aos/events/logging/logger.cc

#include "aos/events/logging/logger.h"

#include <fcntl.h>
#include <limits.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <vector>

#include "absl/strings/escaping.h"
#include "absl/types/span.h"
#include "aos/events/event_loop.h"
#include "aos/events/logging/logfile_sorting.h"
#include "aos/events/logging/logger_generated.h"
#include "aos/events/logging/uuid.h"
#include "aos/flatbuffer_merge.h"
#include "aos/network/multinode_timestamp_filter.h"
#include "aos/network/remote_message_generated.h"
#include "aos/network/remote_message_schema.h"
#include "aos/network/team_number.h"
#include "aos/time/time.h"
#include "aos/util/file.h"
#include "flatbuffers/flatbuffers.h"
#include "openssl/sha.h"

DEFINE_bool(skip_missing_forwarding_entries, false,
            "If true, drop any forwarding entries with missing data.  If "
            "false, CHECK.");

DECLARE_bool(timestamps_to_csv);

DEFINE_bool(skip_order_validation, false,
            "If true, ignore any out of orderness in replay");

DEFINE_double(
    time_estimation_buffer_seconds, 2.0,
    "The time to buffer ahead in the log file to accurately reconstruct time.");

namespace aos {
namespace logger {
namespace {
std::string Sha256(const absl::Span<const uint8_t> str) {
  unsigned char hash[SHA256_DIGEST_LENGTH];
  SHA256_CTX sha256;
  SHA256_Init(&sha256);
  SHA256_Update(&sha256, str.data(), str.size());
  SHA256_Final(hash, &sha256);
  std::stringstream ss;
  for (int i = 0; i < SHA256_DIGEST_LENGTH; i++) {
    ss << std::hex << std::setw(2) << std::setfill('0') << (int)hash[i];
  }
  return ss.str();
}

std::string LogFileVectorToString(std::vector<LogFile> log_files) {
  std::stringstream ss;
  for (const auto &f : log_files) {
    ss << f << "\n";
  }
  return ss.str();
}

// Copies the channel, removing the schema as we go.  If new_name is provided,
// it is used instead of the name inside the channel.  If new_type is provided,
// it is used instead of the type in the channel.
flatbuffers::Offset<Channel> CopyChannel(const Channel *c,
                                         std::string_view new_name,
                                         std::string_view new_type,
                                         flatbuffers::FlatBufferBuilder *fbb) {
  flatbuffers::Offset<flatbuffers::String> name_offset =
      fbb->CreateSharedString(new_name.empty() ? c->name()->string_view()
                                               : new_name);
  flatbuffers::Offset<flatbuffers::String> type_offset =
      fbb->CreateSharedString(new_type.empty() ? c->type()->str() : new_type);
  flatbuffers::Offset<flatbuffers::String> source_node_offset =
      c->has_source_node() ? fbb->CreateSharedString(c->source_node()->str())
                           : 0;

  flatbuffers::Offset<flatbuffers::Vector<flatbuffers::Offset<Connection>>>
      destination_nodes_offset =
          aos::RecursiveCopyVectorTable(c->destination_nodes(), fbb);

  flatbuffers::Offset<
      flatbuffers::Vector<flatbuffers::Offset<flatbuffers::String>>>
      logger_nodes_offset = aos::CopyVectorSharedString(c->logger_nodes(), fbb);

  Channel::Builder channel_builder(*fbb);
  channel_builder.add_name(name_offset);
  channel_builder.add_type(type_offset);
  if (c->has_frequency()) {
    channel_builder.add_frequency(c->frequency());
  }
  if (c->has_max_size()) {
    channel_builder.add_max_size(c->max_size());
  }
  if (c->has_num_senders()) {
    channel_builder.add_num_senders(c->num_senders());
  }
  if (c->has_num_watchers()) {
    channel_builder.add_num_watchers(c->num_watchers());
  }
  if (!source_node_offset.IsNull()) {
    channel_builder.add_source_node(source_node_offset);
  }
  if (!destination_nodes_offset.IsNull()) {
    channel_builder.add_destination_nodes(destination_nodes_offset);
  }
  if (c->has_logger()) {
    channel_builder.add_logger(c->logger());
  }
  if (!logger_nodes_offset.IsNull()) {
    channel_builder.add_logger_nodes(logger_nodes_offset);
  }
  if (c->has_read_method()) {
    channel_builder.add_read_method(c->read_method());
  }
  if (c->has_num_readers()) {
    channel_builder.add_num_readers(c->num_readers());
  }
  return channel_builder.Finish();
}

namespace chrono = std::chrono;
using message_bridge::RemoteMessage;
}  // namespace

Logger::Logger(EventLoop *event_loop, const Configuration *configuration,
               std::function<bool(const Channel *)> should_log)
    : event_loop_(event_loop),
      configuration_(configuration),
      name_(network::GetHostname()),
      timer_handler_(event_loop_->AddTimer(
          [this]() { DoLogData(event_loop_->monotonic_now()); })),
      server_statistics_fetcher_(
          configuration::MultiNode(event_loop_->configuration())
              ? event_loop_->MakeFetcher<message_bridge::ServerStatistics>(
                    "/aos")
              : aos::Fetcher<message_bridge::ServerStatistics>()) {
  VLOG(1) << "Creating logger for " << FlatbufferToJson(event_loop_->node());

  // Find all the nodes which are logging timestamps on our node.  This may
  // over-estimate if should_log is specified.
  std::vector<const Node *> timestamp_logger_nodes =
      configuration::TimestampNodes(configuration_, event_loop_->node());

  std::map<const Channel *, const Node *> timestamp_logger_channels;

  // Now that we have all the nodes accumulated, make remote timestamp loggers
  // for them.
  for (const Node *node : timestamp_logger_nodes) {
    // Note: since we are doing a find using the event loop channel, we need to
    // make sure this channel pointer is part of the event loop configuration,
    // not configuration_.  This only matters when configuration_ !=
    // event_loop->configuration();
    const Channel *channel = configuration::GetChannel(
        event_loop->configuration(),
        absl::StrCat("/aos/remote_timestamps/", node->name()->string_view()),
        RemoteMessage::GetFullyQualifiedName(), event_loop_->name(),
        event_loop_->node());

    CHECK(channel != nullptr)
        << ": Remote timestamps are logged on "
        << event_loop_->node()->name()->string_view()
        << " but can't find channel /aos/remote_timestamps/"
        << node->name()->string_view();
    if (!should_log(channel)) {
      continue;
    }
    timestamp_logger_channels.insert(std::make_pair(channel, node));
  }

  const size_t our_node_index =
      configuration::GetNodeIndex(configuration_, event_loop_->node());

  for (size_t channel_index = 0;
       channel_index < configuration_->channels()->size(); ++channel_index) {
    const Channel *const config_channel =
        configuration_->channels()->Get(channel_index);
    // The MakeRawFetcher method needs a channel which is in the event loop
    // configuration() object, not the configuration_ object.  Go look that up
    // from the config.
    const Channel *channel = aos::configuration::GetChannel(
        event_loop_->configuration(), config_channel->name()->string_view(),
        config_channel->type()->string_view(), "", event_loop_->node());
    CHECK(channel != nullptr)
        << ": Failed to look up channel "
        << aos::configuration::CleanedChannelToString(config_channel);
    if (!should_log(channel)) {
      continue;
    }

    FetcherStruct fs;
    fs.channel_index = channel_index;
    fs.channel = channel;

    const bool is_local =
        configuration::ChannelIsSendableOnNode(channel, event_loop_->node());

    const bool is_readable =
        configuration::ChannelIsReadableOnNode(channel, event_loop_->node());
    const bool is_logged = configuration::ChannelMessageIsLoggedOnNode(
        channel, event_loop_->node());
    const bool log_message = is_logged && is_readable;

    bool log_delivery_times = false;
    if (event_loop_->node() != nullptr) {
      log_delivery_times = configuration::ConnectionDeliveryTimeIsLoggedOnNode(
          channel, event_loop_->node(), event_loop_->node());
    }

    // Now, detect a RemoteMessage timestamp logger where we should just log the
    // contents to a file directly.
    const bool log_contents = timestamp_logger_channels.find(channel) !=
                              timestamp_logger_channels.end();

    if (log_message || log_delivery_times || log_contents) {
      fs.fetcher = event_loop->MakeRawFetcher(channel);
      VLOG(1) << "Logging channel "
              << configuration::CleanedChannelToString(channel);

      if (log_delivery_times) {
        VLOG(1) << "  Delivery times";
        fs.wants_timestamp_writer = true;
        fs.timestamp_node_index = our_node_index;
      }
      if (log_message) {
        VLOG(1) << "  Data";
        fs.wants_writer = true;
        if (!is_local) {
          const Node *source_node = configuration::GetNode(
              configuration_, channel->source_node()->string_view());
          fs.data_node_index =
              configuration::GetNodeIndex(configuration_, source_node);
          fs.log_type = LogType::kLogRemoteMessage;
        } else {
          fs.data_node_index = our_node_index;
        }
      }
      if (log_contents) {
        VLOG(1) << "Timestamp logger channel "
                << configuration::CleanedChannelToString(channel);
        fs.timestamp_node = timestamp_logger_channels.find(channel)->second;
        fs.wants_contents_writer = true;
        fs.contents_node_index =
            configuration::GetNodeIndex(configuration_, fs.timestamp_node);
      }
      fetchers_.emplace_back(std::move(fs));
    }
  }

  // When we are logging remote timestamps, we need to be able to translate from
  // the channel index that the event loop uses to the channel index in the
  // config in the log file.
  event_loop_to_logged_channel_index_.resize(
      event_loop->configuration()->channels()->size(), -1);
  for (size_t event_loop_channel_index = 0;
       event_loop_channel_index <
       event_loop->configuration()->channels()->size();
       ++event_loop_channel_index) {
    const Channel *event_loop_channel =
        event_loop->configuration()->channels()->Get(event_loop_channel_index);

    const Channel *logged_channel = aos::configuration::GetChannel(
        configuration_, event_loop_channel->name()->string_view(),
        event_loop_channel->type()->string_view(), "",
        configuration::GetNode(configuration_, event_loop_->node()));

    if (logged_channel != nullptr) {
      event_loop_to_logged_channel_index_[event_loop_channel_index] =
          configuration::ChannelIndex(configuration_, logged_channel);
    }
  }
}

