video_decode_stats_db_impl.cc

// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "media/capabilities/video_decode_stats_db_impl.h"

#include <memory>
#include <string>
#include <tuple>

#include "base/bind.h"
#include "base/debug/alias.h"
#include "base/files/file_path.h"
#include "base/logging.h"
#include "base/memory/ptr_util.h"
#include "base/metrics/field_trial_params.h"
#include "base/metrics/histogram_functions.h"
#include "base/metrics/histogram_macros.h"
#include "base/sequence_checker.h"
#include "base/task/post_task.h"
#include "base/task/thread_pool.h"
#include "base/threading/thread_task_runner_handle.h"
#include "base/time/default_clock.h"
#include "components/leveldb_proto/public/proto_database_provider.h"
#include "media/base/media_switches.h"
#include "media/capabilities/video_decode_stats.pb.h"

namespace media {

using ProtoDecodeStatsEntry = leveldb_proto::ProtoDatabase<DecodeStatsProto>;

namespace {

// Timeout threshold for DB operations. See OnOperationTimeout().
// NOTE: Used by UmaHistogramOpTime. Change the name if you change the time.
static constexpr base::TimeDelta kPendingOpTimeout =
    base::TimeDelta::FromSeconds(30);

const int kMaxFramesPerBufferDefault = 2500;

const int kMaxDaysToKeepStatsDefault = 30;

const bool kEnableUnweightedEntriesDefault = false;

void UmaHistogramOpTime(const std::string& op_name, base::TimeDelta duration) {
  base::UmaHistogramCustomMicrosecondsTimes(
      "Media.VideoDecodeStatsDB.OpTiming." + op_name, duration,
      base::TimeDelta::FromMilliseconds(1), kPendingOpTimeout, 50);
}

}  // namespace

const char VideoDecodeStatsDBImpl::kMaxFramesPerBufferParamName[] =
    "db_frames_buffer_size";

const char VideoDecodeStatsDBImpl::kMaxDaysToKeepStatsParamName[] =
    "db_days_to_keep_stats";

const char VideoDecodeStatsDBImpl::kEnableUnweightedEntriesParamName[] =
    "db_enable_unweighted_entries";

VideoDecodeStatsDBImpl::PendingOperation::PendingOperation(
    std::string uma_str,
    std::unique_ptr<base::CancelableOnceClosure> timeout_closure)
    : uma_str_(uma_str),
      timeout_closure_(std::move(timeout_closure)),
      start_ticks_(base::TimeTicks::Now()) {
  DVLOG(3) << __func__ << " Started " << uma_str_;
}

VideoDecodeStatsDBImpl::PendingOperation::~PendingOperation() {
  // Destroying a pending operation that hasn't timed out yet implies the
  // operation has completed.
  if (timeout_closure_ && !timeout_closure_->IsCancelled()) {
    base::TimeDelta op_duration = base::TimeTicks::Now() - start_ticks_;
    UmaHistogramOpTime(uma_str_, op_duration);
    DVLOG(3) << __func__ << " Completed " << uma_str_ << " ("
             << op_duration.InMilliseconds() << ")";

    // Ensure the timeout doesn't fire. Destruction should cancel the callback
    // implicitly, but that's not a documented contract, so just taking the safe
    // route.
    timeout_closure_->Cancel();
  }
}

void VideoDecodeStatsDBImpl::PendingOperation::OnTimeout() {
  UmaHistogramOpTime(uma_str_, kPendingOpTimeout);
  LOG(WARNING) << " Timeout performing " << uma_str_
               << " operation on VideoDecodeStatsDB";

  // Cancel the closure to ensure we don't double report the task as completed
  // in ~PendingOperation().
  timeout_closure_->Cancel();
}

// static
int VideoDecodeStatsDBImpl::GetMaxFramesPerBuffer() {
  return base::GetFieldTrialParamByFeatureAsDouble(
      kMediaCapabilitiesWithParameters, kMaxFramesPerBufferParamName,
      kMaxFramesPerBufferDefault);
}

