Add zero-confirmation support to webhooks (only) (#72)

src/db/data.cpp

@@ -34,6 +34,7 @@
 #include "wire/json/write.h"
 #include "wire/msgpack.h"
 #include "wire/uuid.h"
+#include "wire/wrapper/defaulted.h"
 
 namespace lws
 {
@@ -126,9 +127,11 @@ namespace db
     const auto payment_id = payment_bytes.empty() ?
       nullptr : std::addressof(payment_bytes);
 
+    // defaulted will omit "id" and "block" when the output is in the
+    // txpool with no valid values.
     wire::object(dest,
-      wire::field<0>("id", std::cref(self.spend_meta.id)),
-      wire::field<1>("block", self.link.height),
+      wire::optional_field<0>("id", wire::defaulted(std::cref(self.spend_meta.id), output_id::txpool())),
+      wire::optional_field<1>("block", wire::defaulted(self.link.height, block_id::txpool)),
       wire::field<2>("index", self.spend_meta.index),
       wire::field<3>("amount", self.spend_meta.amount),
       wire::field<4>("timestamp", self.timestamp),

src/db/data.h

@@ -30,6 +30,7 @@
 #include <cassert>
 #include <cstdint>
 #include <iosfwd>
+#include <limits>
 #include <string>
 #include <utility>
 
@@ -63,12 +64,19 @@ namespace db
   WIRE_AS_INTEGER(account_time);
 
   //! References a block height
-  enum class block_id : std::uint64_t {};
+  enum class block_id : std::uint64_t
+  {
+    txpool = std::uint64_t(-1) //! Represents not-yet-a-block
+  };
   WIRE_AS_INTEGER(block_id);
 
   //! References a global output number, as determined by the public chain
   struct output_id
   {
+    //! \return Special ID for outputs not yet in a block.
+    static constexpr output_id txpool() noexcept
+    { return {0, std::numeric_limits<std::uint64_t>::max()}; }
+
     std::uint64_t high; //!< Amount on public chain; rct outputs are `0`
     std::uint64_t low;  //!< Offset within `amount` on the public chain
   };

src/db/storage.cpp

@@ -749,6 +749,42 @@ namespace db
     return requests.get_value<request_info>(value);
   }
 
+  expect<std::vector<webhook_value>>
+  storage_reader::find_webhook(webhook_key const& key, crypto::hash8 const& payment_id, cursor::webhooks cur)
+  {
+    MONERO_PRECOND(txn != nullptr);
+    assert(db != nullptr);
+    MONERO_CHECK(check_cursor(*txn, db->tables.webhooks, cur));
+
+    webhook_dupsort dup{};
+
+    static_assert(sizeof(dup.payment_id) == sizeof(payment_id), "bad memcpy");
+    std::memcpy(std::addressof(dup.payment_id), std::addressof(payment_id), sizeof(payment_id));
+
+    MDB_val lkey = lmdb::to_val(key);
+    MDB_val lvalue = lmdb::to_val(dup);
+
+    std::vector<webhook_value> result{};
+    int err = mdb_cursor_get(cur.get(), &lkey, &lvalue, MDB_GET_BOTH_RANGE);
+    for (;;)
+    {
+      if (err)
+      {
+        if (err == MDB_NOTFOUND)
+          break;
+        return {lmdb::error(err)};
+      }
+
+      if (webhooks.get_fixed_value<MONERO_FIELD(webhook_dupsort, payment_id)>(lvalue) != dup.payment_id)
+        break;
+
+      result.push_back(MONERO_UNWRAP(webhooks.get_value(lvalue)));
+      err = mdb_cursor_get(cur.get(), &lkey, &lvalue, MDB_NEXT_DUP);
+    }
+
+    return result;
+  }
+
   expect<std::vector<std::pair<webhook_key, std::vector<webhook_value>>>>
   storage_reader::get_webhooks(cursor::webhooks cur)
   {

src/db/storage.h

@@ -133,6 +133,10 @@ namespace db
     expect<request_info>
       get_request(request type, account_address const& address, cursor::requests cur = nullptr) noexcept;
 