Logger::~Logger() {
  if (log_namer_) {
    // If we are replaying a log file, or in simulation, we want to force the
    // last bit of data to be logged.  The easiest way to deal with this is to
    // poll everything as we go to destroy the class, ie, shut down the logger,
    // and write it to disk.
    StopLogging(event_loop_->monotonic_now());
  }
}

void Logger::StartLogging(std::unique_ptr<LogNamer> log_namer,
                          std::string_view log_start_uuid) {
  CHECK(!log_namer_) << ": Already logging";
  log_namer_ = std::move(log_namer);

  std::string config_sha256;
  if (separate_config_) {
    flatbuffers::FlatBufferBuilder fbb;
    flatbuffers::Offset<aos::Configuration> configuration_offset =
        CopyFlatBuffer(configuration_, &fbb);
    LogFileHeader::Builder log_file_header_builder(fbb);
    log_file_header_builder.add_configuration(configuration_offset);
    fbb.FinishSizePrefixed(log_file_header_builder.Finish());
    aos::SizePrefixedFlatbufferDetachedBuffer<LogFileHeader> config_header(
        fbb.Release());
    config_sha256 = Sha256(config_header.span());
    LOG(INFO) << "Config sha256 of " << config_sha256;
    log_namer_->WriteConfiguration(&config_header, config_sha256);
  }

  log_event_uuid_ = UUID::Random();
  log_start_uuid_ = log_start_uuid;
  VLOG(1) << "Starting logger for " << FlatbufferToJson(event_loop_->node());

  // We want to do as much work as possible before the initial Fetch. Time
  // between that and actually starting to log opens up the possibility of
  // falling off the end of the queue during that time.

  for (FetcherStruct &f : fetchers_) {
    if (f.wants_writer) {
      f.writer = log_namer_->MakeWriter(f.channel);
    }
    if (f.wants_timestamp_writer) {
      f.timestamp_writer = log_namer_->MakeTimestampWriter(f.channel);
    }
    if (f.wants_contents_writer) {
      f.contents_writer = log_namer_->MakeForwardedTimestampWriter(
          f.channel, CHECK_NOTNULL(f.timestamp_node));
    }
  }

  CHECK(node_state_.empty());
  node_state_.resize(configuration::MultiNode(configuration_)
                         ? configuration_->nodes()->size()
                         : 1u);

  for (const Node *node : log_namer_->nodes()) {
    const int node_index = configuration::GetNodeIndex(configuration_, node);

    node_state_[node_index].log_file_header = MakeHeader(node, config_sha256);
  }

  // Grab data from each channel right before we declare the log file started
  // so we can capture the latest message on each channel.  This lets us have
  // non periodic messages with configuration that now get logged.
  for (FetcherStruct &f : fetchers_) {
    const auto start = event_loop_->monotonic_now();
    const bool got_new = f.fetcher->Fetch();
    const auto end = event_loop_->monotonic_now();
    RecordFetchResult(start, end, got_new, &f);

    // If there is a message, we want to write it.
    f.written = f.fetcher->context().data == nullptr;
  }

  // Clear out any old timestamps in case we are re-starting logging.
  for (size_t i = 0; i < node_state_.size(); ++i) {
    SetStartTime(i, monotonic_clock::min_time, realtime_clock::min_time,
                 monotonic_clock::min_time, realtime_clock::min_time);
  }

  WriteHeader();

  LOG(INFO) << "Logging node as " << FlatbufferToJson(event_loop_->node())
            << " start_time " << last_synchronized_time_;

  // Force logging up until the start of the log file now, so the messages at
  // the start are always ordered before the rest of the messages.
  // Note: this ship may have already sailed, but we don't have to make it
  // worse.
  // TODO(austin): Test...
  LogUntil(last_synchronized_time_);

  timer_handler_->Setup(event_loop_->monotonic_now() + polling_period_,
                        polling_period_);
}

std::unique_ptr<LogNamer> Logger::StopLogging(
    aos::monotonic_clock::time_point end_time) {
  CHECK(log_namer_) << ": Not logging right now";

  if (end_time != aos::monotonic_clock::min_time) {
    LogUntil(end_time);
  }
  timer_handler_->Disable();

  for (FetcherStruct &f : fetchers_) {
    f.writer = nullptr;
    f.timestamp_writer = nullptr;
    f.contents_writer = nullptr;
  }
  node_state_.clear();

  log_event_uuid_ = UUID::Zero();
  log_start_uuid_ = std::string();

  return std::move(log_namer_);
}

void Logger::WriteHeader() {
  if (configuration::MultiNode(configuration_)) {
    server_statistics_fetcher_.Fetch();
  }

  aos::monotonic_clock::time_point monotonic_start_time =
      event_loop_->monotonic_now();
  aos::realtime_clock::time_point realtime_start_time =
      event_loop_->realtime_now();

  // We need to pick a point in time to declare the log file "started".  This
  // starts here.  It needs to be after everything is fetched so that the
  // fetchers are all pointed at the most recent message before the start
  // time.
  last_synchronized_time_ = monotonic_start_time;

  for (const Node *node : log_namer_->nodes()) {
    const int node_index = configuration::GetNodeIndex(configuration_, node);
    MaybeUpdateTimestamp(node, node_index, monotonic_start_time,
                         realtime_start_time);
    MaybeWriteHeader(node_index, node);
  }
}

void Logger::MaybeWriteHeader(int node_index) {
  if (configuration::MultiNode(configuration_)) {
    return MaybeWriteHeader(node_index,
                            configuration_->nodes()->Get(node_index));
  } else {
    return MaybeWriteHeader(node_index, nullptr);
  }
}

void Logger::MaybeWriteHeader(int node_index, const Node *node) {
  // This function is responsible for writing the header when the header both
  // has valid data, and when it needs to be written.
  if (node_state_[node_index].header_written &&
      node_state_[node_index].header_valid) {
    // The header has been written and is valid, nothing to do.
    return;
  }
  if (!node_state_[node_index].has_source_node_boot_uuid) {
    // Can't write a header if we don't have the boot UUID.
    return;
  }

  // WriteHeader writes the first header in a log file.  We want to do this only
  // once.
  //
  // Rotate rewrites the same header with a new part ID, but keeps the same part
  // UUID.  We don't want that when things reboot, because that implies that
  // parts go together across a reboot.
  //
  // Reboot resets the parts UUID.  So, once we've written a header the first
  // time, we want to use Reboot to rotate the log and reset the parts UUID.
  //
  // header_valid is cleared whenever the remote reboots.
  if (node_state_[node_index].header_written) {
    log_namer_->Reboot(node, &node_state_[node_index].log_file_header);
  } else {
    log_namer_->WriteHeader(&node_state_[node_index].log_file_header, node);

    node_state_[node_index].header_written = true;
  }
  node_state_[node_index].header_valid = true;
}

void Logger::WriteMissingTimestamps() {
  if (configuration::MultiNode(configuration_)) {
    server_statistics_fetcher_.Fetch();
  } else {
    return;
  }

  if (server_statistics_fetcher_.get() == nullptr) {
    return;
  }

  for (const Node *node : log_namer_->nodes()) {
    const int node_index = configuration::GetNodeIndex(configuration_, node);
    if (MaybeUpdateTimestamp(
            node, node_index,
            server_statistics_fetcher_.context().monotonic_event_time,
            server_statistics_fetcher_.context().realtime_event_time)) {
      CHECK(node_state_[node_index].header_written);
      CHECK(node_state_[node_index].header_valid);
      log_namer_->Rotate(node, &node_state_[node_index].log_file_header);
    } else {
      MaybeWriteHeader(node_index, node);
    }
  }
}

void Logger::SetStartTime(
    size_t node_index, aos::monotonic_clock::time_point monotonic_start_time,
    aos::realtime_clock::time_point realtime_start_time,
    aos::monotonic_clock::time_point logger_monotonic_start_time,
    aos::realtime_clock::time_point logger_realtime_start_time) {
  node_state_[node_index].monotonic_start_time = monotonic_start_time;
  node_state_[node_index].realtime_start_time = realtime_start_time;
  node_state_[node_index]
      .log_file_header.mutable_message()
      ->mutate_monotonic_start_time(
          std::chrono::duration_cast<std::chrono::nanoseconds>(
              monotonic_start_time.time_since_epoch())
              .count());

  // Add logger start times if they are available in the log file header.
  if (node_state_[node_index]
          .log_file_header.mutable_message()
          ->has_logger_monotonic_start_time()) {
    node_state_[node_index]
        .log_file_header.mutable_message()
        ->mutate_logger_monotonic_start_time(
            std::chrono::duration_cast<std::chrono::nanoseconds>(
                logger_monotonic_start_time.time_since_epoch())
                .count());
  }

  if (node_state_[node_index]
          .log_file_header.mutable_message()
          ->has_logger_realtime_start_time()) {
    node_state_[node_index]
        .log_file_header.mutable_message()
        ->mutate_logger_realtime_start_time(
            std::chrono::duration_cast<std::chrono::nanoseconds>(
                logger_realtime_start_time.time_since_epoch())
                .count());
  }

  if (node_state_[node_index]
          .log_file_header.mutable_message()
          ->has_realtime_start_time()) {
    node_state_[node_index]
        .log_file_header.mutable_message()
        ->mutate_realtime_start_time(
            std::chrono::duration_cast<std::chrono::nanoseconds>(
                realtime_start_time.time_since_epoch())
                .count());
  }
}

bool Logger::MaybeUpdateTimestamp(
    const Node *node, int node_index,
    aos::monotonic_clock::time_point monotonic_start_time,
    aos::realtime_clock::time_point realtime_start_time) {
  // Bail early if the start times are already set.
  if (node_state_[node_index].monotonic_start_time !=
      monotonic_clock::min_time) {
    return false;
  }
  if (event_loop_->node() == node ||
      !configuration::MultiNode(configuration_)) {
    // There are no offsets to compute for ourself, so always succeed.
    SetStartTime(node_index, monotonic_start_time, realtime_start_time,
                 monotonic_start_time, realtime_start_time);
    node_state_[node_index].SetBootUUID(event_loop_->boot_uuid().string_view());
    return true;
  } else if (server_statistics_fetcher_.get() != nullptr) {
    // We must be a remote node now.  Look for the connection and see if it is
    // connected.

    for (const message_bridge::ServerConnection *connection :
         *server_statistics_fetcher_->connections()) {
      if (connection->node()->name()->string_view() !=
          node->name()->string_view()) {
        continue;
      }

      if (connection->state() != message_bridge::State::CONNECTED) {
        VLOG(1) << node->name()->string_view()
                << " is not connected, can't start it yet.";
        break;
      }