// static
int VideoDecodeStatsDBImpl::GetMaxDaysToKeepStats() {
  return base::GetFieldTrialParamByFeatureAsDouble(
      kMediaCapabilitiesWithParameters, kMaxDaysToKeepStatsParamName,
      kMaxDaysToKeepStatsDefault);
}

// static
bool VideoDecodeStatsDBImpl::GetEnableUnweightedEntries() {
  return base::GetFieldTrialParamByFeatureAsBool(
      kMediaCapabilitiesWithParameters, kEnableUnweightedEntriesParamName,
      kEnableUnweightedEntriesDefault);
}

// static
base::FieldTrialParams VideoDecodeStatsDBImpl::GetFieldTrialParams() {
  base::FieldTrialParams actual_trial_params;

  const bool result = base::GetFieldTrialParamsByFeature(
      kMediaCapabilitiesWithParameters, &actual_trial_params);
  DCHECK(result);

  return actual_trial_params;
}

// static
std::unique_ptr<VideoDecodeStatsDBImpl> VideoDecodeStatsDBImpl::Create(
    base::FilePath db_dir,
    leveldb_proto::ProtoDatabaseProvider* db_provider) {
  DVLOG(2) << __func__ << " db_dir:" << db_dir;

  auto proto_db = db_provider->GetDB<DecodeStatsProto>(
      leveldb_proto::ProtoDbType::VIDEO_DECODE_STATS_DB, db_dir,
      base::ThreadPool::CreateSequencedTaskRunner(
          {base::MayBlock(), base::TaskPriority::USER_VISIBLE,
           base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN}));

  return base::WrapUnique(new VideoDecodeStatsDBImpl(std::move(proto_db)));
}

constexpr char VideoDecodeStatsDBImpl::kDefaultWriteTime[];

VideoDecodeStatsDBImpl::VideoDecodeStatsDBImpl(
    std::unique_ptr<leveldb_proto::ProtoDatabase<DecodeStatsProto>> db)
    : db_(std::move(db)), wall_clock_(base::DefaultClock::GetInstance()) {
  bool time_parsed =
      base::Time::FromString(kDefaultWriteTime, &default_write_time_);
  DCHECK(time_parsed);

  DCHECK(db_);
}

VideoDecodeStatsDBImpl::~VideoDecodeStatsDBImpl() {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
}

VideoDecodeStatsDBImpl::PendingOpId VideoDecodeStatsDBImpl::StartPendingOp(
    std::string uma_str) {
  PendingOpId op_id = next_op_id_++;

  auto timeout_closure = std::make_unique<base::CancelableOnceClosure>(
      base::BindOnce(&VideoDecodeStatsDBImpl::OnPendingOpTimeout,
                     weak_ptr_factory_.GetWeakPtr(), op_id));

  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
      FROM_HERE, timeout_closure->callback(), kPendingOpTimeout);

  pending_ops_.emplace(op_id, std::make_unique<PendingOperation>(
                                  uma_str, std::move(timeout_closure)));

  return op_id;
}

void VideoDecodeStatsDBImpl::CompletePendingOp(PendingOpId op_id) {
  // Destructing the PendingOperation will trigger UMA for completion timing.
  int count = pending_ops_.erase(op_id);

  // No big deal, but very unusual. Timeout is very generous, so tasks that
  // timeout are generally assumed to be permanently hung.
  if (!count)
    DVLOG(2) << __func__ << " DB operation completed after timeout.";
}

void VideoDecodeStatsDBImpl::OnPendingOpTimeout(PendingOpId op_id) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

  auto it = pending_ops_.find(op_id);
  DCHECK(it != pending_ops_.end());

  it->second->OnTimeout();
  pending_ops_.erase(it);
}

void VideoDecodeStatsDBImpl::Initialize(InitializeCB init_cb) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DCHECK(init_cb);
  DCHECK(!IsInitialized());

