components/metrics/file_metrics_provider.cc

// Copyright 2016 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 "components/metrics/file_metrics_provider.h"

#include "base/command_line.h"
#include "base/files/file.h"
#include "base/files/file_enumerator.h"
#include "base/files/file_util.h"
#include "base/files/memory_mapped_file.h"
#include "base/logging.h"
#include "base/memory/ptr_util.h"
#include "base/metrics/histogram_base.h"
#include "base/metrics/histogram_macros.h"
#include "base/metrics/persistent_histogram_allocator.h"
#include "base/metrics/persistent_memory_allocator.h"
#include "base/strings/string_piece.h"
#include "base/task_runner.h"
#include "base/time/time.h"
#include "components/metrics/metrics_pref_names.h"
#include "components/metrics/metrics_service.h"
#include "components/metrics/persistent_system_profile.h"
#include "components/prefs/pref_registry_simple.h"
#include "components/prefs/pref_service.h"

namespace metrics {

namespace {

// These structures provide values used to define how files are opened and
// accessed. It obviates the need for multiple code-paths within several of
// the methods.
struct SourceOptions {
  // The flags to be used to open a file on disk.
  int file_open_flags;

  // The access mode to be used when mapping a file into memory.
  base::MemoryMappedFile::Access memory_mapped_access;

  // Indicates if the file is to be accessed read-only.
  bool is_read_only;
};

enum : int {
  // Opening a file typically requires at least these flags.
  STD_OPEN = base::File::FLAG_OPEN | base::File::FLAG_READ,
};

constexpr SourceOptions kSourceOptions[] = {
  // SOURCE_HISTOGRAMS_ATOMIC_FILE
  {
    // Ensure that no other process reads this at the same time.
    STD_OPEN | base::File::FLAG_EXCLUSIVE_READ,
    base::MemoryMappedFile::READ_ONLY,
    true
  },
  // SOURCE_HISTOGRAMS_ATOMIC_DIR
  {
    // Ensure that no other process reads this at the same time.
    STD_OPEN | base::File::FLAG_EXCLUSIVE_READ,
    base::MemoryMappedFile::READ_ONLY,
    true
  },
  // SOURCE_HISTOGRAMS_ACTIVE_FILE
  {
    // Allow writing (updated "logged" values) to the file.
    STD_OPEN | base::File::FLAG_WRITE,
    base::MemoryMappedFile::READ_WRITE,
    false
  }
};

enum EmbeddedProfileResult : int {
  EMBEDDED_PROFILE_ATTEMPT,
  EMBEDDED_PROFILE_FOUND,
  EMBEDDED_PROFILE_FALLBACK,
  EMBEDDED_PROFILE_DROPPED,
  EMBEDDED_PROFILE_ACTION_MAX
};

void RecordEmbeddedProfileResult(EmbeddedProfileResult result) {
  UMA_HISTOGRAM_ENUMERATION("UMA.FileMetricsProvider.EmbeddedProfileResult",
                            result, EMBEDDED_PROFILE_ACTION_MAX);
}

void DeleteFileWhenPossible(const base::FilePath& path) {
  // Open (with delete) and then immediately close the file by going out of
  // scope. This is the only cross-platform safe way to delete a file that may
  // be open elsewhere, a distinct possibility given the asynchronous nature
  // of the delete task.
  base::File file(path, base::File::FLAG_OPEN | base::File::FLAG_READ |
                            base::File::FLAG_DELETE_ON_CLOSE);
}

}  // namespace

// This structure stores all the information about the sources being monitored
// and their current reporting state.
struct FileMetricsProvider::SourceInfo {
  SourceInfo(SourceType source_type, SourceAssociation source_association)
      : type(source_type), association(source_association) {}
  ~SourceInfo() {}

  // How to access this source (file/dir, atomic/active).
  const SourceType type;

  // With what run this source is associated.
  const SourceAssociation association;

  // Where on disk the directory is located. This will only be populated when
  // a directory is being monitored.
  base::FilePath directory;

  // Where on disk the file is located. If a directory is being monitored,
  // this will be updated for whatever file is being read.
  base::FilePath path;

  // Name used inside prefs to persistent metadata.
  std::string prefs_key;

  // The last-seen time of this source to detect change.
  base::Time last_seen;