      // Update the boot UUID as soon as we know we are connected.
      if (!connection->has_boot_uuid()) {
        VLOG(1) << "Missing boot_uuid for node " << aos::FlatbufferToJson(node);
        break;
      }

      if (!node_state_[node_index].has_source_node_boot_uuid ||
          node_state_[node_index].source_node_boot_uuid !=
              connection->boot_uuid()->string_view()) {
        node_state_[node_index].SetBootUUID(
            connection->boot_uuid()->string_view());
      }

      if (!connection->has_monotonic_offset()) {
        VLOG(1) << "Missing monotonic offset for setting start time for node "
                << aos::FlatbufferToJson(node);
        break;
      }

      // Found it and it is connected.  Compensate and go.
      SetStartTime(node_index,
                   monotonic_start_time +
                       std::chrono::nanoseconds(connection->monotonic_offset()),
                   realtime_start_time, monotonic_start_time,
                   realtime_start_time);
      return true;
    }
  }
  return false;
}

aos::SizePrefixedFlatbufferDetachedBuffer<LogFileHeader> Logger::MakeHeader(
    const Node *node, std::string_view config_sha256) {
  // Now write the header with this timestamp in it.
  flatbuffers::FlatBufferBuilder fbb;
  fbb.ForceDefaults(true);

  flatbuffers::Offset<aos::Configuration> configuration_offset;
  if (!separate_config_) {
    configuration_offset = CopyFlatBuffer(configuration_, &fbb);
  } else {
    CHECK(!config_sha256.empty());
  }

  const flatbuffers::Offset<flatbuffers::String> name_offset =
      fbb.CreateString(name_);

  CHECK(log_event_uuid_ != UUID::Zero());
  const flatbuffers::Offset<flatbuffers::String> log_event_uuid_offset =
      fbb.CreateString(log_event_uuid_.string_view());

  const flatbuffers::Offset<flatbuffers::String> logger_instance_uuid_offset =
      fbb.CreateString(logger_instance_uuid_.string_view());

  flatbuffers::Offset<flatbuffers::String> log_start_uuid_offset;
  if (!log_start_uuid_.empty()) {
    log_start_uuid_offset = fbb.CreateString(log_start_uuid_);
  }

  flatbuffers::Offset<flatbuffers::String> config_sha256_offset;
  if (!config_sha256.empty()) {
    config_sha256_offset = fbb.CreateString(config_sha256);
  }

  const flatbuffers::Offset<flatbuffers::String> logger_node_boot_uuid_offset =
      fbb.CreateString(event_loop_->boot_uuid().string_view());

  const flatbuffers::Offset<flatbuffers::String> source_node_boot_uuid_offset =
      fbb.CreateString(event_loop_->boot_uuid().string_view());

  const flatbuffers::Offset<flatbuffers::String> parts_uuid_offset =
      fbb.CreateString("00000000-0000-4000-8000-000000000000");

  flatbuffers::Offset<Node> node_offset;
  flatbuffers::Offset<Node> logger_node_offset;

  if (configuration::MultiNode(configuration_)) {
    node_offset = RecursiveCopyFlatBuffer(node, &fbb);
    logger_node_offset = RecursiveCopyFlatBuffer(event_loop_->node(), &fbb);
  }

  aos::logger::LogFileHeader::Builder log_file_header_builder(fbb);

  log_file_header_builder.add_name(name_offset);

  // Only add the node if we are running in a multinode configuration.
  if (node != nullptr) {
    log_file_header_builder.add_node(node_offset);
    log_file_header_builder.add_logger_node(logger_node_offset);
  }

  if (!configuration_offset.IsNull()) {
    log_file_header_builder.add_configuration(configuration_offset);
  }
  // The worst case theoretical out of order is the polling period times 2.
  // One message could get logged right after the boundary, but be for right
  // before the next boundary.  And the reverse could happen for another
  // message.  Report back 3x to be extra safe, and because the cost isn't
  // huge on the read side.
  log_file_header_builder.add_max_out_of_order_duration(
      std::chrono::nanoseconds(3 * polling_period_).count());

  log_file_header_builder.add_monotonic_start_time(
      std::chrono::duration_cast<std::chrono::nanoseconds>(
          monotonic_clock::min_time.time_since_epoch())
          .count());
  if (node == event_loop_->node()) {
    log_file_header_builder.add_realtime_start_time(
        std::chrono::duration_cast<std::chrono::nanoseconds>(
            realtime_clock::min_time.time_since_epoch())
            .count());
  } else {
    log_file_header_builder.add_logger_monotonic_start_time(
        std::chrono::duration_cast<std::chrono::nanoseconds>(
            monotonic_clock::min_time.time_since_epoch())
            .count());
    log_file_header_builder.add_logger_realtime_start_time(
        std::chrono::duration_cast<std::chrono::nanoseconds>(
            realtime_clock::min_time.time_since_epoch())
            .count());
  }

  log_file_header_builder.add_log_event_uuid(log_event_uuid_offset);
  log_file_header_builder.add_logger_instance_uuid(logger_instance_uuid_offset);
  if (!log_start_uuid_offset.IsNull()) {
    log_file_header_builder.add_log_start_uuid(log_start_uuid_offset);
  }
  log_file_header_builder.add_logger_node_boot_uuid(
      logger_node_boot_uuid_offset);
  log_file_header_builder.add_source_node_boot_uuid(
      source_node_boot_uuid_offset);

  log_file_header_builder.add_parts_uuid(parts_uuid_offset);
  log_file_header_builder.add_parts_index(0);

  log_file_header_builder.add_configuration_sha256(0);

  if (!config_sha256_offset.IsNull()) {
    log_file_header_builder.add_configuration_sha256(config_sha256_offset);
  }

  fbb.FinishSizePrefixed(log_file_header_builder.Finish());
  aos::SizePrefixedFlatbufferDetachedBuffer<LogFileHeader> result(
      fbb.Release());

  CHECK(result.Verify()) << ": Built a corrupted header.";

  return result;
}

void Logger::ResetStatisics() {
  max_message_fetch_time_ = std::chrono::nanoseconds::zero();
  max_message_fetch_time_channel_ = -1;
  max_message_fetch_time_size_ = -1;
  total_message_fetch_time_ = std::chrono::nanoseconds::zero();
  total_message_fetch_count_ = 0;
  total_message_fetch_bytes_ = 0;
  total_nop_fetch_time_ = std::chrono::nanoseconds::zero();
  total_nop_fetch_count_ = 0;
  max_copy_time_ = std::chrono::nanoseconds::zero();
  max_copy_time_channel_ = -1;
  max_copy_time_size_ = -1;
  total_copy_time_ = std::chrono::nanoseconds::zero();
  total_copy_count_ = 0;
  total_copy_bytes_ = 0;
}

void Logger::Rotate() {
  for (const Node *node : log_namer_->nodes()) {
    const int node_index = configuration::GetNodeIndex(configuration_, node);
    log_namer_->Rotate(node, &node_state_[node_index].log_file_header);
  }
}

void Logger::LogUntil(monotonic_clock::time_point t) {
  // Grab the latest ServerStatistics message.  This will always have the
  // oppertunity to be >= to the current time, so it will always represent any
  // reboots which may have happened.
  WriteMissingTimestamps();

  // Write each channel to disk, one at a time.
  for (FetcherStruct &f : fetchers_) {
    while (true) {
      if (f.written) {
        const auto start = event_loop_->monotonic_now();
        const bool got_new = f.fetcher->FetchNext();
        const auto end = event_loop_->monotonic_now();
        RecordFetchResult(start, end, got_new, &f);
        if (!got_new) {
          VLOG(2) << "No new data on "
                  << configuration::CleanedChannelToString(
                         f.fetcher->channel());
          break;
        }
        f.written = false;
      }

      // TODO(james): Write tests to exercise this logic.
      if (f.fetcher->context().monotonic_event_time >= t) {
        break;
      }
      if (f.writer != nullptr) {
        // Write!
        const auto start = event_loop_->monotonic_now();
        flatbuffers::FlatBufferBuilder fbb(f.fetcher->context().size +
                                           max_header_size_);
        fbb.ForceDefaults(true);

        fbb.FinishSizePrefixed(PackMessage(&fbb, f.fetcher->context(),
                                           f.channel_index, f.log_type));
        const auto end = event_loop_->monotonic_now();
        RecordCreateMessageTime(start, end, &f);

        VLOG(2) << "Writing data as node "
                << FlatbufferToJson(event_loop_->node()) << " for channel "
                << configuration::CleanedChannelToString(f.fetcher->channel())
                << " to " << f.writer->filename() << " data "
                << FlatbufferToJson(
                       flatbuffers::GetSizePrefixedRoot<MessageHeader>(
                           fbb.GetBufferPointer()));

        max_header_size_ = std::max(max_header_size_,
                                    fbb.GetSize() - f.fetcher->context().size);
        CHECK(node_state_[f.data_node_index].header_valid)
            << ": Can't write data before the header on channel "
            << configuration::CleanedChannelToString(f.fetcher->channel());
        f.writer->QueueSizedFlatbuffer(&fbb);
      }

      if (f.timestamp_writer != nullptr) {
        // And now handle timestamps.
        const auto start = event_loop_->monotonic_now();
        flatbuffers::FlatBufferBuilder fbb;
        fbb.ForceDefaults(true);

        fbb.FinishSizePrefixed(PackMessage(&fbb, f.fetcher->context(),
                                           f.channel_index,
                                           LogType::kLogDeliveryTimeOnly));
        const auto end = event_loop_->monotonic_now();
        RecordCreateMessageTime(start, end, &f);

        VLOG(2) << "Writing timestamps as node "
                << FlatbufferToJson(event_loop_->node()) << " for channel "
                << configuration::CleanedChannelToString(f.fetcher->channel())
                << " to " << f.timestamp_writer->filename() << " timestamp "
                << FlatbufferToJson(
                       flatbuffers::GetSizePrefixedRoot<MessageHeader>(
                           fbb.GetBufferPointer()));

        CHECK(node_state_[f.timestamp_node_index].header_valid)
            << ": Can't write data before the header on channel "
            << configuration::CleanedChannelToString(f.fetcher->channel());
        f.timestamp_writer->QueueSizedFlatbuffer(&fbb);
      }

      if (f.contents_writer != nullptr) {
        const auto start = event_loop_->monotonic_now();
        // And now handle the special message contents channel.  Copy the
        // message into a FlatBufferBuilder and save it to disk.
        // TODO(austin): We can be more efficient here when we start to
        // care...
        flatbuffers::FlatBufferBuilder fbb;
        fbb.ForceDefaults(true);

        const RemoteMessage *msg =
            flatbuffers::GetRoot<RemoteMessage>(f.fetcher->context().data);

        CHECK(msg->has_boot_uuid()) << ": " << aos::FlatbufferToJson(msg);
        if (!node_state_[f.contents_node_index].has_source_node_boot_uuid ||
            node_state_[f.contents_node_index].source_node_boot_uuid !=
                msg->boot_uuid()->string_view()) {
          node_state_[f.contents_node_index].SetBootUUID(
              msg->boot_uuid()->string_view());

          MaybeWriteHeader(f.contents_node_index);
        }

        logger::MessageHeader::Builder message_header_builder(fbb);

        // TODO(austin): This needs to check the channel_index and confirm
        // that it should be logged before squirreling away the timestamp to
        // disk.  We don't want to log irrelevant timestamps.

