wallet2: speed up transactions using remote nodes

Asking for a full histogram from a remote node (since it's
untrusted) is pretty slow, and spams the remote node, so
we replace it by only adding a second input if we have rct
ones, which are for all intents and purposes always mixable.

src/wallet/wallet2.cpp

@@ -4120,7 +4120,7 @@ std::vector<size_t> wallet2::pick_preferred_rct_inputs(uint64_t needed_money) co
   return picks;
 }
 
-static bool should_pick_a_second_output(bool use_rct, size_t n_transfers, const std::vector<size_t> &unused_transfers_indices, const std::vector<size_t> &unused_dust_indices)
+bool wallet2::should_pick_a_second_output(bool use_rct, size_t n_transfers, const std::vector<size_t> &unused_transfers_indices, const std::vector<size_t> &unused_dust_indices) const
 {
   if (!use_rct)
     return false;
@@ -4128,9 +4128,43 @@ static bool should_pick_a_second_output(bool use_rct, size_t n_transfers, const
     return false;
   if (unused_dust_indices.empty() && unused_transfers_indices.empty())
     return false;
+  // we want at least one free rct output to avoid a corner case where
+  // we'd choose a non rct output which doesn't have enough "siblings"
+  // value-wise on the chain, and thus can't be mixed
+  bool found = false;
+  for (auto i: unused_dust_indices)
+  {
+    if (m_transfers[i].is_rct())
+    {
+      found = true;
+      break;
+    }
+  }
+  if (!found) for (auto i: unused_transfers_indices)
+  {
+    if (m_transfers[i].is_rct())
+    {
+      found = true;
+      break;
+    }
+  }
+  if (!found)
+    return false;
   return true;
 }
 
+std::vector<size_t> wallet2::get_only_rct(const std::vector<size_t> &unused_dust_indices, const std::vector<size_t> &unused_transfers_indices) const
+{
+  std::vector<size_t> indices;
+  for (size_t n: unused_dust_indices)
+    if (m_transfers[n].is_rct())
+      indices.push_back(n);
+  for (size_t n: unused_transfers_indices)
+    if (m_transfers[n].is_rct())
+      indices.push_back(n);
+  return indices;
+}
+
 // Another implementation of transaction creation that is hopefully better
 // While there is anything left to pay, it goes through random outputs and tries
 // to fill the next destination/amount. If it fully fills it, it will use the
@@ -4195,8 +4229,7 @@ std::vector<wallet2::pending_tx> wallet2::create_transactions_2(std::vector<cryp
   THROW_WALLET_EXCEPTION_IF(needed_money == 0, error::zero_destination);
 
   // gather all our dust and non dust outputs
-  const std::vector<size_t> unused_indices = select_available_outputs_from_histogram(fake_outs_count + 1, true, true, true, trusted_daemon);
+  for (size_t i = 0; i < m_transfers.size(); ++i)
-  for (size_t i: unused_indices)
   {
     const transfer_details& td = m_transfers[i];
     if (!td.m_spent && (use_rct ? true : !td.is_rct()) && is_transfer_unlocked(td))
@@ -4279,7 +4312,8 @@ std::vector<wallet2::pending_tx> wallet2::create_transactions_2(std::vector<cryp
     size_t idx;
     if ((dsts.empty() || dsts[0].amount == 0) && !adding_fee) {
       // the "make rct txes 2/2" case - we pick a small value output to "clean up" the wallet too
-      idx = pop_best_value(unused_dust_indices.empty() ? unused_transfers_indices : unused_dust_indices, tx.selected_transfers, true);
+      std::vector<size_t> indices = get_only_rct(unused_dust_indices, unused_transfers_indices);
+      idx = pop_best_value(indices, tx.selected_transfers, true);
 
       // since we're trying to add a second output which is not strictly needed,
       // we only add it if it's unrelated enough to the first one
@@ -4289,6 +4323,8 @@ std::vector<wallet2::pending_tx> wallet2::create_transactions_2(std::vector<cryp
         LOG_PRINT_L2("Second outout was not strictly needed, and relatedness " << relatedness << ", not adding");
         break;
       }
+      pop_if_present(unused_transfers_indices, idx);
+      pop_if_present(unused_dust_indices, idx);
     } else if (!prefered_inputs.empty()) {
       idx = pop_back(prefered_inputs);
       pop_if_present(unused_transfers_indices, idx);

src/wallet/wallet2.h

@@ -633,6 +633,8 @@ namespace tools
     void get_outs(std::vector<std::vector<get_outs_entry>> &outs, const std::list<size_t> &selected_transfers, size_t fake_outputs_count);
     bool wallet_generate_key_image_helper(const cryptonote::account_keys& ack, const crypto::public_key& tx_public_key, size_t real_output_index, cryptonote::keypair& in_ephemeral, crypto::key_image& ki);
     crypto::public_key get_tx_pub_key_from_received_outs(const tools::wallet2::transfer_details &td) const;
+    bool should_pick_a_second_output(bool use_rct, size_t n_transfers, const std::vector<size_t> &unused_transfers_indices, const std::vector<size_t> &unused_dust_indices) const;
+    std::vector<size_t> get_only_rct(const std::vector<size_t> &unused_dust_indices, const std::vector<size_t> &unused_transfers_indices) const;
 
     cryptonote::account_base m_account;
     boost::optional<epee::net_utils::http::login> m_daemon_login;