+    //! \return All webhook values associated with user `key` and `payment_id`.
+    expect<std::vector<webhook_value>>
+      find_webhook(webhook_key const& key, crypto::hash8 const& payment_id, cursor::webhooks cur = nullptr);
+
     //! \return All webhooks in the DB
     expect<std::vector<std::pair<webhook_key, std::vector<webhook_value>>>>
       get_webhooks(cursor::webhooks cur = nullptr);

src/rpc/client.cpp

@@ -51,6 +51,7 @@ namespace rpc
     constexpr const char abort_scan_signal[] = "SCAN";
     constexpr const char abort_process_signal[] = "PROCESS";
     constexpr const char minimal_chain_topic[] = "json-minimal-chain_main";
+    constexpr const char full_txpool_topic[] = "json-full-txpool_add";
     constexpr const int daemon_zmq_linger = 0;
     constexpr const std::chrono::seconds chain_poll_timeout{20};
     constexpr const std::chrono::minutes chain_sub_timeout{2};
@@ -225,10 +226,9 @@ namespace rpc
       if (out.daemon_sub.get() == nullptr)
         return net::zmq::get_error_code();
 
-      option = 1; // keep only last pub message from daemon
       MONERO_ZMQ_CHECK(zmq_connect(out.daemon_sub.get(), out.ctx->sub_addr.c_str()));
-      MONERO_ZMQ_CHECK(zmq_setsockopt(out.daemon_sub.get(), ZMQ_CONFLATE, &option, sizeof(option)));
       MONERO_CHECK(do_subscribe(out.daemon_sub.get(), minimal_chain_topic));
+      MONERO_CHECK(do_subscribe(out.daemon_sub.get(), full_txpool_topic));
     }
 
     out.signal_sub.reset(zmq_socket(out.ctx->comm.get(), ZMQ_SUB));
@@ -250,7 +250,7 @@ namespace rpc
     return do_subscribe(signal_sub.get(), abort_scan_signal);
   }
 
-  expect<minimal_chain_pub> client::wait_for_block()
+  expect<std::vector<std::pair<client::topic, std::string>>> client::wait_for_block()
   {
     MONERO_PRECOND(ctx != nullptr);
     assert(daemon != nullptr);
@@ -271,14 +271,45 @@ namespace rpc
         return ready.error();
       }
     }
-    expect<std::string> pub = net::zmq::receive(daemon_sub.get(), ZMQ_DONTWAIT);
-    if (!pub)
-      return pub.error();
 
-    if (!boost::string_ref{*pub}.starts_with(minimal_chain_topic))
-      return {lws::error::bad_daemon_response};
-    pub->erase(0, sizeof(minimal_chain_topic));
-    return minimal_chain_pub::from_json(std::move(*pub));
+    std::vector<std::pair<topic, std::string>> messages{};
+    for (; /*every message */ ;)
+    {
+      expect<std::string> pub = net::zmq::receive(daemon_sub.get(), ZMQ_DONTWAIT);
+      if (!pub)
+      {
+        if (pub == net::zmq::make_error_code(EAGAIN))
+          return {std::move(messages)};
+      	return pub.error();
+      }
+      if (pub->size() < 5)
+        break; // for loop
+
+      switch (pub->at(5))
+      {
+        case 'm': // json-minimal-chain_main
+          if (boost::string_ref{*pub}.starts_with(minimal_chain_topic))
+          {
+            pub->erase(0, sizeof(minimal_chain_topic));
+            messages.emplace_back(topic::block, std::move(*pub));
+          }
+          else
+            MWARNING("Unexpected pub/sub message");
+          break;
+        case 'f': // json-full-txpool_add
+          if (boost::string_ref{*pub}.starts_with(full_txpool_topic))
+          {
+            pub->erase(0, sizeof(full_txpool_topic));
+            messages.emplace_back(topic::txpool, std::move(*pub));
+          }
+          else
+            MWARNING("Unexpected pub/sub message");
+          break;
+        default:
+          break;
+      }
+    } // for every message
+    return {lws::error::bad_daemon_response};
   }
 
   expect<void> client::send(epee::byte_slice message, std::chrono::seconds timeout) noexcept

src/rpc/client.h

@@ -75,6 +75,12 @@ namespace rpc
     expect<void> get_response(cryptonote::rpc::Message& response, std::chrono::seconds timeout, source_location loc);
 
   public:
+
+    enum class topic : std::uint8_t
+    {
+      block = 0, txpool
+    };
+
     //! A client with no connection (all send/receive functions fail).
     explicit client() noexcept
       : ctx(), daemon(), daemon_sub(), signal_sub()
@@ -110,7 +116,7 @@ namespace rpc
     expect<void> watch_scan_signals() noexcept;
 