        // Note: this must match the same order as MessageBridgeServer and
        // PackMessage.  We want identical headers to have identical
        // on-the-wire formats to make comparing them easier.

        // Translate from the channel index that the event loop uses to the
        // channel index in the log file.
        message_header_builder.add_channel_index(
            event_loop_to_logged_channel_index_[msg->channel_index()]);

        message_header_builder.add_queue_index(msg->queue_index());
        message_header_builder.add_monotonic_sent_time(
            msg->monotonic_sent_time());
        message_header_builder.add_realtime_sent_time(
            msg->realtime_sent_time());

        message_header_builder.add_monotonic_remote_time(
            msg->monotonic_remote_time());
        message_header_builder.add_realtime_remote_time(
            msg->realtime_remote_time());
        message_header_builder.add_remote_queue_index(
            msg->remote_queue_index());

        message_header_builder.add_monotonic_timestamp_time(
            f.fetcher->context()
                .monotonic_event_time.time_since_epoch()
                .count());

        fbb.FinishSizePrefixed(message_header_builder.Finish());
        const auto end = event_loop_->monotonic_now();
        RecordCreateMessageTime(start, end, &f);

        CHECK(node_state_[f.contents_node_index].header_valid)
            << ": Can't write data before the header on channel "
            << configuration::CleanedChannelToString(f.fetcher->channel());
        f.contents_writer->QueueSizedFlatbuffer(&fbb);
      }

      f.written = true;
    }
  }
  last_synchronized_time_ = t;
}

void Logger::DoLogData(const monotonic_clock::time_point end_time) {
  // We want to guarantee that messages aren't out of order by more than
  // max_out_of_order_duration.  To do this, we need sync points.  Every write
  // cycle should be a sync point.

  do {
    // Move the sync point up by at most polling_period.  This forces one sync
    // per iteration, even if it is small.
    LogUntil(std::min(last_synchronized_time_ + polling_period_, end_time));

    on_logged_period_();

    // If we missed cycles, we could be pretty far behind.  Spin until we are
    // caught up.
  } while (last_synchronized_time_ + polling_period_ < end_time);
}

void Logger::RecordFetchResult(aos::monotonic_clock::time_point start,
                               aos::monotonic_clock::time_point end,
                               bool got_new, FetcherStruct *fetcher) {
  const auto duration = end - start;
  if (!got_new) {
    ++total_nop_fetch_count_;
    total_nop_fetch_time_ += duration;
    return;
  }
  ++total_message_fetch_count_;
  total_message_fetch_bytes_ += fetcher->fetcher->context().size;
  total_message_fetch_time_ += duration;
  if (duration > max_message_fetch_time_) {
    max_message_fetch_time_ = duration;
    max_message_fetch_time_channel_ = fetcher->channel_index;
    max_message_fetch_time_size_ = fetcher->fetcher->context().size;
  }
}

void Logger::RecordCreateMessageTime(aos::monotonic_clock::time_point start,
                                     aos::monotonic_clock::time_point end,
                                     FetcherStruct *fetcher) {
  const auto duration = end - start;
  total_copy_time_ += duration;
  ++total_copy_count_;
  total_copy_bytes_ += fetcher->fetcher->context().size;
  if (duration > max_copy_time_) {
    max_copy_time_ = duration;
    max_copy_time_channel_ = fetcher->channel_index;
    max_copy_time_size_ = fetcher->fetcher->context().size;
  }
}

std::vector<std::vector<std::string>> ToLogReaderVector(
    const std::vector<LogFile> &log_files) {
  std::vector<std::vector<std::string>> result;
  for (const LogFile &log_file : log_files) {
    for (const LogParts &log_parts : log_file.parts) {
      std::vector<std::string> parts;
      for (const std::string &part : log_parts.parts) {
        parts.emplace_back(part);
      }
      result.emplace_back(std::move(parts));
    }
  }
  return result;
}

LogReader::LogReader(std::string_view filename,
                     const Configuration *replay_configuration)
    : LogReader(SortParts({std::string(filename)}), replay_configuration) {}

LogReader::LogReader(std::vector<LogFile> log_files,
                     const Configuration *replay_configuration)
    : log_files_(std::move(log_files)),
      replay_configuration_(replay_configuration) {
  CHECK_GT(log_files_.size(), 0u);
  {
    // Validate that we have the same config everwhere.  This will be true if
    // all the parts were sorted together and the configs match.
    const Configuration *config = nullptr;
    for (const LogFile &log_file : log_files_) {
      if (log_file.config.get() == nullptr) {
        LOG(FATAL) << "Couldn't find a config in " << log_file;
      }
      if (config == nullptr) {
        config = log_file.config.get();
      } else {
        CHECK_EQ(config, log_file.config.get());
      }
    }
  }

  MakeRemappedConfig();

  // Remap all existing remote timestamp channels.  They will be recreated, and
  // the data logged isn't relevant anymore.
  for (const Node *node : configuration::GetNodes(logged_configuration())) {
    std::vector<const Node *> timestamp_logger_nodes =
        configuration::TimestampNodes(logged_configuration(), node);
    for (const Node *remote_node : timestamp_logger_nodes) {
      const std::string channel = absl::StrCat(
          "/aos/remote_timestamps/", remote_node->name()->string_view());
      // See if the log file is an old log with MessageHeader channels in it, or
      // a newer log with RemoteMessage.  If we find an older log, rename the
      // type too along with the name.
      if (HasChannel<MessageHeader>(channel, node)) {
        CHECK(!HasChannel<RemoteMessage>(channel, node))
            << ": Can't have both a MessageHeader and RemoteMessage remote "
               "timestamp channel.";
        // In theory, we should check NOT_LOGGED like RemoteMessage and be more
        // careful about updating the config, but there are fewer and fewer logs
        // with MessageHeader remote messages, so it isn't worth the effort.
        RemapLoggedChannel<MessageHeader>(channel, node, "/original",
                                          "aos.message_bridge.RemoteMessage");
      } else {
        CHECK(HasChannel<RemoteMessage>(channel, node))
            << ": Failed to find {\"name\": \"" << channel << "\", \"type\": \""
            << RemoteMessage::GetFullyQualifiedName() << "\"} for node "
            << node->name()->string_view();
        // Only bother to remap if there's something on the channel.  We can
        // tell if the channel was marked NOT_LOGGED or not.  This makes the
        // config not change un-necesarily when we replay a log with NOT_LOGGED
        // messages.
        if (HasLoggedChannel<RemoteMessage>(channel, node)) {
          RemapLoggedChannel<RemoteMessage>(channel, node);
        }
      }
    }
  }

  if (replay_configuration) {
    CHECK_EQ(configuration::MultiNode(configuration()),
             configuration::MultiNode(replay_configuration))
        << ": Log file and replay config need to both be multi or single "
           "node.";
  }

  if (!configuration::MultiNode(configuration())) {
    states_.emplace_back(std::make_unique<State>(
        std::make_unique<TimestampMapper>(FilterPartsForNode(log_files_, ""))));
  } else {
    if (replay_configuration) {
      CHECK_EQ(logged_configuration()->nodes()->size(),
               replay_configuration->nodes()->size())
          << ": Log file and replay config need to have matching nodes "
             "lists.";
      for (const Node *node : *logged_configuration()->nodes()) {
        if (configuration::GetNode(replay_configuration, node) == nullptr) {
          LOG(FATAL) << "Found node " << FlatbufferToJson(node)
                     << " in logged config that is not present in the replay "
                        "config.";
        }
      }
    }
    states_.resize(configuration()->nodes()->size());
  }
}

LogReader::~LogReader() {
  if (event_loop_factory_unique_ptr_) {
    Deregister();
  } else if (event_loop_factory_ != nullptr) {
    LOG(FATAL) << "Must call Deregister before the SimulatedEventLoopFactory "
                  "is destroyed";
  }
  // Zero out some buffers. It's easy to do use-after-frees on these, so make
  // it more obvious.
  if (remapped_configuration_buffer_) {
    remapped_configuration_buffer_->Wipe();
  }
}

const Configuration *LogReader::logged_configuration() const {
  return log_files_[0].config.get();
}

const Configuration *LogReader::configuration() const {
  return remapped_configuration_;
}

std::vector<const Node *> LogReader::LoggedNodes() const {
  return configuration::GetNodes(logged_configuration());
}

monotonic_clock::time_point LogReader::monotonic_start_time(
    const Node *node) const {
  State *state =
      states_[configuration::GetNodeIndex(configuration(), node)].get();
  CHECK(state != nullptr) << ": Unknown node " << FlatbufferToJson(node);

  return state->monotonic_start_time();
}

realtime_clock::time_point LogReader::realtime_start_time(
    const Node *node) const {
  State *state =
      states_[configuration::GetNodeIndex(configuration(), node)].get();
  CHECK(state != nullptr) << ": Unknown node " << FlatbufferToJson(node);

  return state->realtime_start_time();
}

void LogReader::Register() {
  event_loop_factory_unique_ptr_ =
      std::make_unique<SimulatedEventLoopFactory>(configuration());
  Register(event_loop_factory_unique_ptr_.get());
}

void LogReader::Register(SimulatedEventLoopFactory *event_loop_factory) {
  event_loop_factory_ = event_loop_factory;
  remapped_configuration_ = event_loop_factory_->configuration();
  filters_ =
      std::make_unique<message_bridge::MultiNodeNoncausalOffsetEstimator>(
          event_loop_factory_->configuration(), logged_configuration(),
          FLAGS_skip_order_validation,
          chrono::duration_cast<chrono::nanoseconds>(
              chrono::duration<double>(FLAGS_time_estimation_buffer_seconds)));

  for (const Node *node : configuration::GetNodes(configuration())) {
    const size_t node_index =
        configuration::GetNodeIndex(configuration(), node);
    std::vector<LogParts> filtered_parts = FilterPartsForNode(
        log_files_, node != nullptr ? node->name()->string_view() : "");

