Persist setup metrics and have Chrome report them during UMA upload.

components/metrics/file_metrics_provider.h

Index: components/metrics/file_metrics_provider.h
diff --git a/components/metrics/file_metrics_provider.h b/components/metrics/file_metrics_provider.h
new file mode 100644
index 0000000000000000000000000000000000000000..2962b0d688b2d40d503830301aded4e2a18ac83d
--- /dev/null
+++ b/components/metrics/file_metrics_provider.h
@@ -0,0 +1,118 @@
+// 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.
+
+#ifndef COMPONENTS_METRICS_FILE_METRICS_PROVIDER_H_
+#define COMPONENTS_METRICS_FILE_METRICS_PROVIDER_H_
+
+#include <list>
+#include <string>
+
+#include "base/callback.h"
+#include "base/files/file_path.h"
+#include "base/gtest_prod_util.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/synchronization/lock.h"
+#include "base/time/time.h"
+#include "components/metrics/metrics_provider.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace base {
+class MemoryMappedFile;
+class PersistentMemoryAllocator;
+class TaskRunner;
+}
+
+namespace metrics {
+
+// FileMetricsProvider gathers and logs histograms written to files on disk.
+// Any number of files can be registered and will be polled once per upload
+// cycle (at startup and about every 30 minutes thereafter) for data to send.
+class FileMetricsProvider
+    : public metrics::MetricsProvider {
+ public:
+  enum FileType {
+    // "Atomic" files are a collection of histograms that are written
+    // completely in a single atomic operation (typically a write followed
+    // by an atomic rename) and the file is never updated again except to
+    // be replaced by a completely new set of histograms. This is the only
+    // option that can be used if the file is not writeable by *this*
+    // process.
+    FILE_HISTOGRAMS_ATOMIC,
+
+    // "Active" files may be open by one or more other processes and updated
+    // at any time with new samples or new histograms. Such files may also be
+    // inactive for any period of time only to be opened again and have new
+    // data written to them. Because the file will be actively watched by *this*
+    // process, the underlying file cannot be deleted on operating systems
+    // that do not support delete-while-open (e.g. Windows).
+    // TODO(bcwhite): Enable when read/write mem-mapped files are supported.
+    //FILE_HISTOGRAMS_ACTIVE,
+  };
+
+ private:
+  // This is fully defined in the header file (rather than just a forward
+  // declaration) so it can be known to tests.

[grt (UTC plus 2), 2016/02/15 15:42:39]

this is no longer true. is now a good time to move it into the .cc file?

[bcwhite, 2016/02/15 19:22:11]

Done.

+  struct FileInformation {
+    FileInformation();
+    ~FileInformation();
+
+    base::FilePath path;
+    FileType type;
+    std::string prefs_key;
+    base::Time last_seen;
+    scoped_ptr<base::MemoryMappedFile> mapped;

[grt (UTC plus 2), 2016/02/15 15:42:39]

i think it would be useful to document the various valid states for these
members. it's a bit complicated.

[bcwhite, 2016/02/15 19:22:11]

Done.

+    scoped_ptr<base::MemoryMappedFile> file_to_release;
+    scoped_ptr<base::PersistentMemoryAllocator> allocator;
+    scoped_ptr<base::PersistentMemoryAllocator> allocator_to_release;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(FileInformation);
+  };
+
+ public:
+  using FileInformationList = std::list<scoped_ptr<FileInformation>>;
+
+  FileMetricsProvider(const scoped_refptr<base::TaskRunner>& task_runner,
+                      PrefService* local_state);
+  ~FileMetricsProvider() override;
+
+  // Indicate a file to be monitored and how the file is used. Because some
+  // metadata must persist across process restarts, preferences entries are
+  // used based on the |prefs_key| name. Call RegisterPrefs() with the same
+  // name to create the necessary keys in advance.
+  void RegisterFile(const base::FilePath& path, FileType type,
+                    const base::StringPiece& prefs_key);
+
+  static void RegisterPrefs(PrefRegistrySimple* prefs,
+                            const base::StringPiece& key);
+
+ private:
+  friend class FileMetricsProviderTest;
+  FRIEND_TEST_ALL_PREFIXES(FileMetricsProviderTest, AccessMetrics);
+
+  static void CheckAndMapNewMetricFilesOnTaskRunner(
+      FileInformationList* files);
+  static bool CheckAndMapNewMetrics(FileInformation* file);
+
+  void ScheduleFilesCheck();
+  void RecordFilesChecked(FileInformationList* checked);
+  void RecordFileAsSeen(FileInformation* file);
+
+  // metrics::MetricsDataProvider:
+  void OnDidCreateMetricsLog() override;
+  void RecordHistogramSnapshots(base::HistogramSnapshotManager* hsm) override;
+
+  scoped_refptr<base::TaskRunner> task_runner_;
+  FileInformationList files_to_check_;
+  FileInformationList files_to_read_;
+  PrefService* pref_service_;
+
+  DISALLOW_COPY_AND_ASSIGN(FileMetricsProvider);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_FILE_METRICS_PROVIDER_H_