     //! Wait for new block announce or internal timeout.
-    expect<minimal_chain_pub> wait_for_block();
+    expect<std::vector<std::pair<topic, std::string>>> wait_for_block();
 
     //! \return A JSON message for RPC request `M`.
     template<typename M>

src/rpc/daemon_pub.cpp

@@ -27,6 +27,8 @@
 
 #include "daemon_pub.h"
 
+#include "cryptonote_basic/cryptonote_basic.h" // monero/src
+#include "rpc/daemon_zmq.h"
 #include "wire/crypto.h"
 #include "wire/error.h"
 #include "wire/field.h"
@@ -83,5 +85,19 @@ namespace rpc
       return err;
     return {std::move(out)};
   }
+
+  static void read_bytes(wire::json_reader& source, full_txpool_pub& self)
+  {
+    wire_read::array(source, self.txes);
+  }
+
+  expect<full_txpool_pub> full_txpool_pub::from_json(std::string&& source)
+  {
+    full_txpool_pub out{};
+    std::error_code err = wire::json::from_bytes(std::move(source), out);
+    if (err)
+      return err;
+    return {std::move(out)};
+  }
 }
 }

src/rpc/daemon_pub.h

@@ -34,6 +34,7 @@
 #include "crypto/hash.h"   // monero/src
 #include "wire/json/fwd.h"
 
+namespace cryptonote { class transaction; }
 namespace lws
 {
 namespace rpc
@@ -46,5 +47,12 @@ namespace rpc
 
     static expect<minimal_chain_pub> from_json(std::string&&);
   };
+
+  struct full_txpool_pub
+  {
+    std::vector<cryptonote::transaction> txes;
+
+    static expect<full_txpool_pub> from_json(std::string&&);
+  };
 }
 }

src/rpc/daemon_zmq.cpp

@@ -42,6 +42,7 @@ namespace
   constexpr const std::size_t default_inputs = 2;
   constexpr const std::size_t default_outputs = 4;
   constexpr const std::size_t default_txextra_size = 2048;
+  constexpr const std::size_t default_txpool_size = 32;
 }
 
 namespace rct
@@ -141,7 +142,7 @@ namespace cryptonote
     );
   }
 
-  static void read_bytes(wire::json_reader& source, transaction& self)
+  void read_bytes(wire::json_reader& source, transaction& self)
   {
     self.vin.reserve(default_inputs);
     self.vout.reserve(default_outputs);
@@ -177,6 +178,11 @@ namespace cryptonote
       self.transactions.reserve(default_transaction_count);
       wire::object(source, WIRE_FIELD(block), WIRE_FIELD(transactions));
     }
+
+    static void read_bytes(wire::json_reader& source, tx_in_pool& self)
+    {
+      wire::object(source, WIRE_FIELD(tx), WIRE_FIELD(tx_hash));
+    }
   } // rpc
 } // cryptonote
 
@@ -187,3 +193,8 @@ void lws::rpc::read_bytes(wire::json_reader& source, get_blocks_fast_response& s
   wire::object(source, WIRE_FIELD(blocks), WIRE_FIELD(output_indices), WIRE_FIELD(start_height), WIRE_FIELD(current_height));
 }
 
+void lws::rpc::read_bytes(wire::json_reader& source, get_transaction_pool_response& self)
+{
+  self.transactions.reserve(default_txpool_size);
+  wire::object(source, WIRE_FIELD(transactions));
+}

src/rpc/daemon_zmq.h

@@ -40,9 +40,13 @@ namespace crypto
 
 namespace cryptonote
 {
+  class transaction;
+  void read_bytes(wire::json_reader& source, transaction& self);
+
   namespace rpc
   {
     struct block_with_transactions;
+    struct tx_in_pool;
   }
 }
 