    // Confirm that all the parts are from the same boot if there are enough
    // parts to not be from the same boot.
    if (filtered_parts.size() > 1u) {
      for (size_t i = 1; i < filtered_parts.size(); ++i) {
        CHECK_EQ(filtered_parts[i].source_boot_uuid,
                 filtered_parts[0].source_boot_uuid)
            << ": Found parts from different boots "
            << LogFileVectorToString(log_files_);
      }
      if (!filtered_parts[0].source_boot_uuid.empty()) {
        event_loop_factory_->GetNodeEventLoopFactory(node)->set_boot_uuid(
            filtered_parts[0].source_boot_uuid);
      }
    }

    states_[node_index] = std::make_unique<State>(
        filtered_parts.size() == 0u
            ? nullptr
            : std::make_unique<TimestampMapper>(std::move(filtered_parts)));
    State *state = states_[node_index].get();
    state->set_event_loop(state->SetNodeEventLoopFactory(
        event_loop_factory_->GetNodeEventLoopFactory(node)));

    state->SetChannelCount(logged_configuration()->channels()->size());
  }
  event_loop_factory_->SetTimeConverter(filters_.get());

  for (const Node *node : configuration::GetNodes(configuration())) {
    const size_t node_index =
        configuration::GetNodeIndex(configuration(), node);
    State *state = states_[node_index].get();
    for (const Node *other_node : configuration::GetNodes(configuration())) {
      const size_t other_node_index =
          configuration::GetNodeIndex(configuration(), other_node);
      State *other_state = states_[other_node_index].get();
      if (other_state != state) {
        state->AddPeer(other_state);
      }
    }
  }

  // Register after making all the State objects so we can build references
  // between them.
  for (const Node *node : configuration::GetNodes(configuration())) {
    const size_t node_index =
        configuration::GetNodeIndex(configuration(), node);
    State *state = states_[node_index].get();

    Register(state->event_loop());
  }

  if (live_nodes_ == 0) {
    LOG(FATAL)
        << "Don't have logs from any of the nodes in the replay config--are "
           "you sure that the replay config matches the original config?";
  }

  filters_->CheckGraph();

  for (std::unique_ptr<State> &state : states_) {
    state->SeedSortedMessages();
  }

  // We want to start the log file at the last start time of the log files
  // from all the nodes.  Compute how long each node's simulation needs to run
  // to move time to this point.
  distributed_clock::time_point start_time = distributed_clock::min_time;

  // TODO(austin): We want an "OnStart" callback for each node rather than
  // running until the last node.

  for (std::unique_ptr<State> &state : states_) {
    VLOG(1) << "Start time is " << state->monotonic_start_time() << " for node "
            << MaybeNodeName(state->event_loop()->node()) << "now "
            << state->monotonic_now();
    if (state->monotonic_start_time() == monotonic_clock::min_time) {
      continue;
    }
    // And start computing the start time on the distributed clock now that
    // that works.
    start_time = std::max(
        start_time, state->ToDistributedClock(state->monotonic_start_time()));
  }

  // TODO(austin): If a node doesn't have a start time, we might not queue
  // enough.  If this happens, we'll explode with a frozen error eventually.

  CHECK_GE(start_time, distributed_clock::epoch())
      << ": Hmm, we have a node starting before the start of time.  Offset "
         "everything.";

  // Forwarding is tracked per channel.  If it is enabled, we want to turn it
  // off.  Otherwise messages replayed will get forwarded across to the other
  // nodes, and also replayed on the other nodes.  This may not satisfy all
  // our users, but it'll start the discussion.
  if (configuration::MultiNode(event_loop_factory_->configuration())) {
    for (size_t i = 0; i < logged_configuration()->channels()->size(); ++i) {
      const Channel *channel = logged_configuration()->channels()->Get(i);
      const Node *node = configuration::GetNode(
          configuration(), channel->source_node()->string_view());

      State *state =
          states_[configuration::GetNodeIndex(configuration(), node)].get();

      const Channel *remapped_channel =
          RemapChannel(state->event_loop(), channel);

      event_loop_factory_->DisableForwarding(remapped_channel);
    }

    // If we are replaying a log, we don't want a bunch of redundant messages
    // from both the real message bridge and simulated message bridge.
    event_loop_factory_->DisableStatistics();
  }

  // While we are starting the system up, we might be relying on matching data
  // to timestamps on log files where the timestamp log file starts before the
  // data.  In this case, it is reasonable to expect missing data.
  {
    const bool prior_ignore_missing_data = ignore_missing_data_;
    ignore_missing_data_ = true;
    VLOG(1) << "Running until " << start_time << " in Register";
    event_loop_factory_->RunFor(start_time.time_since_epoch());
    VLOG(1) << "At start time";
    // Now that we are running for real, missing data means that the log file is
    // corrupted or went wrong.
    ignore_missing_data_ = prior_ignore_missing_data;
  }

  for (std::unique_ptr<State> &state : states_) {
    // Make the RT clock be correct before handing it to the user.
    if (state->realtime_start_time() != realtime_clock::min_time) {
      state->SetRealtimeOffset(state->monotonic_start_time(),
                               state->realtime_start_time());
    }
    VLOG(1) << "Start time is " << state->monotonic_start_time() << " for node "
            << MaybeNodeName(state->event_loop()->node()) << "now "
            << state->monotonic_now();
  }

  if (FLAGS_timestamps_to_csv) {
    filters_->Start(event_loop_factory);
  }
}

message_bridge::NoncausalOffsetEstimator *LogReader::GetFilter(
    const Node *node_a, const Node *node_b) {
  if (filters_) {
    return filters_->GetFilter(node_a, node_b);
  }
  return nullptr;
}

void LogReader::Register(EventLoop *event_loop) {
  State *state =
      states_[configuration::GetNodeIndex(configuration(), event_loop->node())]
          .get();

  state->set_event_loop(event_loop);

  // We don't run timing reports when trying to print out logged data, because
  // otherwise we would end up printing out the timing reports themselves...
  // This is only really relevant when we are replaying into a simulation.
  event_loop->SkipTimingReport();
  event_loop->SkipAosLog();

  for (size_t logged_channel_index = 0;
       logged_channel_index < logged_configuration()->channels()->size();
       ++logged_channel_index) {
    const Channel *channel = RemapChannel(
        event_loop,
        logged_configuration()->channels()->Get(logged_channel_index));

    if (channel->logger() == LoggerConfig::NOT_LOGGED) {
      continue;
    }

    message_bridge::NoncausalOffsetEstimator *filter = nullptr;
    RemoteMessageSender *remote_timestamp_sender = nullptr;

    State *source_state = nullptr;

    if (!configuration::ChannelIsSendableOnNode(channel, event_loop->node()) &&
        configuration::ChannelIsReadableOnNode(channel, event_loop->node())) {
      // We've got a message which is being forwarded to this node.
      const Node *source_node = configuration::GetNode(
          event_loop->configuration(), channel->source_node()->string_view());
      filter = GetFilter(event_loop->node(), source_node);

      // Delivery timestamps are supposed to be logged back on the source node.
      // Configure remote timestamps to be sent.
      const bool delivery_time_is_logged =
          configuration::ConnectionDeliveryTimeIsLoggedOnNode(
              channel, event_loop->node(), source_node);

      source_state =
          states_[configuration::GetNodeIndex(configuration(), source_node)]
              .get();

      if (delivery_time_is_logged) {
        remote_timestamp_sender =
            source_state->RemoteTimestampSender(event_loop->node());
      }
    }

    state->SetChannel(
        logged_channel_index,
        configuration::ChannelIndex(event_loop->configuration(), channel),
        event_loop->MakeRawSender(channel), filter, remote_timestamp_sender,
        source_state);
  }

  // If we didn't find any log files with data in them, we won't ever get a
  // callback or be live.  So skip the rest of the setup.
  if (state->OldestMessageTime() == monotonic_clock::max_time) {
    return;
  }

  state->set_timer_handler(event_loop->AddTimer([this, state]() {
    VLOG(1) << "Starting sending " << MaybeNodeName(state->event_loop()->node())
            << "at " << state->event_loop()->context().monotonic_event_time
            << " now " << state->monotonic_now();
    if (state->OldestMessageTime() == monotonic_clock::max_time) {
      --live_nodes_;
      VLOG(1) << MaybeNodeName(state->event_loop()->node()) << "Node down!";
      if (exit_on_finish_ && live_nodes_ == 0) {
        event_loop_factory_->Exit();
      }
      return;
    }

