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

base/metrics/persistent_histogram_allocator.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 "base/metrics/persistent_histogram_allocator.h"
 
 #include <memory>
 
+#include "base/files/important_file_writer.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/synchronization/lock.h"
 
@@ skipping 537 matching lines @@
     // the method GetCreateHistogramResultHistogram() *before* setting
     // the (temporary) memory allocator via SetGlobalAllocator() so that
     // histogram is instead allocated from the process heap.
     DCHECK_NE(kResultHistogram, histogram_data->name);
   }
 
   g_allocator = nullptr;
   return WrapUnique(histogram_allocator);
 };
 
+void GlobalHistogramAllocator::SetPersistentLocation(FilePath location) {
+  persistent_location_ = std::move(location);
+}
+
+bool GlobalHistogramAllocator::WritePersistentLocation() {
+#if defined(OS_NACL)
+  // NACL doesn't support file operations, including ImportantFileWriter.
+  NOTREACHED();
+  return false;
+#else
+  // Stop if no destination is set, perhaps because it has not been enabled.
+  if (persistent_location_.empty())
+    return false;

[Ilya Sherman, 2016/04/19 00:57:46]

Hmm.  Could this be a DCHECK instead?  That is, are there legitimate expected
use cases where the destination would not be set?

[bcwhite, 2016/04/19 16:33:37]

Right now, it's mostly unset because the experiment is only at 10%.  It could
also be unset if for some reason the process starts to exit (crashes?) before
higher layers start and set the destination.

+
+  StringPiece contents(static_cast<const char*>(data()), used());
+  if (!ImportantFileWriter::WriteFileAtomically(persistent_location_,
+                                                contents)) {
+    LOG(ERROR) << "Could not write persistent histograms to file: "
+               << persistent_location_.value();
+    return false;
+  }
+
+  return true;
+#endif
+}
+
 GlobalHistogramAllocator::GlobalHistogramAllocator(
     std::unique_ptr<PersistentMemoryAllocator> memory)
     : PersistentHistogramAllocator(std::move(memory)),
       import_iterator_(this) {}
 
 void GlobalHistogramAllocator::ImportHistogramsToStatisticsRecorder() {
   // Skip the import if it's the histogram that was last created. Should a
   // race condition cause the "last created" to be overwritten before it
   // is recognized here then the histogram will be created and be ignored
   // when it is detected as a duplicate by the statistics-recorder. This
   // simple check reduces the time of creating persistent histograms by
   // about 40%.
   Reference record_to_ignore = last_created();
 
   // There is no lock on this because the iterator is lock-free while still
   // guaranteed to only return each entry only once. The StatisticsRecorder
   // has its own lock so the Register operation is safe.
   while (true) {
     std::unique_ptr<HistogramBase> histogram =
         import_iterator_.GetNextWithIgnore(record_to_ignore);
     if (!histogram)
       break;
     StatisticsRecorder::RegisterOrDeleteDuplicate(histogram.release());
   }
 }
 
 }  // namespace base