Support experiment to store persistent metrics in memory-mapped file.

chrome/browser/metrics/chrome_metrics_service_client.cc

Index: chrome/browser/metrics/chrome_metrics_service_client.cc
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.cc b/chrome/browser/metrics/chrome_metrics_service_client.cc
index 2861db7cdd25856894100c062c4ab3cc835428f0..b7d9607b68794c06b08544bdb981df8317caf112 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.cc
+++ b/chrome/browser/metrics/chrome_metrics_service_client.cc
@@ -182,6 +182,22 @@ CreateInstallerFileMetricsProvider(bool metrics_reporting_enabled) {
   return file_metrics_provider;
 }
 
+void CleanupGlobalPersistentHistogramStorage() {

[Ilya Sherman, 2016/06/17 19:54:32]

nit: s/Cleanup/CleanUp

[bcwhite, 2016/06/20 16:21:28]

Done.

+  // If there is a global metrics file being updated on disk, mark it to be
+  // deleted when the process exits. A normal shutdown is almost complete
+  // so there is no benefit in keeping a file with no new data to be processed
+  // during the next startup sequence.

[Ilya Sherman, 2016/06/17 19:54:32]

To clarify: Is there any harm in keeping the file?  Would metrics be
doubly-reported if it were to remain?  Or is this purely an optimization?  If
it's purely an optimization, I wonder whether it's important enough to do -- it
slows down shutdown a bit, and makes the code a bit harder to reason about. 
OTOH, if it's to avoid duplicated data, I worry that it might not be robust
enough.

[bcwhite, 2016/06/20 16:21:28]

It's very important so that startup doesn't process all the data in the file
when there is nothing in it.  Due to a metrics collection just previous in the
shutdown process, everything should be zeros.

Collection of the "previous run" information is done in the foreground during
startup so gathering this data delays the appearance of the first browser
window.  Removing this file during clean exit means no penalty on the next
startup.

+  base::GlobalHistogramAllocator* allocator =
+      base::GlobalHistogramAllocator::Get();
+  if (allocator) {
+    const base::FilePath& path = allocator->GetPersistentLocation();
+    if (!path.empty()) {
+      base::File file(path, base::File::FLAG_OPEN | base::File::FLAG_READ |
+                            base::File::FLAG_DELETE_ON_CLOSE);

[Ilya Sherman, 2016/06/17 19:54:32]

nit: This seems like a somewhat indirect way to delete a file.  Is there not
something more direct in our APIs?

[bcwhite, 2016/06/20 16:21:28]

Unfortunately no.  base::DeleteFile() won't work on an open file on Windows.

+    }
+  }
+}
+
 }  // namespace
 
 
@@ -213,6 +229,7 @@ ChromeMetricsServiceClient::ChromeMetricsServiceClient(
 
 ChromeMetricsServiceClient::~ChromeMetricsServiceClient() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  CleanupGlobalPersistentHistogramStorage();
 }
 
 // static