  // "Simple options" will use the default global cache of 8MB. In the worst
  // case our whole DB will be less than 35K, so we aren't worried about
  // spamming the cache.
  // TODO(chcunningham): Keep an eye on the size as the table evolves.
  db_->Init(base::BindOnce(&VideoDecodeStatsDBImpl::OnInit,
                           weak_ptr_factory_.GetWeakPtr(),
                           StartPendingOp("Initialize"), std::move(init_cb)));
}

void VideoDecodeStatsDBImpl::OnInit(PendingOpId op_id,
                                    InitializeCB init_cb,
                                    leveldb_proto::Enums::InitStatus status) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DCHECK_NE(status, leveldb_proto::Enums::InitStatus::kInvalidOperation);
  bool success = status == leveldb_proto::Enums::InitStatus::kOK;
  DVLOG(2) << __func__ << (success ? " succeeded" : " FAILED!");
  CompletePendingOp(op_id);
  UMA_HISTOGRAM_BOOLEAN("Media.VideoDecodeStatsDB.OpSuccess.Initialize",
                        success);

  db_init_ = true;

  // Can't use DB when initialization fails.
  if (!success)
    db_.reset();

  std::move(init_cb).Run(success);
}

bool VideoDecodeStatsDBImpl::IsInitialized() {
  // |db_| will be null if Initialization failed.
  return db_init_ && db_;
}

void VideoDecodeStatsDBImpl::AppendDecodeStats(
    const VideoDescKey& key,
    const DecodeStatsEntry& entry,
    AppendDecodeStatsCB append_done_cb) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DCHECK(IsInitialized());

  DVLOG(3) << __func__ << " Reading key " << key.ToLogString()
           << " from DB with intent to update with " << entry.ToLogString();

  db_->GetEntry(
      key.Serialize(),
      base::BindOnce(&VideoDecodeStatsDBImpl::WriteUpdatedEntry,
                     weak_ptr_factory_.GetWeakPtr(), StartPendingOp("Read"),
                     key, entry, std::move(append_done_cb)));
}

void VideoDecodeStatsDBImpl::GetDecodeStats(const VideoDescKey& key,
                                            GetDecodeStatsCB get_stats_cb) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DCHECK(IsInitialized());

  DVLOG(3) << __func__ << " " << key.ToLogString();

  db_->GetEntry(
      key.Serialize(),
      base::BindOnce(&VideoDecodeStatsDBImpl::OnGotDecodeStats,
                     weak_ptr_factory_.GetWeakPtr(), StartPendingOp("Read"),
                     std::move(get_stats_cb)));
}

bool VideoDecodeStatsDBImpl::AreStatsUsable(
    const DecodeStatsProto* const stats_proto) {
  // CHECK FOR CORRUPTION
  // We've observed this in a tiny fraction of reports, but the consequences can
  // lead to crashes due floating point math exceptions. http://crbug.com/982009

  bool are_stats_valid =
      // All frame counts should be capped by |frames_decoded|.
      stats_proto->frames_dropped() <= stats_proto->frames_decoded() &&
      stats_proto->frames_power_efficient() <= stats_proto->frames_decoded() &&

      // You can't drop or power-efficiently decode more than 100% of frames.
      stats_proto->unweighted_average_frames_dropped() <= 1 &&
      stats_proto->unweighted_average_frames_efficient() <= 1 &&

      // |last_write_date| represents base::Time::ToJsTime(), a number of msec
      // since the epoch, so it should never be negative (zero is valid, as a
      // default for this field, indicating the last write was made before we
      // added time stamping). The converted time should also never be in the
      // future.
      stats_proto->last_write_date() >= 0 &&
      base::Time::FromJsTime(stats_proto->last_write_date()) <=
          wall_clock_->Now();

  UMA_HISTOGRAM_BOOLEAN("Media.VideoDecodeStatsDB.OpSuccess.Validate",
                        are_stats_valid);

  if (!are_stats_valid)
    return false;

  // CHECK FOR EXPIRATION
  // Avoid keeping old data forever so users aren't stuck with predictions after
  // upgrading their machines (e.g. driver updates or new hardware).

