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 c867c27f30fc36fd14ce8270a654f4d426cd6ec4..20f1afd71ac9cd0f3487c60bd17931ddc4f8aaae 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.cc
+++ b/chrome/browser/metrics/chrome_metrics_service_client.cc
@@ -180,6 +180,27 @@ CreateInstallerFileMetricsProvider(bool metrics_reporting_enabled) {
   return file_metrics_provider;
 }
 
+void CleanUpGlobalPersistentHistogramStorage() {
+  // 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, 2016/06/29 20:41:38]

nit: I'd move this comment to be just above the function, and also add a bit of
text mentioning that this basically trades off slower shutdown vs. slower
startup.

[bcwhite, 2016/06/30 22:56:12]

Done.

+  base::GlobalHistogramAllocator* allocator =
+      base::GlobalHistogramAllocator::Get();
+  if (!allocator)
+    return;
+
+  const base::FilePath& path = allocator->GetPersistentLocation();
+  if (path.empty())
+    return;
+
+  // Open (with delete) and then immediately close the file by going out

[Alexei Svitkine (slow), 2016/06/29 19:13:43]

Nit: Wrapping is off? I think some words below should fit on this line.

[bcwhite, 2016/06/30 22:56:12]

One word.

+  // of scope. This is the only cross-platform safe way to delete a file
+  // that may be open elsewhere.

[Ilya Sherman, 2016/06/29 20:41:38]

If the file is open elsewhere, is there anything that guarantees that there
won't be any further attempts to read from or write to the file?

[bcwhite, 2016/06/30 22:56:12]

None.  Existing opens will continue to operate normally.  New opens will fail
and the actual contents will be free'd once the last open handle gets closed.

[Ilya Sherman, 2016/06/30 23:16:48]

Ah, nice.  I think this would be great to document explicitly in this comment --
that behavior wasn't obvious to me.

[bcwhite, 2016/07/11 18:06:39]

Will do.

+  base::File file(path, base::File::FLAG_OPEN | base::File::FLAG_READ |
+                        base::File::FLAG_DELETE_ON_CLOSE);
+}
+
 }  // namespace
 
 
@@ -211,6 +232,7 @@ ChromeMetricsServiceClient::ChromeMetricsServiceClient(
 
 ChromeMetricsServiceClient::~ChromeMetricsServiceClient() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  CleanUpGlobalPersistentHistogramStorage();
 }
 
 // static