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

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_util.h"
 #include "base/files/memory_mapped_file.h"
@@ skipping 165 matching lines @@
   DCHECK(thread_checker_.CalledOnValidThread());
   base::PersistentHistogramAllocator::Iterator histogram_iter(
       file->allocator.get());
 
   int histogram_count = 0;
   while (true) {
     scoped_ptr<base::HistogramBase> histogram = histogram_iter.GetNext();
     if (!histogram)
       break;
     if (file->type == FILE_HISTOGRAMS_ATOMIC)
-      snapshot_manager->PrepareAbsoluteTakingOwnership(std::move(histogram));
+      snapshot_manager->PrepareDifferenceTakingOwnership(std::move(histogram));

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

So, why is it important here that we take the (idempotent) difference, rather
than a non-idempotent delta?  It seems kind of wrong to me, but maybe there's
something I'm just not thinking of properly.

[bcwhite, 2016/04/19 16:33:38]

Because in some situations, the data may be read-only.  SetupMetrics is not
writable because it's created by an Administrator process.

Though...  The current implementation is to copy the file into RAM anyway so any
written changes would be discarded anyway.  Using PrepareDelta() could cause a
problem if we ever go back to using a memory-mapped file instead of RAM copy,
but I guess I could back-out the PrepareDifference changes.

Thoughts?

[Ilya Sherman, 2016/04/20 07:12:14]

I'd prefer to back-out the PrepareDifference changes for now if they're not
needed.

For a read-only file, I don't understand how PrepareDelta() could possibly be
different than PrepareAbsolute() -- if it's strictly read-only, then there can't
have been any deltas performed on it.  Right?

[bcwhite, 2016/04/20 16:47:45]

Done.
Uploads could have been done on the data _before_ it was written out to disk. 
It's dumped as a single memory block with both "active" and "logged" counts held
side-by-side.

     else
       snapshot_manager->PrepareDeltaTakingOwnership(std::move(histogram));
     ++histogram_count;
   }
 
   DVLOG(1) << "Reported " << histogram_count << " histograms from "
            << file->path.value();
 }
 
 void FileMetricsProvider::CreateAllocatorForFile(FileInfo* file) {
@@ skipping 85 matching lines @@
     if (file->type == FILE_HISTOGRAMS_ATOMIC) {
       DCHECK(!file->mapped);
       file->allocator.reset();
       file->data.clear();
       RecordFileAsSeen(file.get());
     }
   }
 }
 
 }  // namespace metrics