    TimestampedMessage timestamped_message = state->PopOldest();

    const monotonic_clock::time_point monotonic_now =
        state->event_loop()->context().monotonic_event_time;
    if (!FLAGS_skip_order_validation) {
      CHECK(monotonic_now == timestamped_message.monotonic_event_time)
          << ": " << FlatbufferToJson(state->event_loop()->node()) << " Now "
          << monotonic_now << " trying to send "
          << timestamped_message.monotonic_event_time << " failure "
          << state->DebugString();
    } else if (monotonic_now != timestamped_message.monotonic_event_time) {
      LOG(WARNING) << "Check failed: monotonic_now == "
                      "timestamped_message.monotonic_event_time) ("
                   << monotonic_now << " vs. "
                   << timestamped_message.monotonic_event_time
                   << "): " << FlatbufferToJson(state->event_loop()->node())
                   << " Now " << monotonic_now << " trying to send "
                   << timestamped_message.monotonic_event_time << " failure "
                   << state->DebugString();
    }

    if (timestamped_message.monotonic_event_time >
            state->monotonic_start_time() ||
        event_loop_factory_ != nullptr) {
      if (timestamped_message.data.span().size() != 0u) {
        if (timestamped_message.monotonic_remote_time !=
            monotonic_clock::min_time) {
          // Confirm that the message was sent on the sending node before the
          // destination node (this node).  As a proxy, do this by making sure
          // that time on the source node is past when the message was sent.
          //
          // TODO(austin): <= means that the cause message (which we know) could
          // happen after the effect even though we know they are at the same
          // time.  I doubt anyone will notice for a bit, but we should really
          // fix that.
          if (!FLAGS_skip_order_validation) {
            CHECK_LE(
                timestamped_message.monotonic_remote_time,
                state->monotonic_remote_now(timestamped_message.channel_index))
                << state->event_loop()->node()->name()->string_view() << " to "
                << state->remote_node(timestamped_message.channel_index)
                       ->name()
                       ->string_view()
                << " while trying to send a message on "
                << configuration::CleanedChannelToString(
                       logged_configuration()->channels()->Get(
                           timestamped_message.channel_index))
                << " " << state->DebugString();
          } else if (timestamped_message.monotonic_remote_time >
                     state->monotonic_remote_now(
                         timestamped_message.channel_index)) {
            LOG(WARNING)
                << "Check failed: timestamped_message.monotonic_remote_time < "
                   "state->monotonic_remote_now(timestamped_message.channel_"
                   "index) ("
                << timestamped_message.monotonic_remote_time << " vs. "
                << state->monotonic_remote_now(
                       timestamped_message.channel_index)
                << ") " << state->event_loop()->node()->name()->string_view()
                << " to "
                << state->remote_node(timestamped_message.channel_index)
                       ->name()
                       ->string_view()
                << " currently " << timestamped_message.monotonic_event_time
                << " ("
                << state->ToDistributedClock(
                       timestamped_message.monotonic_event_time)
                << ") remote event time "
                << timestamped_message.monotonic_remote_time << " ("
                << state->RemoteToDistributedClock(
                       timestamped_message.channel_index,
                       timestamped_message.monotonic_remote_time)
                << ") " << state->DebugString();
          }
        }

        // If we have access to the factory, use it to fix the realtime time.
        state->SetRealtimeOffset(timestamped_message.monotonic_event_time,
                                 timestamped_message.realtime_event_time);

        VLOG(1) << MaybeNodeName(state->event_loop()->node()) << "Sending "
                << timestamped_message.monotonic_event_time;
        // TODO(austin): std::move channel_data in and make that efficient in
        // simulation.
        state->Send(std::move(timestamped_message));
      } else if (!ignore_missing_data_ &&
                 !FLAGS_skip_missing_forwarding_entries) {
        // We've found a timestamp without data.  This likely means that we are
        // at the end of the log file.  Record it and CHECK that in the rest of
        // the log file, we don't find any more data on that channel.  Not all
        // channels will end at the same point in time since they can be in
        // different files.
        VLOG(1) << "Found the last message on channel "
                << timestamped_message.channel_index;

        // Vector storing if we've seen a nullptr message or not per channel.
        std::vector<bool> last_message;
        last_message.resize(logged_configuration()->channels()->size(), false);

        last_message[timestamped_message.channel_index] = true;

        // Now that we found the end of one channel, artificially stop the
        // rest.  It is confusing when part of your data gets replayed but not
        // all.
        while (state->OldestMessageTime() != monotonic_clock::max_time) {
          TimestampedMessage next = state->PopOldest();
          // Make sure that once we have seen the last message on a channel,
          // data doesn't start back up again.  If the user wants to play
          // through events like this, they can set
          // --skip_missing_forwarding_entries or ignore_missing_data_.
          CHECK_LT(next.channel_index, last_message.size());
          if (next.data.span().size() == 0u) {
            last_message[next.channel_index] = true;
          } else {
            if (last_message[next.channel_index]) {
              LOG(FATAL)
                  << "Found missing data in the middle of the log file on "
                     "channel "
                  << next.channel_index << " Last "
                  << last_message[next.channel_index] << state->DebugString();
            }
          }
        }
      }
    } else {
      LOG(WARNING)
          << "Not sending data from before the start of the log file. "
          << timestamped_message.monotonic_event_time.time_since_epoch().count()
          << " start " << monotonic_start_time().time_since_epoch().count()
          << " "
          << FlatbufferToJson(timestamped_message.data,
                              {.multi_line = false, .max_vector_size = 100});
    }

    const monotonic_clock::time_point next_time = state->OldestMessageTime();
    if (next_time != monotonic_clock::max_time) {
      VLOG(1) << "Scheduling " << MaybeNodeName(state->event_loop()->node())
              << "wakeup for " << next_time << "("
              << state->ToDistributedClock(next_time)
              << " distributed), now is " << state->monotonic_now();
      state->Setup(next_time);
    } else {
      VLOG(1) << MaybeNodeName(state->event_loop()->node())
              << "No next message, scheduling shutdown";
      // Set a timer up immediately after now to die. If we don't do this,
      // then the senders waiting on the message we just read will never get
      // called.
      if (event_loop_factory_ != nullptr) {
        state->Setup(monotonic_now + event_loop_factory_->send_delay() +
                     std::chrono::nanoseconds(1));
      }
    }

    VLOG(1) << MaybeNodeName(state->event_loop()->node()) << "Done sending at "
            << state->event_loop()->context().monotonic_event_time << " now "
            << state->monotonic_now();
  }));

  ++live_nodes_;

  if (state->OldestMessageTime() != monotonic_clock::max_time) {
    event_loop->OnRun([state]() { state->Setup(state->OldestMessageTime()); });
  }
}

void LogReader::Deregister() {
  // Make sure that things get destroyed in the correct order, rather than
  // relying on getting the order correct in the class definition.
  for (std::unique_ptr<State> &state : states_) {
    state->Deregister();
  }

  event_loop_factory_unique_ptr_.reset();
  event_loop_factory_ = nullptr;
}

void LogReader::RemapLoggedChannel(std::string_view name, std::string_view type,
                                   std::string_view add_prefix,
                                   std::string_view new_type) {
  for (size_t ii = 0; ii < logged_configuration()->channels()->size(); ++ii) {
    const Channel *const channel = logged_configuration()->channels()->Get(ii);
    if (channel->name()->str() == name &&
        channel->type()->string_view() == type) {
      CHECK_EQ(0u, remapped_channels_.count(ii))
          << "Already remapped channel "
          << configuration::CleanedChannelToString(channel);
      RemappedChannel remapped_channel;
      remapped_channel.remapped_name =
          std::string(add_prefix) + std::string(name);
      remapped_channel.new_type = new_type;
      remapped_channels_[ii] = std::move(remapped_channel);
      VLOG(1) << "Remapping channel "
              << configuration::CleanedChannelToString(channel)
              << " to have name " << remapped_channels_[ii].remapped_name;
      MakeRemappedConfig();
      return;
    }
  }
  LOG(FATAL) << "Unabled to locate channel with name " << name << " and type "
             << type;
}

void LogReader::RemapLoggedChannel(std::string_view name, std::string_view type,
                                   const Node *node,
                                   std::string_view add_prefix,
                                   std::string_view new_type) {
  VLOG(1) << "Node is " << aos::FlatbufferToJson(node);
  const Channel *remapped_channel =
      configuration::GetChannel(logged_configuration(), name, type, "", node);
  CHECK(remapped_channel != nullptr) << ": Failed to find {\"name\": \"" << name
                                     << "\", \"type\": \"" << type << "\"}";
  VLOG(1) << "Original {\"name\": \"" << name << "\", \"type\": \"" << type
          << "\"}";
  VLOG(1) << "Remapped "
          << aos::configuration::StrippedChannelToString(remapped_channel);

  // We want to make /spray on node 0 go to /0/spray by snooping the maps.  And
  // we want it to degrade if the heuristics fail to just work.
  //
  // The easiest way to do this is going to be incredibly specific and verbose.
  // Look up /spray, to /0/spray.  Then, prefix the result with /original to get
  // /original/0/spray.  Then, create a map from /original/spray to
  // /original/0/spray for just the type we were asked for.
  if (name != remapped_channel->name()->string_view()) {
    MapT new_map;
    new_map.match = std::make_unique<ChannelT>();
    new_map.match->name = absl::StrCat(add_prefix, name);
    new_map.match->type = type;
    if (node != nullptr) {
      new_map.match->source_node = node->name()->str();
    }
    new_map.rename = std::make_unique<ChannelT>();
    new_map.rename->name =
        absl::StrCat(add_prefix, remapped_channel->name()->string_view());
    maps_.emplace_back(std::move(new_map));
  }

  const size_t channel_index =
      configuration::ChannelIndex(logged_configuration(), remapped_channel);
  CHECK_EQ(0u, remapped_channels_.count(channel_index))
      << "Already remapped channel "
      << configuration::CleanedChannelToString(remapped_channel);

  RemappedChannel remapped_channel_struct;
  remapped_channel_struct.remapped_name =
      std::string(add_prefix) +
      std::string(remapped_channel->name()->string_view());
  remapped_channel_struct.new_type = new_type;
  remapped_channels_[channel_index] = std::move(remapped_channel_struct);
  MakeRemappedConfig();
}

void LogReader::MakeRemappedConfig() {
  for (std::unique_ptr<State> &state : states_) {
    if (state) {
      CHECK(!state->event_loop())
          << ": Can't change the mapping after the events are scheduled.";
    }
  }

  // If no remapping occurred and we are using the original config, then there
  // is nothing interesting to do here.
  if (remapped_channels_.empty() && replay_configuration_ == nullptr) {
    remapped_configuration_ = logged_configuration();
    return;
  }
  // Config to copy Channel definitions from. Use the specified
  // replay_configuration_ if it has been provided.
  const Configuration *const base_config = replay_configuration_ == nullptr
                                               ? logged_configuration()
                                               : replay_configuration_;

  // Create a config with all the channels, but un-sorted/merged.  Collect up
  // the schemas while we do this.  Call MergeConfiguration to sort everything,
  // and then merge it all in together.