  double last_write_date = stats_proto->last_write_date();
  if (last_write_date == 0) {
    // Set a default time if the write date is zero (no write since proto was
    // updated to include the time stamp).
    last_write_date = default_write_time_.ToJsTime();
  }

  const int kMaxDaysToKeepStats = GetMaxDaysToKeepStats();
  DCHECK_GT(kMaxDaysToKeepStats, 0);

  return wall_clock_->Now() - base::Time::FromJsTime(last_write_date) <=
         base::TimeDelta::FromDays(kMaxDaysToKeepStats);
}

void VideoDecodeStatsDBImpl::WriteUpdatedEntry(
    PendingOpId op_id,
    const VideoDescKey& key,
    const DecodeStatsEntry& new_entry,
    AppendDecodeStatsCB append_done_cb,
    bool read_success,
    std::unique_ptr<DecodeStatsProto> stats_proto) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DCHECK(IsInitialized());
  CompletePendingOp(op_id);

  // Note: outcome of "Write" operation logged in OnEntryUpdated().
  UMA_HISTOGRAM_BOOLEAN("Media.VideoDecodeStatsDB.OpSuccess.Read",
                        read_success);

  if (!read_success) {
    DVLOG(2) << __func__ << " FAILED DB read for " << key.ToLogString()
             << "; ignoring update!";
    std::move(append_done_cb).Run(false);
    return;
  }

  if (!stats_proto || !AreStatsUsable(stats_proto.get())) {
    // Default instance will have all zeros for numeric types.
    stats_proto = std::make_unique<DecodeStatsProto>();
  }

  // Debug alias the various counts so we can get them in dumps to catch
  // lingering crashes in http://crbug.com/982009
  uint64_t old_frames_decoded = stats_proto->frames_decoded();
  uint64_t old_frames_dropped = stats_proto->frames_dropped();
  uint64_t old_frames_power_efficient = stats_proto->frames_power_efficient();
  uint64_t new_frames_decoded = new_entry.frames_decoded;
  uint64_t new_frames_dropped = new_entry.frames_dropped;
  uint64_t new_frames_power_efficient = new_entry.frames_power_efficient;
  base::debug::Alias(&old_frames_decoded);
  base::debug::Alias(&old_frames_dropped);
  base::debug::Alias(&old_frames_power_efficient);
  base::debug::Alias(&new_frames_decoded);
  base::debug::Alias(&new_frames_dropped);
  base::debug::Alias(&new_frames_power_efficient);

  const uint64_t kMaxFramesPerBuffer = GetMaxFramesPerBuffer();
  DCHECK_GT(kMaxFramesPerBuffer, 0UL);

  double new_entry_dropped_ratio = 0;
  double new_entry_efficient_ratio = 0;
  if (new_entry.frames_decoded) {
    new_entry_dropped_ratio = static_cast<double>(new_entry.frames_dropped) /
                              new_entry.frames_decoded;
    new_entry_efficient_ratio =
        static_cast<double>(new_entry.frames_power_efficient) /
        new_entry.frames_decoded;
  } else {
    // Callers shouldn't ask DB to save empty records. See
    // VideoDecodeStatsRecorder.
    NOTREACHED() << __func__ << " saving empty stats record";
  }

  if (old_frames_decoded + new_entry.frames_decoded > kMaxFramesPerBuffer) {
    // The |new_entry| is pushing out some or all of the old data. Achieve this
    // by weighting the dropped and power efficiency stats by the ratio of the
    // the buffer that new entry fills.
    double fill_ratio = std::min(
        static_cast<double>(new_entry.frames_decoded) / kMaxFramesPerBuffer,
        1.0);

    double old_dropped_ratio = 0;
    double old_efficient_ratio = 0;
    if (old_frames_decoded) {
      old_dropped_ratio =
          static_cast<double>(old_frames_dropped) / old_frames_decoded;
      old_efficient_ratio =
          static_cast<double>(old_frames_power_efficient) / old_frames_decoded;
    }

    double agg_dropped_ratio = fill_ratio * new_entry_dropped_ratio +
                               (1 - fill_ratio) * old_dropped_ratio;
    double agg_efficient_ratio = fill_ratio * new_entry_efficient_ratio +
                                 (1 - fill_ratio) * old_efficient_ratio;