  // Indicates if the data has been read out or not.
  bool read_complete = false;

  // Once a file has been recognized as needing to be read, it is mapped
  // into memory and assigned to an |allocator| object.
  std::unique_ptr<base::PersistentHistogramAllocator> allocator;

 private:
  DISALLOW_COPY_AND_ASSIGN(SourceInfo);
};

FileMetricsProvider::FileMetricsProvider(
    const scoped_refptr<base::TaskRunner>& task_runner,
    PrefService* local_state)
    : task_runner_(task_runner),
      pref_service_(local_state),
      weak_factory_(this) {
  base::StatisticsRecorder::RegisterHistogramProvider(
      weak_factory_.GetWeakPtr());
}

FileMetricsProvider::~FileMetricsProvider() {}

void FileMetricsProvider::RegisterSource(const base::FilePath& path,
                                         SourceType type,
                                         SourceAssociation source_association,
                                         const base::StringPiece prefs_key) {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Ensure that kSourceOptions has been filled for this type.
  DCHECK_GT(arraysize(kSourceOptions), static_cast<size_t>(type));

  std::unique_ptr<SourceInfo> source(new SourceInfo(type, source_association));
  source->prefs_key = prefs_key.as_string();

  switch (source->type) {
    case SOURCE_HISTOGRAMS_ACTIVE_FILE:
      DCHECK(prefs_key.empty());
    // fall through
    case SOURCE_HISTOGRAMS_ATOMIC_FILE:
      source->path = path;
      break;
    case SOURCE_HISTOGRAMS_ATOMIC_DIR:
      source->directory = path;
      break;
  }

  // |prefs_key| may be empty if the caller does not wish to persist the
  // state across instances of the program.
  if (pref_service_ && !prefs_key.empty()) {
    source->last_seen = base::Time::FromInternalValue(
        pref_service_->GetInt64(metrics::prefs::kMetricsLastSeenPrefix +
                                source->prefs_key));
  }

  switch (source_association) {
    case ASSOCIATE_CURRENT_RUN:
    case ASSOCIATE_INTERNAL_PROFILE:
      sources_to_check_.push_back(std::move(source));
      break;
    case ASSOCIATE_PREVIOUS_RUN:
    case ASSOCIATE_INTERNAL_PROFILE_OR_PREVIOUS_RUN:
      DCHECK_EQ(SOURCE_HISTOGRAMS_ATOMIC_FILE, source->type);
      sources_for_previous_run_.push_back(std::move(source));
      break;
  }
}

// static
void FileMetricsProvider::RegisterPrefs(PrefRegistrySimple* prefs,
                                        const base::StringPiece prefs_key) {
  prefs->RegisterInt64Pref(metrics::prefs::kMetricsLastSeenPrefix +
                           prefs_key.as_string(), 0);
}

// static
bool FileMetricsProvider::LocateNextFileInDirectory(SourceInfo* source) {
  DCHECK_EQ(SOURCE_HISTOGRAMS_ATOMIC_DIR, source->type);
  DCHECK(!source->directory.empty());

  // Open the directory and find all the files, remembering the oldest that
  // has not been read. They can be removed and/or ignored if they're older
  // than the last-check time.
  base::Time oldest_file_time = base::Time::Now();
  base::FilePath oldest_file_path;
  base::FilePath file_path;
  int file_count = 0;
  int delete_count = 0;
  base::FileEnumerator file_iter(source->directory, /*recursive=*/false,
                                 base::FileEnumerator::FILES);
  for (file_path = file_iter.Next(); !file_path.empty();
       file_path = file_iter.Next()) {
    base::FileEnumerator::FileInfo file_info = file_iter.GetInfo();

    // Ignore directories and zero-sized files.
    if (file_info.IsDirectory() || file_info.GetSize() == 0)
      continue;

    // Ignore temporary files.
    base::FilePath::CharType first_character =
        file_path.BaseName().value().front();
    if (first_character == FILE_PATH_LITERAL('.') ||
        first_character == FILE_PATH_LITERAL('_')) {
      continue;
    }

    // Ignore non-PMA (Persistent Memory Allocator) files.
    if (file_path.Extension() !=
        base::PersistentMemoryAllocator::kFileExtension) {
      continue;
    }

