Support saving browser metrics to disk and reading them during next run.

components/metrics/file_metrics_provider.cc

Index: components/metrics/file_metrics_provider.cc
diff --git a/components/metrics/file_metrics_provider.cc b/components/metrics/file_metrics_provider.cc
index 44df25c5fd67a62230ded244d2844314996e24ad..d085d8e2dea9a6ac3c5a6eaf24fa41239ab3ca75 100644
--- a/components/metrics/file_metrics_provider.cc
+++ b/components/metrics/file_metrics_provider.cc
@@ -69,6 +69,7 @@ FileMetricsProvider::~FileMetricsProvider() {}
 
 void FileMetricsProvider::RegisterFile(const base::FilePath& path,
                                        FileType type,
+                                       FileAssociation file_association,
                                        const base::StringPiece prefs_key) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
@@ -84,7 +85,15 @@ void FileMetricsProvider::RegisterFile(const base::FilePath& path,
                                 file->prefs_key));
   }
 
-  files_to_check_.push_back(std::move(file));
+  switch (file_association) {
+    case ASSOCIATE_CURRENT_RUN:
+      files_to_check_.push_back(std::move(file));
+      break;
+    case ASSOCIATE_PREVIOUS_RUN:
+      DCHECK_EQ(FILE_HISTOGRAMS_ATOMIC, file->type);
+      files_for_previous_run_.push_back(std::move(file));
+      break;
+  }
 }
 
 // static
@@ -187,7 +196,7 @@ void FileMetricsProvider::RecordHistogramSnapshotsFromFile(
     if (!histogram)
       break;
     if (file->type == FILE_HISTOGRAMS_ATOMIC)
-      snapshot_manager->PrepareAbsoluteTakingOwnership(std::move(histogram));
+      snapshot_manager->PrepareFinalDeltaTakingOwnership(std::move(histogram));
     else
       snapshot_manager->PrepareDeltaTakingOwnership(std::move(histogram));
     ++histogram_count;
@@ -268,6 +277,46 @@ void FileMetricsProvider::OnDidCreateMetricsLog() {
   // 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).
   ScheduleFilesCheck();
+
+  // Clear any data for initial metrics since they're always reported
+  // before the first call to this method.

[Ilya Sherman, 2016/05/05 22:22:52]

Please document here why it's not appropriate for the data to be cleared sooner.

[bcwhite, 2016/05/06 16:59:27]

Done.

+#if DCHECK_IS_ON()
+  for (const std::unique_ptr<FileInfo>& file : files_for_previous_run_)
+    DCHECK(file->read_complete);
+#endif
+  files_for_previous_run_.clear();
+}
+
+bool FileMetricsProvider::HasInitialStabilityMetrics() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // Check all files for previous run to see if they need to be read.
+  for (auto iter = files_for_previous_run_.begin();
+       iter != files_for_previous_run_.end();) {
+    SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.InitialCheckTime");
+
+    auto temp = iter++;
+    FileInfo* file = 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 = CheckAndMapNewMetrics(file);
+    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 no point keeping it around for later. Also mark

[Ilya Sherman, 2016/05/05 22:22:52]

nit: "it no" -> "it, so no"

[bcwhite, 2016/05/06 16:59:27]

Done.

+    // it as having been read since uploading it with a future browser run
+    // would associate it the previous run which would no longer be the run
+    // from which it came.
+    if (result != ACCESS_RESULT_SUCCESS) {
+      RecordFileAsSeen(file);
+      files_for_previous_run_.erase(temp);
+    }
+  }
+
+  return !files_for_previous_run_.empty();
 }
 
 void FileMetricsProvider::RecordHistogramSnapshots(
@@ -279,6 +328,8 @@ void FileMetricsProvider::RecordHistogramSnapshots(
     if (file->read_complete)
       continue;
 
+    SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.SnapshotTime");

[Ilya Sherman, 2016/05/05 22:22:52]

Please also add a histogram to measure the snapshot time for all files, in case
we somehow pile up a lot of files, each of which is individually not too slow to
be read.  (Ditto for the other timing histograms.)

[bcwhite, 2016/05/06 16:59:27]

Done.

+
     // If the file is mapped or loaded then it needs to have an allocator
     // attached to it in order to read histograms out of it.
     if (file->mapped || !file->data.empty())
@@ -300,4 +351,25 @@ void FileMetricsProvider::RecordHistogramSnapshots(
   }
 }
 
+void FileMetricsProvider::RecordInitialHistogramSnapshots(
+    base::HistogramSnapshotManager* snapshot_manager) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  for (const std::unique_ptr<FileInfo>& file : files_for_previous_run_) {
+    SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.InitialSnapshotTime");
+
+    // The file needs to have an allocator attached to it in order to read
+    // histograms out of it.
+    DCHECK(file->mapped || !file->data.empty());
+    CreateAllocatorForFile(file.get());
+    DCHECK(file->allocator);
+
+    // Dump all histograms contained within the file to the snapshot-manager.
+    RecordHistogramSnapshotsFromFile(snapshot_manager, file.get());
+
+    // Update the last-seen time so it isn't read again unless it changes.
+    RecordFileAsSeen(file.get());
+  }
+}
+
 }  // namespace metrics