    // Debug alias the various counts so we can get them in dumps to catch
    // lingering crashes in http://crbug.com/982009
    base::debug::Alias(&fill_ratio);
    base::debug::Alias(&old_dropped_ratio);
    base::debug::Alias(&old_efficient_ratio);
    base::debug::Alias(&agg_dropped_ratio);
    base::debug::Alias(&agg_efficient_ratio);

    stats_proto->set_frames_decoded(kMaxFramesPerBuffer);
    stats_proto->set_frames_dropped(
        std::round(agg_dropped_ratio * kMaxFramesPerBuffer));
    stats_proto->set_frames_power_efficient(
        std::round(agg_efficient_ratio * kMaxFramesPerBuffer));
  } else {
    // Adding |new_entry| does not exceed |kMaxFramesPerfBuffer|. Simply sum the
    // stats.
    stats_proto->set_frames_decoded(new_entry.frames_decoded +
                                    old_frames_decoded);
    stats_proto->set_frames_dropped(new_entry.frames_dropped +
                                    old_frames_dropped);
    stats_proto->set_frames_power_efficient(new_entry.frames_power_efficient +
                                            old_frames_power_efficient);
  }

  if (GetEnableUnweightedEntries()) {
    uint64_t old_num_unweighted_playbacks =
        stats_proto->num_unweighted_playbacks();
    double old_unweighted_drop_avg =
        stats_proto->unweighted_average_frames_dropped();
    double old_unweighted_efficient_avg =
        stats_proto->unweighted_average_frames_efficient();

    uint64_t new_num_unweighted_playbacks = old_num_unweighted_playbacks + 1;
    double new_unweighted_drop_avg =
        ((old_unweighted_drop_avg * old_num_unweighted_playbacks) +
         new_entry_dropped_ratio) /
        new_num_unweighted_playbacks;
    double new_unweighted_efficient_avg =
        ((old_unweighted_efficient_avg * old_num_unweighted_playbacks) +
         new_entry_efficient_ratio) /
        new_num_unweighted_playbacks;

    stats_proto->set_num_unweighted_playbacks(new_num_unweighted_playbacks);
    stats_proto->set_unweighted_average_frames_dropped(new_unweighted_drop_avg);
    stats_proto->set_unweighted_average_frames_efficient(
        new_unweighted_efficient_avg);

    DVLOG(2) << __func__ << " Updating unweighted averages. dropped:"
             << new_unweighted_drop_avg
             << " efficient:" << new_unweighted_efficient_avg
             << " num_playbacks:" << new_num_unweighted_playbacks;
  }

  // Update the time stamp for the current write.
  stats_proto->set_last_write_date(wall_clock_->Now().ToJsTime());

  // Make sure we never write bogus stats into the DB! While its possible the DB
  // may experience some corruption (disk), we should have detected that above
  // and discarded any bad data prior to this upcoming save.
  DCHECK(AreStatsUsable(stats_proto.get()));

  // Push the update to the DB.
  using DBType = leveldb_proto::ProtoDatabase<DecodeStatsProto>;
  std::unique_ptr<DBType::KeyEntryVector> entries =
      std::make_unique<DBType::KeyEntryVector>();
  entries->emplace_back(key.Serialize(), *stats_proto);
  db_->UpdateEntries(
      std::move(entries), std::make_unique<leveldb_proto::KeyVector>(),
      base::BindOnce(&VideoDecodeStatsDBImpl::OnEntryUpdated,
                     weak_ptr_factory_.GetWeakPtr(), StartPendingOp("Write"),
                     std::move(append_done_cb)));
}

void VideoDecodeStatsDBImpl::OnEntryUpdated(PendingOpId op_id,
                                            AppendDecodeStatsCB append_done_cb,
                                            bool success) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DVLOG(3) << __func__ << " update " << (success ? "succeeded" : "FAILED!");
  CompletePendingOp(op_id);
  UMA_HISTOGRAM_BOOLEAN("Media.VideoDecodeStatsDB.OpSuccess.Write", success);
  std::move(append_done_cb).Run(success);
}

void VideoDecodeStatsDBImpl::OnGotDecodeStats(
    PendingOpId op_id,
    GetDecodeStatsCB get_stats_cb,
    bool success,
    std::unique_ptr<DecodeStatsProto> stats_proto) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DVLOG(3) << __func__ << " get " << (success ? "succeeded" : "FAILED!");
  CompletePendingOp(op_id);
  UMA_HISTOGRAM_BOOLEAN("Media.VideoDecodeStatsDB.OpSuccess.Read", success);