@@ -71,5 +75,21 @@ namespace rpc
     using response = get_blocks_fast_response;
   };
   void read_bytes(wire::json_reader&, get_blocks_fast_response&);
+
+  struct get_transaction_pool_request
+  {
+    get_transaction_pool_request() = delete;
+  };
+  struct get_transaction_pool_response
+  {
+    get_transaction_pool_response() = delete;
+    std::vector<cryptonote::rpc::tx_in_pool> transactions;
+  };
+  struct get_transaction_pool
+  {
+    using request = get_transaction_pool_request;
+    using response = get_transaction_pool_response;
+  };
+  void read_bytes(wire::json_reader&, get_transaction_pool_response&);
 } // rpc
 } // lws

src/scanner.cpp

@@ -35,6 +35,8 @@
 #include <cassert>
 #include <chrono>
 #include <cstring>
+#include <functional>
+#include <limits>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -50,7 +52,8 @@
 #include "misc_log_ex.h"             // monero/contrib/epee/include
 #include "net/http_client.h"
 #include "net/net_parse_helpers.h"
-#include "rpc/daemon_messages.h"     // monero/src
+#include "rpc/daemon_messages.h"      // monero/src
+#include "rpc/message_data_structs.h" // monero/src
 #include "rpc/daemon_zmq.h"
 #include "rpc/json.h"
 #include "util/source_location.h"
@@ -119,9 +122,16 @@ namespace lws
       }
     }
 
-    bool is_new_block(db::storage& disk, const account& user, const rpc::minimal_chain_pub& chain)
+    bool is_new_block(std::string&& chain_msg, db::storage& disk, const account& user)
     {
-      if (user.scan_height() < db::block_id(chain.top_block_height))
+      const auto chain = rpc::minimal_chain_pub::from_json(std::move(chain_msg));
+      if (!chain)
+      {
+        MERROR("Unable to parse blockchain notification: " << chain.error());
+        return false;
+      }
+
+      if (user.scan_height() < db::block_id(chain->top_block_height))
         return true;
 
       auto reader = disk.start_read();
@@ -132,8 +142,8 @@ namespace lws
       }
 
       // check possible chain rollback daemon side
-      const expect<crypto::hash> id = reader->get_block_hash(db::block_id(chain.top_block_height));
-      if (!id || *id != chain.top_block_id)
+      const expect<crypto::hash> id = reader->get_block_hash(db::block_id(chain->top_block_height));
+      if (!id || *id != chain->top_block_id)
         return true;
 
       // check possible chain rollback from other thread
@@ -241,13 +251,103 @@ namespace lws
       }
     };
 