  // This is the builder that we use for the config containing all the new
  // channels.
  flatbuffers::FlatBufferBuilder fbb;
  fbb.ForceDefaults(true);
  std::vector<flatbuffers::Offset<Channel>> channel_offsets;

  CHECK_EQ(Channel::MiniReflectTypeTable()->num_elems, 13u)
      << ": Merging logic needs to be updated when the number of channel "
         "fields changes.";

  // List of schemas.
  std::map<std::string_view, FlatbufferVector<reflection::Schema>> schema_map;
  // Make sure our new RemoteMessage schema is in there for old logs without it.
  schema_map.insert(std::make_pair(
      RemoteMessage::GetFullyQualifiedName(),
      FlatbufferVector<reflection::Schema>(FlatbufferSpan<reflection::Schema>(
          message_bridge::RemoteMessageSchema()))));

  // Reconstruct the remapped channels.
  for (auto &pair : remapped_channels_) {
    const Channel *const c = CHECK_NOTNULL(configuration::GetChannel(
        base_config, logged_configuration()->channels()->Get(pair.first), "",
        nullptr));
    channel_offsets.emplace_back(
        CopyChannel(c, pair.second.remapped_name, "", &fbb));
  }

  // Now reconstruct the original channels, translating types as needed
  for (const Channel *c : *base_config->channels()) {
    // Search for a mapping channel.
    std::string_view new_type = "";
    for (auto &pair : remapped_channels_) {
      const Channel *const remapped_channel =
          logged_configuration()->channels()->Get(pair.first);
      if (remapped_channel->name()->string_view() == c->name()->string_view() &&
          remapped_channel->type()->string_view() == c->type()->string_view()) {
        new_type = pair.second.new_type;
        break;
      }
    }

    // Copy everything over.
    channel_offsets.emplace_back(CopyChannel(c, "", new_type, &fbb));

    // Add the schema if it doesn't exist.
    if (schema_map.find(c->type()->string_view()) == schema_map.end()) {
      CHECK(c->has_schema());
      schema_map.insert(std::make_pair(c->type()->string_view(),
                                       RecursiveCopyFlatBuffer(c->schema())));
    }
  }

  // The MergeConfiguration API takes a vector, not a map.  Convert.
  std::vector<FlatbufferVector<reflection::Schema>> schemas;
  while (!schema_map.empty()) {
    schemas.emplace_back(std::move(schema_map.begin()->second));
    schema_map.erase(schema_map.begin());
  }

  // Create the Configuration containing the new channels that we want to add.
  const flatbuffers::Offset<flatbuffers::Vector<flatbuffers::Offset<Channel>>>
      channels_offset =
          channel_offsets.empty() ? 0 : fbb.CreateVector(channel_offsets);

  // Copy over the old maps.
  std::vector<flatbuffers::Offset<Map>> map_offsets;
  if (base_config->maps()) {
    for (const Map *map : *base_config->maps()) {
      map_offsets.emplace_back(aos::RecursiveCopyFlatBuffer(map, &fbb));
    }
  }

  // Now create the new maps.  These are second so they take effect first.
  for (const MapT &map : maps_) {
    const flatbuffers::Offset<flatbuffers::String> match_name_offset =
        fbb.CreateString(map.match->name);
    const flatbuffers::Offset<flatbuffers::String> match_type_offset =
        fbb.CreateString(map.match->type);
    const flatbuffers::Offset<flatbuffers::String> rename_name_offset =
        fbb.CreateString(map.rename->name);
    flatbuffers::Offset<flatbuffers::String> match_source_node_offset;
    if (!map.match->source_node.empty()) {
      match_source_node_offset = fbb.CreateString(map.match->source_node);
    }
    Channel::Builder match_builder(fbb);
    match_builder.add_name(match_name_offset);
    match_builder.add_type(match_type_offset);
    if (!map.match->source_node.empty()) {
      match_builder.add_source_node(match_source_node_offset);
    }
    const flatbuffers::Offset<Channel> match_offset = match_builder.Finish();

    Channel::Builder rename_builder(fbb);
    rename_builder.add_name(rename_name_offset);
    const flatbuffers::Offset<Channel> rename_offset = rename_builder.Finish();

    Map::Builder map_builder(fbb);
    map_builder.add_match(match_offset);
    map_builder.add_rename(rename_offset);
    map_offsets.emplace_back(map_builder.Finish());
  }

  flatbuffers::Offset<flatbuffers::Vector<flatbuffers::Offset<Map>>>
      maps_offsets = map_offsets.empty() ? 0 : fbb.CreateVector(map_offsets);

  // And copy everything else over.
  flatbuffers::Offset<flatbuffers::Vector<flatbuffers::Offset<Node>>>
      nodes_offset = aos::RecursiveCopyVectorTable(base_config->nodes(), &fbb);

  flatbuffers::Offset<flatbuffers::Vector<flatbuffers::Offset<Application>>>
      applications_offset =
          aos::RecursiveCopyVectorTable(base_config->applications(), &fbb);

  // Now insert everything else in unmodified.
  ConfigurationBuilder configuration_builder(fbb);
  if (!channels_offset.IsNull()) {
    configuration_builder.add_channels(channels_offset);
  }
  if (!maps_offsets.IsNull()) {
    configuration_builder.add_maps(maps_offsets);
  }
  if (!nodes_offset.IsNull()) {
    configuration_builder.add_nodes(nodes_offset);
  }
  if (!applications_offset.IsNull()) {
    configuration_builder.add_applications(applications_offset);
  }

  if (base_config->has_channel_storage_duration()) {
    configuration_builder.add_channel_storage_duration(
        base_config->channel_storage_duration());
  }

  CHECK_EQ(Configuration::MiniReflectTypeTable()->num_elems, 6u)
      << ": Merging logic needs to be updated when the number of configuration "
         "fields changes.";

  fbb.Finish(configuration_builder.Finish());

  // Clean it up and return it!  By using MergeConfiguration here, we'll
  // actually get a deduplicated config for free too.
  FlatbufferDetachedBuffer<Configuration> new_merged_config =
      configuration::MergeConfiguration(
          FlatbufferDetachedBuffer<Configuration>(fbb.Release()));

  remapped_configuration_buffer_ =
      std::make_unique<FlatbufferDetachedBuffer<Configuration>>(
          configuration::MergeConfiguration(new_merged_config, schemas));

  remapped_configuration_ = &remapped_configuration_buffer_->message();

  // TODO(austin): Lazily re-build to save CPU?
}

const Channel *LogReader::RemapChannel(const EventLoop *event_loop,
                                       const Channel *channel) {
  std::string_view channel_name = channel->name()->string_view();
  std::string_view channel_type = channel->type()->string_view();
  const int channel_index =
      configuration::ChannelIndex(logged_configuration(), channel);
  // If the channel is remapped, find the correct channel name to use.
  if (remapped_channels_.count(channel_index) > 0) {
    VLOG(3) << "Got remapped channel on "
            << configuration::CleanedChannelToString(channel);
    channel_name = remapped_channels_[channel_index].remapped_name;
  }

  VLOG(2) << "Going to remap channel " << channel_name << " " << channel_type;
  const Channel *remapped_channel = configuration::GetChannel(
      event_loop->configuration(), channel_name, channel_type,
      event_loop->name(), event_loop->node());

  CHECK(remapped_channel != nullptr)
      << ": Unable to send {\"name\": \"" << channel_name << "\", \"type\": \""
      << channel_type << "\"} because it is not in the provided configuration.";

  return remapped_channel;
}

LogReader::State::State(std::unique_ptr<TimestampMapper> timestamp_mapper)
    : timestamp_mapper_(std::move(timestamp_mapper)) {}

void LogReader::State::AddPeer(State *peer) {
  if (timestamp_mapper_ && peer->timestamp_mapper_) {
    timestamp_mapper_->AddPeer(peer->timestamp_mapper_.get());
  }
}

EventLoop *LogReader::State::SetNodeEventLoopFactory(
    NodeEventLoopFactory *node_event_loop_factory) {
  node_event_loop_factory_ = node_event_loop_factory;
  event_loop_unique_ptr_ =
      node_event_loop_factory_->MakeEventLoop("log_reader");
  return event_loop_unique_ptr_.get();
}

void LogReader::State::SetChannelCount(size_t count) {
  channels_.resize(count);
  remote_timestamp_senders_.resize(count);
  filters_.resize(count);
  channel_source_state_.resize(count);
  factory_channel_index_.resize(count);
  queue_index_map_.resize(count);
}

void LogReader::State::SetChannel(
    size_t logged_channel_index, size_t factory_channel_index,
    std::unique_ptr<RawSender> sender,
    message_bridge::NoncausalOffsetEstimator *filter,
    RemoteMessageSender *remote_timestamp_sender, State *source_state) {
  channels_[logged_channel_index] = std::move(sender);
  filters_[logged_channel_index] = filter;
  remote_timestamp_senders_[logged_channel_index] = remote_timestamp_sender;

  if (source_state) {
    channel_source_state_[logged_channel_index] = source_state;

    if (remote_timestamp_sender != nullptr) {
      source_state->queue_index_map_[logged_channel_index] =
          std::make_unique<std::vector<State::ContiguousSentTimestamp>>();
    }
  }

  factory_channel_index_[logged_channel_index] = factory_channel_index;
}

bool LogReader::State::Send(const TimestampedMessage &timestamped_message) {
  aos::RawSender *sender = channels_[timestamped_message.channel_index].get();
  uint32_t remote_queue_index = 0xffffffff;

  if (remote_timestamp_senders_[timestamped_message.channel_index] != nullptr) {
    std::vector<ContiguousSentTimestamp> *queue_index_map = CHECK_NOTNULL(
        CHECK_NOTNULL(channel_source_state_[timestamped_message.channel_index])
            ->queue_index_map_[timestamped_message.channel_index]
            .get());

    struct SentTimestamp {
      monotonic_clock::time_point monotonic_event_time;
      uint32_t queue_index;
    } search;

    search.monotonic_event_time = timestamped_message.monotonic_remote_time;
    search.queue_index = timestamped_message.remote_queue_index;

    // Find the sent time if available.
    auto element = std::lower_bound(
        queue_index_map->begin(), queue_index_map->end(), search,
        [](ContiguousSentTimestamp a, SentTimestamp b) {
          if (a.ending_monotonic_event_time < b.monotonic_event_time) {
            return true;
          }
          if (a.starting_monotonic_event_time > b.monotonic_event_time) {
            return false;
          }

          if (a.ending_queue_index < b.queue_index) {
            return true;
          }
          if (a.starting_queue_index >= b.queue_index) {
            return false;
          }

          // If it isn't clearly below or above, it is below.  Since we return
          // the last element <, this will return a match.
          return false;
        });

    // TODO(austin): Be a bit more principled here, but we will want to do that
    // after the logger rewrite.  We hit this when one node finishes, but the
    // other node isn't done yet.  So there is no send time, but there is a
    // receive time.
    if (element != queue_index_map->end()) {
      CHECK_GE(timestamped_message.monotonic_remote_time,
               element->starting_monotonic_event_time);
      CHECK_LE(timestamped_message.monotonic_remote_time,
               element->ending_monotonic_event_time);
      CHECK_GE(timestamped_message.remote_queue_index,
               element->starting_queue_index);
      CHECK_LE(timestamped_message.remote_queue_index,
               element->ending_queue_index);

      remote_queue_index = timestamped_message.remote_queue_index +
                           element->actual_queue_index -
                           element->starting_queue_index;
    } else {
      VLOG(1) << "No timestamp match in the map.";
    }
  }