  std::unique_ptr<DecodeStatsEntry> entry;

  if (stats_proto && AreStatsUsable(stats_proto.get())) {
    DCHECK(success);

    if (GetEnableUnweightedEntries()) {
      DCHECK_GE(stats_proto->unweighted_average_frames_dropped(), 0);
      DCHECK_LE(stats_proto->unweighted_average_frames_dropped(), 1);
      DCHECK_GE(stats_proto->unweighted_average_frames_efficient(), 0);
      DCHECK_LE(stats_proto->unweighted_average_frames_efficient(), 1);

      DVLOG(2) << __func__ << " Using unweighted averages. dropped:"
               << stats_proto->unweighted_average_frames_dropped()
               << " efficient:"
               << stats_proto->unweighted_average_frames_efficient()
               << " num_playbacks:" << stats_proto->num_unweighted_playbacks();

      // The meaning of DecodStatsEntry is a little different for folks in the
      // unweighted experiment group
      // - The *ratios* of dropped / decoded and efficient / decoded are valid,
      //   which means no change to any math in the upper layer. The ratio is
      //   internally computed as an unweighted average of the dropped frames
      //   ratio over all the playbacks in this bucket.
      // - The denominator "decoded" is actually the number of entries
      //   accumulated by this key scaled by 100,000. Scaling by 100,000
      //   preserves the precision of the dropped / decoded ratio to the 5th
      //   decimal place (i.e. 0.01234, or 1.234%)
      // - The numerator "dropped" or "efficient" doesn't represent anything and
      //   is simply chosen to create the correct ratio.
      //
      // This is obviously not the most efficient or readable way to do this,
      // but  allows us to continue using the same proto and UKM reporting
      // while we experiment with the unweighted approach. If this approach
      // proves successful we will refactor the API and proto.
      uint64_t frames_decoded_lie =
          100000 * stats_proto->num_unweighted_playbacks();
      entry = std::make_unique<DecodeStatsEntry>(
          frames_decoded_lie,
          frames_decoded_lie * stats_proto->unweighted_average_frames_dropped(),
          frames_decoded_lie *
              stats_proto->unweighted_average_frames_efficient());
    } else {
      entry = std::make_unique<DecodeStatsEntry>(
          stats_proto->frames_decoded(), stats_proto->frames_dropped(),
          stats_proto->frames_power_efficient());
    }
  }

  DVLOG(3) << __func__ << " read " << (success ? "succeeded" : "FAILED!")
           << " entry: " << (entry ? entry->ToLogString() : "nullptr");

  std::move(get_stats_cb).Run(success, std::move(entry));
}

void VideoDecodeStatsDBImpl::ClearStats(base::OnceClosure clear_done_cb) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DVLOG(2) << __func__;

  db_->UpdateEntriesWithRemoveFilter(
      std::make_unique<ProtoDecodeStatsEntry::KeyEntryVector>(),
      base::BindRepeating([](const std::string& key) { return true; }),
      base::BindOnce(&VideoDecodeStatsDBImpl::OnStatsCleared,
                     weak_ptr_factory_.GetWeakPtr(), StartPendingOp("Clear"),
                     std::move(clear_done_cb)));
}

void VideoDecodeStatsDBImpl::OnStatsCleared(PendingOpId op_id,
                                            base::OnceClosure clear_done_cb,
                                            bool success) {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  DVLOG(2) << __func__ << (success ? " succeeded" : " FAILED!");

  CompletePendingOp(op_id);

  UMA_HISTOGRAM_BOOLEAN("Media.VideoDecodeStatsDB.OpSuccess.Clear", success);

  // We don't pass success to |clear_done_cb|. Clearing is best effort and
  // there is no additional action for callers to take in case of failure.
  // TODO(chcunningham): Monitor UMA and consider more aggressive action like
  // deleting the DB directory.
  std::move(clear_done_cb).Run();
}

}  // namespace media