-    void scan_transaction(
+    struct add_spend
+    {
+      void operator()(lws::account& user, const db::spend& spend) const
+      { user.add_spend(spend); }
+    };
+    struct add_output
+    {
+      bool operator()(lws::account& user, const db::output& out) const
+      { return user.add_out(out); }
+    };
+
+    struct null_spend
+    {
+      void operator()(lws::account&, const db::spend&) const noexcept
+      {}
+    };
+    struct send_webhook
+    {
+      db::storage const& disk_;
+      rpc::client& client_;
+      net::ssl_verification_t verify_mode_;
+      std::unordered_map<crypto::hash, crypto::hash> txpool_;
+
+      bool operator()(lws::account& user, const db::output& out)
+      {
+        /* Upstream monerod does not send all fields for a transaction, so
+           mempool notifications cannot compute tx_hash correctly (it is not
+           sent separately, a further blunder). Instead, if there are matching
+           outputs with webhooks, fetch mempool to compare tx_prefix_hash and
+           then use corresponding tx_hash. */
+        const db::webhook_key key{user.id(), db::webhook_type::tx_confirmation};
+        std::vector<db::webhook_value> hooks{};
+        {
+          auto reader = disk_.start_read();
+          if (!reader)
+          {
+            MERROR("Unable to lookup webhook on tx in pool: " << reader.error().message());
+            return false;
+          }
+          auto found = reader->find_webhook(key, out.payment_id.short_);
+          if (!found)
+          {
+            MERROR("Failed db lookup for webhooks: " << found.error().message());
+            return false;
+          }
+          hooks = std::move(*found);
+        }
+
+        if (!hooks.empty() && txpool_.empty())
+        {
+          cryptonote::rpc::GetTransactionPool::Request req{};
+          if (!send(client_, rpc::client::make_message("get_transaction_pool", req)))
+          {
+            MERROR("Unable to compute tx hash for webhook, aborting");
+            return false;
+          }
+          auto resp = client_.get_message(std::chrono::seconds{3});
+          if (!resp)
+          {
+            MERROR("Unable to get txpool: " << resp.error().message());
+            return false;
+          }
+
+          rpc::json<rpc::get_transaction_pool>::response txpool{};
+          const std::error_code err = wire::json::from_bytes(std::move(*resp), txpool);
+          if (err)
+            MONERO_THROW(err, "Invalid json-rpc");
+          for (auto& tx : txpool.result.transactions)
+            txpool_.emplace(get_transaction_prefix_hash(tx.tx), tx.tx_hash);
+        }
+
+        std::vector<db::webhook_tx_confirmation> events{};
+        for (auto& hook : hooks)
+        {
+          events.push_back(db::webhook_tx_confirmation{key, std::move(hook), out});
+          events.back().value.second.confirmations = 0;
+
+          const auto hash = txpool_.find(out.tx_prefix_hash);
+          if (hash != txpool_.end())
+            events.back().tx_info.link.tx_hash = hash->second;
+          else
+            events.pop_back(); //cannot compute tx_hash
+        }
+        send_via_http(epee::to_span(events), std::chrono::seconds{5}, verify_mode_);
+        return true;
+      }
+    };
+
+    void scan_transaction_base(
       epee::span<lws::account> users,
       const db::block_id height,
       const std::uint64_t timestamp,
       crypto::hash const& tx_hash,
       cryptonote::transaction const& tx,
-      std::vector<std::uint64_t> const& out_ids)
+      std::vector<std::uint64_t> const& out_ids,
+      std::function<void(lws::account&, const db::spend&)> spend_action,
+      std::function<bool(lws::account&, const db::output&)> output_action)
     {
       if (2 < tx.version)
         throw std::runtime_error{"Unsupported tx version"};
@@ -302,7 +402,8 @@ namespace lws
               goffset += offset;
               if (user.has_spendable(db::output_id{in_data->amount, goffset}))
               {
-                user.add_spend(
+                spend_action(
+                  user,
                   db::spend{
                     db::transaction_link{height, tx_hash},
                     in_data->k_image,
@@ -377,7 +478,8 @@ namespace lws
             }
           }
 
-          const bool added = user.add_out(
+          const bool added = output_action(
+            user,
             db::output{
               db::transaction_link{height, tx_hash},
               db::output::spend_meta_{
@@ -405,6 +507,39 @@ namespace lws
       } // for all users
     }
 