    // Process real files.
    base::Time modified = file_info.GetLastModifiedTime();
    if (modified > source->last_seen) {
      // This file hasn't been read. Remember it if it is older than others.
      if (modified < oldest_file_time) {
        oldest_file_path = std::move(file_path);
        oldest_file_time = modified;
      }
      ++file_count;
    } else {
      // This file has been read. Try to delete it. Ignore any errors because
      // the file may be un-removeable by this process. It could, for example,
      // have been created by a privileged process like setup.exe. Even if it
      // is not removed, it will continue to be ignored bacuse of the older
      // modification time.
      base::DeleteFile(file_path, /*recursive=*/false);
      ++delete_count;
    }
  }

  UMA_HISTOGRAM_COUNTS_100("UMA.FileMetricsProvider.DirectoryFiles",
                           file_count);
  UMA_HISTOGRAM_COUNTS_100("UMA.FileMetricsProvider.DeletedFiles",
                           delete_count);

  // Stop now if there are no files to read.
  if (oldest_file_path.empty())
    return false;

  // Set the active file to be the oldest modified file that has not yet
  // been read.
  source->path = std::move(oldest_file_path);
  return true;
}

// static
void FileMetricsProvider::FinishedWithSource(SourceInfo* source,
                                             AccessResult result) {
  // Different source types require different post-processing.
  switch (source->type) {
    case SOURCE_HISTOGRAMS_ATOMIC_FILE:
    case SOURCE_HISTOGRAMS_ATOMIC_DIR:
      // Done with this file so delete the allocator and its owned file.
      source->allocator.reset();
      // Remove the file if has been recorded. This prevents them from
      // accumulating or also being recorded by different instances of
      // the browser.
      if (result == ACCESS_RESULT_SUCCESS ||
          result == ACCESS_RESULT_NOT_MODIFIED) {
        DeleteFileWhenPossible(source->path);
      }
      break;
    case SOURCE_HISTOGRAMS_ACTIVE_FILE:
      // Keep the allocator open so it doesn't have to be re-mapped each
      // time. This also allows the contents to be merged on-demand.
      break;
  }
}

// static
void FileMetricsProvider::CheckAndMergeMetricSourcesOnTaskRunner(
    SourceInfoList* sources) {
  // This method has all state information passed in |sources| and is intended
  // to run on a worker thread rather than the UI thread.
  for (std::unique_ptr<SourceInfo>& source : *sources) {
    AccessResult result = CheckAndMapMetricSource(source.get());

    // Some results are not reported in order to keep the dashboard clean.
    if (result != ACCESS_RESULT_DOESNT_EXIST &&
        result != ACCESS_RESULT_NOT_MODIFIED) {
      UMA_HISTOGRAM_ENUMERATION(
          "UMA.FileMetricsProvider.AccessResult", result, ACCESS_RESULT_MAX);
    }

    // Metrics associated with internal profiles have to be fetched directly
    // so just keep the mapping for use by the main thread.
    if (source->association == ASSOCIATE_INTERNAL_PROFILE)
      continue;

    // Mapping was successful. Merge it.
    if (result == ACCESS_RESULT_SUCCESS) {
      MergeHistogramDeltasFromSource(source.get());
      DCHECK(source->read_complete);
    }

    // All done with this source.
    FinishedWithSource(source.get(), result);
  }
}

// This method has all state information passed in |source| and is intended
// to run on a worker thread rather than the UI thread.
// static
FileMetricsProvider::AccessResult FileMetricsProvider::CheckAndMapMetricSource(
    SourceInfo* source) {
  // If source was read, clean up after it.
  if (source->read_complete)
    FinishedWithSource(source, ACCESS_RESULT_SUCCESS);
  source->read_complete = false;
  DCHECK(!source->allocator);

  // If the source is a directory, look for files within it.
  if (!source->directory.empty() && !LocateNextFileInDirectory(source))
    return ACCESS_RESULT_DOESNT_EXIST;

  // Do basic validation on the file metadata.
  base::File::Info info;
  if (!base::GetFileInfo(source->path, &info))
    return ACCESS_RESULT_DOESNT_EXIST;

  if (info.is_directory || info.size == 0)
    return ACCESS_RESULT_INVALID_FILE;

  if (source->last_seen >= info.last_modified)
    return ACCESS_RESULT_NOT_MODIFIED;

