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 151948)
+++ 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 +540,10 @@
   return client_id_;
 }
 
-std::string MetricsService::GetEntropySource(bool reporting_will_be_enabled) {
+base::FieldTrial::EntropyProvider* MetricsService::GetEntropyProvider(
+    bool reporting_will_be_enabled) {
+  const int low_entropy_source = GetLowEntropySource();
+
   // 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 +551,15 @@
   //  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);
+    return new SHA1EntropyProvider(high_entropy_source);

[Ilya Sherman, 2012/08/17 07:34:28]

I'm not a fan of functions that return heap-allocated memory.  This places the
burden on the caller to remember to free the memory, or else we leak it.  Can
you return a scoped_ptr instead, e.g. using the Pass() method to transfer
ownership?

[Alexei Svitkine (slow), 2012/08/17 14:08:59]

Done.

   }
+
   entropy_source_returned_ = LAST_ENTROPY_LOW;
-  return low_entropy_source;
+  return new PermutedEntropyProvider(GetLowEntropySource(), kMaxEntropySize);
 }
 
 void MetricsService::ForceClientIdCreation() {