Use a different algorithm with the low entropy source for field trials.

chrome/browser/metrics/metrics_service.cc

Index: chrome/browser/metrics/metrics_service.cc
===================================================================
--- chrome/browser/metrics/metrics_service.cc	(revision 152081)
+++ chrome/browser/metrics/metrics_service.cc	(working copy)
@@ -185,6 +185,7 @@
 #include "chrome/common/chrome_notification_types.h"
 #include "chrome/common/chrome_result_codes.h"
 #include "chrome/common/chrome_switches.h"
+#include "chrome/common/metrics/entropy_provider.h"
 #include "chrome/common/metrics/metrics_log_manager.h"
 #include "chrome/common/net/test_server_locations.h"
 #include "chrome/common/pref_names.h"
@@ -279,7 +280,7 @@
 // Generates a new non-identifying entropy source used to seed persistent
 // activities.
 int GenerateLowEntropySource() {
-  return base::RandInt(1, kMaxEntropySize);
+  return base::RandInt(0, kMaxEntropySize - 1);
 }
 
 // Converts an exit code into something that can be inserted into our
@@ -344,7 +345,7 @@
   return codes;
 }
 
-}
+}  // namespace
 
 // static
 MetricsService::ShutdownCleanliness MetricsService::clean_shutdown_status_ =
@@ -539,7 +540,11 @@
   return client_id_;
 }
 
-std::string MetricsService::GetEntropySource(bool reporting_will_be_enabled) {
+scoped_ptr<base::FieldTrial::EntropyProvider>
+    MetricsService::GetEntropyProvider(bool reporting_will_be_enabled) {
+  scoped_ptr<base::FieldTrial::EntropyProvider> entropy_provider;

[Ilya Sherman, 2012/08/17 17:53:20]

Optional nit: I think you can still return the entropy provider directly, rather
than storing it in a local variable, like so:

return scoped_ptr<base::FieldTrial::EntropyProvider(new
SHA1EntropyProvider(high_entropy_source));

(Note that .Pass() is not required if you declare the scoped_ptr<> as part of
the return statement.)

[Alexei Svitkine (slow), 2012/08/20 15:57:40]

Done.

+  const int low_entropy_source = GetLowEntropySource();

[jar (doing other things), 2012/08/17 19:06:19]

nit: move this definition closer to first (and only?) use (which is inside a
conditional).

[Alexei Svitkine (slow), 2012/08/20 15:57:40]

I've inlined this into the two callers.

+
   // For metrics reporting-enabled users, we combine the client ID and low
   // entropy source to get the final entropy source. Otherwise, only use the low
   // entropy source.
@@ -547,13 +552,18 @@
   //  1) It makes the entropy source less identifiable for parties that do not
   //     know the low entropy source.
   //  2) It makes the final entropy source resettable.
-  std::string low_entropy_source = base::IntToString(GetLowEntropySource());
   if (reporting_will_be_enabled) {
     entropy_source_returned_ = LAST_ENTROPY_HIGH;
-    return client_id_ + low_entropy_source;
+    const std::string high_entropy_source =
+        client_id_ + base::IntToString(low_entropy_source);
+    entropy_provider.reset(new SHA1EntropyProvider(high_entropy_source));
+  } else {
+    entropy_source_returned_ = LAST_ENTROPY_LOW;
+    entropy_provider.reset(new PermutedEntropyProvider(GetLowEntropySource(),

[hfung, 2012/08/17 17:35:40]

GetLowEntropySource() -> low_entropy_source?

[Alexei Svitkine (slow), 2012/08/17 17:57:40]

Done.

+                                                       kMaxEntropySize));
   }
-  entropy_source_returned_ = LAST_ENTROPY_LOW;
-  return low_entropy_source;
+
+  return entropy_provider.Pass();
 }
 
 void MetricsService::ForceClientIdCreation() {