  // A new file of metrics has been found.
  base::File file(source->path, kSourceOptions[source->type].file_open_flags);
  if (!file.IsValid())
    return ACCESS_RESULT_NO_OPEN;

  std::unique_ptr<base::MemoryMappedFile> mapped(new base::MemoryMappedFile());
  if (!mapped->Initialize(std::move(file),
                          kSourceOptions[source->type].memory_mapped_access)) {
    return ACCESS_RESULT_SYSTEM_MAP_FAILURE;
  }

  // Ensure any problems below don't occur repeatedly.
  source->last_seen = info.last_modified;

  // Test the validity of the file contents.
  const bool read_only = kSourceOptions[source->type].is_read_only;
  if (!base::FilePersistentMemoryAllocator::IsFileAcceptable(*mapped,
                                                             read_only)) {
    return ACCESS_RESULT_INVALID_CONTENTS;
  }

  // Map the file and validate it.
  std::unique_ptr<base::PersistentMemoryAllocator> memory_allocator =
      base::MakeUnique<base::FilePersistentMemoryAllocator>(
          std::move(mapped), 0, 0, base::StringPiece(), read_only);
  if (memory_allocator->GetMemoryState() ==
      base::PersistentMemoryAllocator::MEMORY_DELETED) {
    return ACCESS_RESULT_MEMORY_DELETED;
  }

  // Create an allocator for the mapped file. Ownership passes to the allocator.
  source->allocator = base::MakeUnique<base::PersistentHistogramAllocator>(
      std::move(memory_allocator));

  return ACCESS_RESULT_SUCCESS;
}

// static
void FileMetricsProvider::MergeHistogramDeltasFromSource(SourceInfo* source) {
  DCHECK(source->allocator);
  base::PersistentHistogramAllocator::Iterator histogram_iter(
      source->allocator.get());

  const bool read_only = kSourceOptions[source->type].is_read_only;
  int histogram_count = 0;
  while (true) {
    std::unique_ptr<base::HistogramBase> histogram = histogram_iter.GetNext();
    if (!histogram)
      break;
    if (read_only) {
      source->allocator->MergeHistogramFinalDeltaToStatisticsRecorder(
          histogram.get());
    } else {
      source->allocator->MergeHistogramDeltaToStatisticsRecorder(
          histogram.get());
    }
    ++histogram_count;
  }

  source->read_complete = true;
  DVLOG(1) << "Reported " << histogram_count << " histograms from "
           << source->path.value();
}

// static
void FileMetricsProvider::RecordHistogramSnapshotsFromSource(
    base::HistogramSnapshotManager* snapshot_manager,
    SourceInfo* source) {
  DCHECK_EQ(SOURCE_HISTOGRAMS_ATOMIC_FILE, source->type);

  base::PersistentHistogramAllocator::Iterator histogram_iter(
      source->allocator.get());

  int histogram_count = 0;
  while (true) {
    std::unique_ptr<base::HistogramBase> histogram = histogram_iter.GetNext();
    if (!histogram)
      break;
    snapshot_manager->PrepareFinalDelta(histogram.get());
    ++histogram_count;
  }

  source->read_complete = true;
  DVLOG(1) << "Reported " << histogram_count << " histograms from "
           << source->path.value();
}

void FileMetricsProvider::ScheduleSourcesCheck() {
  DCHECK(thread_checker_.CalledOnValidThread());
  if (sources_to_check_.empty())
    return;

  // Create an independent list of sources for checking. This will be Owned()
  // by the reply call given to the task-runner, to be deleted when that call
  // has returned. It is also passed Unretained() to the task itself, safe
  // because that must complete before the reply runs.
  SourceInfoList* check_list = new SourceInfoList();
  std::swap(sources_to_check_, *check_list);
  task_runner_->PostTaskAndReply(
      FROM_HERE,
      base::Bind(&FileMetricsProvider::CheckAndMergeMetricSourcesOnTaskRunner,
                 base::Unretained(check_list)),
      base::Bind(&FileMetricsProvider::RecordSourcesChecked,
                 weak_factory_.GetWeakPtr(), base::Owned(check_list)));
}

