOG_ERROR("key image " << epee::string_tools::pod_to_hex(ki)
+              << " from received " << print_money(tx.vout[o].amount) << " output already exists with "
+              << print_money(m_transfers[kit->second].amount()) << ", replacing with new output");
+          // The new larger output replaced a previous smaller one
+          tx_money_got_in_outs -= tx.vout[o].amount;
+
+          transfer_details &td = m_transfers[kit->second];
+	  td.m_block_height = height;
+	  td.m_internal_output_index = o;
+	  td.m_global_output_index = res.o_indexes[o];
+	  td.m_tx = tx;
+          THROW_WALLET_EXCEPTION_IF(td.m_key_image != ki, error::wallet_internal_error, "Inconsistent key images");
+	  THROW_WALLET_EXCEPTION_IF(td.m_spent, error::wallet_internal_error, "Inconsistent spent status");
+
+	  LOG_PRINT_L0("Received money: " << print_money(td.amount()) << ", with tx: " << get_transaction_hash(tx));
+	  if (0 != m_callback)
+	    m_callback->on_money_received(height, td.m_tx, td.m_internal_output_index);
+        }
+      }
     }
   }