+    void scan_transaction(
+      epee::span<lws::account> users,
+      const db::block_id height,
+      const std::uint64_t timestamp,
+      crypto::hash const& tx_hash,
+      cryptonote::transaction const& tx,
+      std::vector<std::uint64_t> const& out_ids)
+    {
+      scan_transaction_base(users, height, timestamp, tx_hash, tx, out_ids, add_spend{}, add_output{});
+    }
+
+    void scan_transactions(std::string&& txpool_msg, epee::span<lws::account> users, db::storage const& disk, rpc::client& client, const net::ssl_verification_t verify_mode)
+    {
+      // uint64::max is for txpool
+      static const std::vector<std::uint64_t> fake_outs(
+        256, std::numeric_limits<std::uint64_t>::max()
+      );
+
+      const auto parsed = rpc::full_txpool_pub::from_json(std::move(txpool_msg));
+      if (!parsed)
+      {
+        MERROR("Failed parsing txpool pub: " << parsed.error().message());
+        return;
+      }
+
+      const auto time =
+        boost::numeric_cast<std::uint64_t>(std::chrono::system_clock::to_time_t(std::chrono::system_clock::now()));
+
+      send_webhook sender{disk, client, verify_mode};
+      for (const auto& tx : parsed->txes)
+        scan_transaction_base(users, db::block_id::txpool, time, crypto::hash{}, tx, fake_outs, null_spend{}, sender);
+    }
+
     void update_rates(rpc::context& ctx)
     {
       const expect<boost::optional<lws::rates>> new_rates = ctx.retrieve_rates();
@@ -489,20 +624,40 @@ namespace lws
           if (fetched.result.blocks.size() <= 1)
           {
             // synced to top of chain, wait for next blocks
-            for (;;)
+            for (bool wait_for_block = true; wait_for_block; )
             {
-              const expect<rpc::minimal_chain_pub> new_block = client.wait_for_block();
-              if (new_block.matches(std::errc::interrupted))
-                return;
-              if (!new_block || is_new_block(disk, users.front(), *new_block))
-                break;
-            }
+              expect<std::vector<std::pair<rpc::client::topic, std::string>>> new_pubs = client.wait_for_block();
+              if (new_pubs.matches(std::errc::interrupted))
+                return; // reset entire state (maybe shutdown)
+
+              if (!new_pubs)
+                break; // exit wait for block loop, and try fetching new blocks
+
+              // put txpool messages before block messages
+              static_assert(rpc::client::topic::block < rpc::client::topic::txpool, "bad sort");
+              std::sort(new_pubs->begin(), new_pubs->end(), std::greater<>{});
+
+              // process txpool first
+              auto message = new_pubs->begin();
+              for ( ; message != new_pubs->end(); ++message)
+              {
+                if (message->first != rpc::client::topic::txpool)
+                  break; // inner for loop
+                scan_transactions(std::move(message->second), epee::to_mut_span(users), disk, client, webhook_verify);
+              }
+
+              for ( ; message != new_pubs->end(); ++message)
+              {
+                if (message->first == rpc::client::topic::block && is_new_block(std::move(message->second), disk, users.front()))
+                  wait_for_block = false;
+              }
+            } // wait for block
 
             // request next chunk of blocks
             if (!send(client, block_request.clone()))
               return;
             continue; // to next get_blocks_fast read
-          }
+          } // if only one block was fetched
 
           // request next chunk of blocks
           if (!send(client, block_request.clone()))

src/wire/wrapper/defaulted.h

@@ -0,0 +1,78 @@
+// Copyright (c) 2021-2023, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <functional>
+#include <utility>
+
+#include "wire/field.h"
+#include "wire/traits.h"
+
+//! An optional field that is omitted when a default value is used
+#define WIRE_FIELD_DEFAULTED(name, default_)                            \
+  ::wire::optional_field( #name , ::wire::defaulted(std::ref( self . name ), default_ ))
+
+namespace wire
+{
+  /*! A wrapper that tells `wire::writer`s to skip field generation when default
+    value, and tells `wire::reader`s to use default value when field not present. */
+  template<typename T, typename U>
+  struct defaulted_
+  {
+    using value_type = typename unwrap_reference<T>::type;
+
+    T value;
+    U default_;
+
+    constexpr const value_type& get_value() const noexcept { return value; }
+    value_type& get_value() noexcept { return value; }
+
+    // concept requirements for optional fields
+
+    constexpr explicit operator bool() const { return get_value() != default_; }
+    value_type& emplace() noexcept { return get_value(); }
+
+    constexpr const value_type& operator*() const noexcept { return get_value(); }
+    value_type& operator*() noexcept { return get_value(); }
+
+    void reset() { get_value() = default_; }
+  };
+
+  //! Links `value` with `default_`.
+  template<typename T, typename U>
+  inline constexpr defaulted_<T, U> defaulted(T value, U default_)
+  {
+    return {std::move(value), std::move(default_)};
+  }
+
+  /* read/write functions not needed since `defaulted_` meets the concept
+     requirements for an optional type (optional fields are handled
+     directly by the generic read/write code because the field name is omitted
+     entirely when the value is "empty"). */
+} // wire
+