void FileMetricsProvider::RecordSourcesChecked(SourceInfoList* checked) {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Sources that still have an allocator at this point are read/write "active"
  // files that may need their contents merged on-demand. If there is no
  // allocator (not a read/write file) but a read was done on the task-runner,
  // try again immediately to see if more is available (in a directory of
  // files). Otherwise, remember the source for checking again at a later time.
  bool did_read = false;
  for (auto iter = checked->begin(); iter != checked->end();) {
    auto temp = iter++;
    SourceInfo* source = temp->get();
    if (source->read_complete) {
      RecordSourceAsRead(source);
      did_read = true;
    }
    if (source->allocator) {
      if (source->association == ASSOCIATE_INTERNAL_PROFILE) {
        sources_with_profile_.splice(sources_with_profile_.end(), *checked,
                                     temp);
      } else {
        sources_mapped_.splice(sources_mapped_.end(), *checked, temp);
      }
    } else {
      sources_to_check_.splice(sources_to_check_.end(), *checked, temp);
    }
  }

  // If a read was done, schedule another one immediately. In the case of a
  // directory of files, this ensures that all entries get processed. It's
  // done here instead of as a loop in CheckAndMergeMetricSourcesOnTaskRunner
  // so that (a) it gives the disk a rest and (b) testing of individual reads
  // is possible.
  if (did_read)
    ScheduleSourcesCheck();
}

void FileMetricsProvider::DeleteFileAsync(const base::FilePath& path) {
  task_runner_->PostTask(FROM_HERE, base::Bind(DeleteFileWhenPossible, path));
}

void FileMetricsProvider::RecordSourceAsRead(SourceInfo* source) {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Persistently record the "last seen" timestamp of the source file to
  // ensure that the file is never read again unless it is modified again.
  if (pref_service_ && !source->prefs_key.empty()) {
    pref_service_->SetInt64(
        metrics::prefs::kMetricsLastSeenPrefix + source->prefs_key,
        source->last_seen.ToInternalValue());
  }
}

void FileMetricsProvider::OnDidCreateMetricsLog() {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Schedule a check to see if there are new metrics to load. If so, they
  // will be reported during the next collection run after this one. The
  // check is run off of the worker-pool so as to not cause delays on the
  // main UI thread (which is currently where metric collection is done).
  ScheduleSourcesCheck();

  // Clear any data for initial metrics since they're always reported
  // before the first call to this method. It couldn't be released after
  // being reported in RecordInitialHistogramSnapshots because the data
  // will continue to be used by the caller after that method returns. Once
  // here, though, all actions to be done on the data have been completed.
  for (const std::unique_ptr<SourceInfo>& source : sources_for_previous_run_)
    DeleteFileAsync(source->path);
  sources_for_previous_run_.clear();
}

bool FileMetricsProvider::ProvideIndependentMetrics(
    SystemProfileProto* system_profile_proto,
    base::HistogramSnapshotManager* snapshot_manager) {
  DCHECK(thread_checker_.CalledOnValidThread());

  while (!sources_with_profile_.empty()) {
    SourceInfo* source = sources_with_profile_.begin()->get();
    DCHECK(source->allocator);

    bool success = false;
    RecordEmbeddedProfileResult(EMBEDDED_PROFILE_ATTEMPT);
    if (PersistentSystemProfile::GetSystemProfile(
            *source->allocator->memory_allocator(), system_profile_proto)) {
      RecordHistogramSnapshotsFromSource(snapshot_manager, source);
      success = true;
      RecordEmbeddedProfileResult(EMBEDDED_PROFILE_FOUND);
    } else {
      RecordEmbeddedProfileResult(EMBEDDED_PROFILE_DROPPED);

      // TODO(bcwhite): Remove these once crbug/695880 is resolved.

      int histogram_count = 0;
      base::PersistentHistogramAllocator::Iterator histogram_iter(
          source->allocator.get());
      while (histogram_iter.GetNext()) {
        ++histogram_count;
      }
      UMA_HISTOGRAM_COUNTS_10000(
          "UMA.FileMetricsProvider.EmbeddedProfile.DroppedHistogramCount",
          histogram_count);

      base::File::Info info;
      if (base::GetFileInfo(source->path, &info)) {
        UMA_HISTOGRAM_CUSTOM_COUNTS(
            "UMA.FileMetricsProvider.EmbeddedProfile.DroppedFileAge",
            (base::Time::Now() - info.last_modified).InMinutes(), 1,
            base::TimeDelta::FromDays(30).InMinutes(), 50);
      }
    }

