core: new /getrandom_rctouts.bin binary RPC call

to get random ringct outputs to mix with

src/cryptonote_core/blockchain.cpp

@@ -1597,6 +1597,104 @@ bool Blockchain::get_random_outs_for_amounts(const COMMAND_RPC_GET_RANDOM_OUTPUT
   return true;
 }
 //------------------------------------------------------------------
+// This function adds the ringct output at index i to the list
+// unlocked and other such checks should be done by here.
+void Blockchain::add_out_to_get_rct_random_outs(std::list<COMMAND_RPC_GET_RANDOM_RCT_OUTPUTS::out_entry>& outs, size_t i) const
+{
+  LOG_PRINT_L3("Blockchain::" << __func__);
+  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+
+  COMMAND_RPC_GET_RANDOM_RCT_OUTPUTS::out_entry& oen = *outs.insert(outs.end(), COMMAND_RPC_GET_RANDOM_RCT_OUTPUTS::out_entry());
+  oen.global_amount_index = i;
+  output_data_t data = m_db->get_output_key(0, i);
+  oen.out_key = data.pubkey;
+  oen.commitment = m_db->get_rct_commitment(i);
+}
+//------------------------------------------------------------------
+// This function takes an RPC request for mixins and creates an RPC response
+// with the requested mixins.
+// TODO: figure out why this returns boolean / if we should be returning false
+// in some cases
+bool Blockchain::get_random_rct_outs(const COMMAND_RPC_GET_RANDOM_RCT_OUTPUTS::request& req, COMMAND_RPC_GET_RANDOM_RCT_OUTPUTS::response& res) const
+{
+  LOG_PRINT_L3("Blockchain::" << __func__);
+  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+
+  // for each amount that we need to get mixins for, get <n> random outputs
+  // from BlockchainDB where <n> is req.outs_count (number of mixins).
+  auto num_outs = m_db->get_num_outputs(0);
+  // ensure we don't include outputs that aren't yet eligible to be used
+  // outpouts are sorted by height
+  while (num_outs > 0)
+  {
+    const tx_out_index toi = m_db->get_output_tx_and_index(0, num_outs - 1);
+    const uint64_t height = m_db->get_tx_block_height(toi.first);
+    if (height + CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE <= m_db->height())
+      break;
+    --num_outs;
+  }
+
+  std::unordered_set<uint64_t> seen_indices;
+
+  // if there aren't enough outputs to mix with (or just enough),
+  // use all of them.  Eventually this should become impossible.
+  if (num_outs <= req.outs_count)
+  {
+    for (uint64_t i = 0; i < num_outs; i++)
+    {
+      // get tx_hash, tx_out_index from DB
+      tx_out_index toi = m_db->get_output_tx_and_index(0, i);
+
+      // if tx is unlocked, add output to result_outs
+      if (is_tx_spendtime_unlocked(m_db->get_tx_unlock_time(toi.first)))
+      {
+        add_out_to_get_rct_random_outs(res.outs, i);
+      }
+    }
+  }
+  else
+  {
+    // while we still need more mixins
+    while (res.outs.size() < req.outs_count)
+    {
+      // if we've gone through every possible output, we've gotten all we can
+      if (seen_indices.size() == num_outs)
+      {
+        break;
+      }
+
+      // get a random output index from the DB.  If we've already seen it,
+      // return to the top of the loop and try again, otherwise add it to the
+      // list of output indices we've seen.
+
+      // triangular distribution over [a,b) with a=0, mode c=b=up_index_limit
+      uint64_t r = crypto::rand<uint64_t>() % ((uint64_t)1 << 53);
+      double frac = std::sqrt((double)r / ((uint64_t)1 << 53));
+      uint64_t i = (uint64_t)(frac*num_outs);
+      // just in case rounding up to 1 occurs after sqrt
+      if (i == num_outs)
+        --i;
+
+      if (seen_indices.count(i))
+      {
+        continue;
+      }
+      seen_indices.emplace(i);
+
+      // get tx_hash, tx_out_index from DB
+      tx_out_index toi = m_db->get_output_tx_and_index(0, i);
+
+      // if the output's transaction is unlocked, add the output's index to
+      // our list.
+      if (is_tx_spendtime_unlocked(m_db->get_tx_unlock_time(toi.first)))
+      {
+        add_out_to_get_rct_random_outs(res.outs, i);
+      }
+    }
+  }
+  return true;
+}
+//------------------------------------------------------------------
 bool Blockchain::get_outs(const COMMAND_RPC_GET_OUTPUTS::request& req, COMMAND_RPC_GET_OUTPUTS::response& res) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);