  // Send!  Use the replayed queue index here instead of the logged queue index
  // for the remote queue index.  This makes re-logging work.
  const bool sent = sender->Send(
      timestamped_message.data.message().data()->Data(),
      timestamped_message.data.message().data()->size(),
      timestamped_message.monotonic_remote_time,
      timestamped_message.realtime_remote_time, remote_queue_index);
  if (!sent) return false;

  if (queue_index_map_[timestamped_message.channel_index]) {
    if (queue_index_map_[timestamped_message.channel_index]->empty()) {
      // Nothing here, start a range with 0 length.
      ContiguousSentTimestamp timestamp;
      timestamp.starting_monotonic_event_time =
          timestamp.ending_monotonic_event_time =
              timestamped_message.monotonic_event_time;
      timestamp.starting_queue_index = timestamp.ending_queue_index =
          timestamped_message.queue_index;
      timestamp.actual_queue_index = sender->sent_queue_index();
      queue_index_map_[timestamped_message.channel_index]->emplace_back(
          timestamp);
    } else {
      // We've got something.  See if the next timestamp is still contiguous. If
      // so, grow it.
      ContiguousSentTimestamp *back =
          &queue_index_map_[timestamped_message.channel_index]->back();
      if ((back->starting_queue_index - back->actual_queue_index) ==
          (timestamped_message.queue_index - sender->sent_queue_index())) {
        back->ending_queue_index = timestamped_message.queue_index;
        back->ending_monotonic_event_time =
            timestamped_message.monotonic_event_time;
      } else {
        // Otherwise, make a new one.
        ContiguousSentTimestamp timestamp;
        timestamp.starting_monotonic_event_time =
            timestamp.ending_monotonic_event_time =
                timestamped_message.monotonic_event_time;
        timestamp.starting_queue_index = timestamp.ending_queue_index =
            timestamped_message.queue_index;
        timestamp.actual_queue_index = sender->sent_queue_index();
        queue_index_map_[timestamped_message.channel_index]->emplace_back(
            timestamp);
      }
    }

    // TODO(austin): Should we prune the map?  On a many day log, I only saw the
    // queue index diverge a couple of elements, which would be a very small
    // map.
  } else if (remote_timestamp_senders_[timestamped_message.channel_index] !=
             nullptr) {
    flatbuffers::FlatBufferBuilder fbb;
    fbb.ForceDefaults(true);
    flatbuffers::Offset<flatbuffers::String> boot_uuid_offset =
        fbb.CreateString(event_loop_->boot_uuid().string_view());

    RemoteMessage::Builder message_header_builder(fbb);

    message_header_builder.add_channel_index(
        factory_channel_index_[timestamped_message.channel_index]);

    // Swap the remote and sent metrics.  They are from the sender's
    // perspective, not the receiver's perspective.
    message_header_builder.add_monotonic_sent_time(
        sender->monotonic_sent_time().time_since_epoch().count());
    message_header_builder.add_realtime_sent_time(
        sender->realtime_sent_time().time_since_epoch().count());
    message_header_builder.add_queue_index(sender->sent_queue_index());

    message_header_builder.add_monotonic_remote_time(
        timestamped_message.monotonic_remote_time.time_since_epoch().count());
    message_header_builder.add_realtime_remote_time(
        timestamped_message.realtime_remote_time.time_since_epoch().count());

    message_header_builder.add_remote_queue_index(remote_queue_index);
    message_header_builder.add_boot_uuid(boot_uuid_offset);

    fbb.Finish(message_header_builder.Finish());

    remote_timestamp_senders_[timestamped_message.channel_index]->Send(
        FlatbufferDetachedBuffer<RemoteMessage>(fbb.Release()),
        timestamped_message.monotonic_timestamp_time);
  }

  return true;
}

LogReader::RemoteMessageSender::RemoteMessageSender(
    aos::Sender<message_bridge::RemoteMessage> sender, EventLoop *event_loop)
    : event_loop_(event_loop),
      sender_(std::move(sender)),
      timer_(event_loop->AddTimer([this]() { SendTimestamp(); })) {}

void LogReader::RemoteMessageSender::ScheduleTimestamp() {
  if (remote_timestamps_.empty()) {
    CHECK_NOTNULL(timer_);
    timer_->Disable();
    scheduled_time_ = monotonic_clock::min_time;
    return;
  }

  if (scheduled_time_ != remote_timestamps_.front().monotonic_timestamp_time) {
    CHECK_NOTNULL(timer_);
    timer_->Setup(remote_timestamps_.front().monotonic_timestamp_time);
    scheduled_time_ = remote_timestamps_.front().monotonic_timestamp_time;
  }
}

void LogReader::RemoteMessageSender::Send(
    FlatbufferDetachedBuffer<RemoteMessage> remote_message,
    monotonic_clock::time_point monotonic_timestamp_time) {
  // There are 2 cases.  Either we have a monotonic_timestamp_time and need to
  // resend the timestamp at the correct time, or we don't and can send it
  // immediately.
  if (monotonic_timestamp_time == monotonic_clock::min_time) {
    CHECK(remote_timestamps_.empty())
        << ": Unsupported mix of timestamps and no timestamps.";
    sender_.Send(std::move(remote_message));
  } else {
    remote_timestamps_.emplace_back(std::move(remote_message),
                                    monotonic_timestamp_time);
    ScheduleTimestamp();
  }
}

void LogReader::RemoteMessageSender::SendTimestamp() {
  CHECK_EQ(event_loop_->context().monotonic_event_time, scheduled_time_);
  CHECK(!remote_timestamps_.empty());

  // Send out all timestamps at the currently scheduled time.
  while (remote_timestamps_.front().monotonic_timestamp_time ==
         scheduled_time_) {
    sender_.Send(std::move(remote_timestamps_.front().remote_message));
    remote_timestamps_.pop_front();
    if (remote_timestamps_.empty()) {
      break;
    }
  }
  scheduled_time_ = monotonic_clock::min_time;

  ScheduleTimestamp();
}

LogReader::RemoteMessageSender *LogReader::State::RemoteTimestampSender(
    const Node *delivered_node) {
  auto sender = remote_timestamp_senders_map_.find(delivered_node);

  if (sender == remote_timestamp_senders_map_.end()) {
    sender =
        remote_timestamp_senders_map_
            .emplace(delivered_node,
                     std::make_unique<RemoteMessageSender>(
                         event_loop()->MakeSender<RemoteMessage>(absl::StrCat(
                             "/aos/remote_timestamps/",
                             delivered_node->name()->string_view())),
                         event_loop()))
            .first;
  }

  return sender->second.get();
}

TimestampedMessage LogReader::State::PopOldest() {
  CHECK_GT(sorted_messages_.size(), 0u);

  std::tuple<TimestampedMessage, message_bridge::NoncausalOffsetEstimator *>
      result = std::move(sorted_messages_.front());
  VLOG(2) << MaybeNodeName(event_loop_->node()) << "PopOldest Popping "
          << std::get<0>(result).monotonic_event_time;
  sorted_messages_.pop_front();
  SeedSortedMessages();

  if (std::get<1>(result) != nullptr) {
    std::get<1>(result)->Pop(event_loop_->node(),
                             std::get<0>(result).monotonic_event_time);
  }
  return std::move(std::get<0>(result));
}

monotonic_clock::time_point LogReader::State::OldestMessageTime() const {
  if (sorted_messages_.size() > 0) {
    VLOG(2) << MaybeNodeName(event_loop_->node()) << "oldest message at "
            << std::get<0>(sorted_messages_.front()).monotonic_event_time;
    return std::get<0>(sorted_messages_.front()).monotonic_event_time;
  }

  TimestampedMessage *m =
      timestamp_mapper_ ? timestamp_mapper_->Front() : nullptr;
  if (m == nullptr) {
    return monotonic_clock::max_time;
  }
  return m->monotonic_event_time;
}

void LogReader::State::SeedSortedMessages() {
  if (!timestamp_mapper_) return;
  const aos::monotonic_clock::time_point end_queue_time =
      (sorted_messages_.size() > 0
           ? std::get<0>(sorted_messages_.front()).monotonic_event_time
           : timestamp_mapper_->monotonic_start_time()) +
      chrono::duration_cast<chrono::seconds>(
          chrono::duration<double>(FLAGS_time_estimation_buffer_seconds));

  while (true) {
    TimestampedMessage *m = timestamp_mapper_->Front();
    if (m == nullptr) {
      return;
    }
    if (sorted_messages_.size() > 0) {
      // Stop placing sorted messages on the list once we have
      // --time_estimation_buffer_seconds seconds queued up (but queue at least
      // until the log starts.
      if (end_queue_time <
          std::get<0>(sorted_messages_.back()).monotonic_event_time) {
        return;
      }
    }

    message_bridge::NoncausalOffsetEstimator *filter = nullptr;

    TimestampedMessage timestamped_message = std::move(*m);
    timestamp_mapper_->PopFront();

    // Skip any messages without forwarding information.
    if (timestamped_message.monotonic_remote_time !=
        monotonic_clock::min_time) {
      // Got a forwarding timestamp!
      filter = filters_[timestamped_message.channel_index];

      CHECK(filter != nullptr);

      // Call the correct method depending on if we are the forward or
      // reverse direction here.
      filter->Sample(event_loop_->node(),
                     timestamped_message.monotonic_event_time,
                     timestamped_message.monotonic_remote_time);

      if (timestamped_message.monotonic_timestamp_time !=
          monotonic_clock::min_time) {
        // TODO(austin): This assumes that this timestamp is only logged on the
        // node which sent the data.  That is correct for now, but should be
        // explicitly checked somewhere.
        filter->ReverseSample(event_loop_->node(),
                              timestamped_message.monotonic_event_time,
                              timestamped_message.monotonic_timestamp_time);
      }
    }
    sorted_messages_.emplace_back(std::move(timestamped_message), filter);
  }
}

void LogReader::State::Deregister() {
  for (size_t i = 0; i < channels_.size(); ++i) {
    channels_[i].reset();
  }
  remote_timestamp_senders_map_.clear();
  event_loop_unique_ptr_.reset();
  event_loop_ = nullptr;
  timer_handler_ = nullptr;
  node_event_loop_factory_ = nullptr;
}

}  // namespace logger
}  // namespace aos