    // Regardless of whether this source was successfully recorded, it is never
    // read again.
    source->read_complete = true;
    RecordSourceAsRead(source);
    sources_to_check_.splice(sources_to_check_.end(), sources_with_profile_,
                             sources_with_profile_.begin());
    if (success)
      return true;
  }

  return false;
}

bool FileMetricsProvider::HasInitialStabilityMetrics() {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Measure the total time spent checking all sources as well as the time
  // per individual file. This method is called during startup and thus blocks
  // the initial showing of the browser window so it's important to know the
  // total delay.
  SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.InitialCheckTime.Total");

  // Check all sources for previous run to see if they need to be read.
  for (auto iter = sources_for_previous_run_.begin();
       iter != sources_for_previous_run_.end();) {
    SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.InitialCheckTime.File");

    auto temp = iter++;
    SourceInfo* source = temp->get();

    // This would normally be done on a background I/O thread but there
    // hasn't been a chance to run any at the time this method is called.
    // Do the check in-line.
    AccessResult result = CheckAndMapMetricSource(source);
    UMA_HISTOGRAM_ENUMERATION("UMA.FileMetricsProvider.InitialAccessResult",
                              result, ACCESS_RESULT_MAX);

    // If it couldn't be accessed, remove it from the list. There is only ever
    // one chance to record it so no point keeping it around for later. Also
    // mark it as having been read since uploading it with a future browser
    // run would associate it with the then-previous run which would no longer
    // be the run from which it came.
    if (result != ACCESS_RESULT_SUCCESS) {
      DCHECK(!source->allocator);
      RecordSourceAsRead(source);
      DeleteFileAsync(source->path);
      sources_for_previous_run_.erase(temp);
      continue;
    }

    DCHECK(source->allocator);

    // If the source should be associated with an existing internal profile,
    // move it to |sources_with_profile_| for later upload.
    if (source->association == ASSOCIATE_INTERNAL_PROFILE_OR_PREVIOUS_RUN) {
      if (PersistentSystemProfile::HasSystemProfile(
              *source->allocator->memory_allocator())) {
        RecordEmbeddedProfileResult(EMBEDDED_PROFILE_ATTEMPT);
        RecordEmbeddedProfileResult(EMBEDDED_PROFILE_FALLBACK);
        sources_with_profile_.splice(sources_with_profile_.end(),
                                     sources_for_previous_run_, temp);
      }
    }
  }

  return !sources_for_previous_run_.empty();
}

void FileMetricsProvider::RecordInitialHistogramSnapshots(
    base::HistogramSnapshotManager* snapshot_manager) {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Measure the total time spent processing all sources as well as the time
  // per individual file. This method is called during startup and thus blocks
  // the initial showing of the browser window so it's important to know the
  // total delay.
  SCOPED_UMA_HISTOGRAM_TIMER(
      "UMA.FileMetricsProvider.InitialSnapshotTime.Total");

  for (const std::unique_ptr<SourceInfo>& source : sources_for_previous_run_) {
    SCOPED_UMA_HISTOGRAM_TIMER(
        "UMA.FileMetricsProvider.InitialSnapshotTime.File");

    // The source needs to have an allocator attached to it in order to read
    // histograms out of it.
    DCHECK(!source->read_complete);
    DCHECK(source->allocator);

    // Dump all histograms contained within the source to the snapshot-manager.
    RecordHistogramSnapshotsFromSource(snapshot_manager, source.get());

    // Update the last-seen time so it isn't read again unless it changes.
    RecordSourceAsRead(source.get());
  }
}

void FileMetricsProvider::MergeHistogramDeltas() {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Measure the total time spent processing all sources as well as the time
  // per individual file. This method is called on the UI thread so it's
  // important to know how much total "jank" may be introduced.
  SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.SnapshotTime.Total");

  for (std::unique_ptr<SourceInfo>& source : sources_mapped_) {
    SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.SnapshotTime.File");
    MergeHistogramDeltasFromSource(source.get());
  }
}

}  // namespace metrics