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

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
===================================================================
--- base/metrics/field_trial.cc	(revision 151948)
+++ base/metrics/field_trial.cc	(working copy)
@@ -53,10 +53,12 @@
 //------------------------------------------------------------------------------
 // FieldTrial methods and members.
 
-FieldTrial::FieldTrial(const std::string& name,
+FieldTrial::FieldTrial(EntropyProvider* entropy_provider,
+                       const std::string& name,
                        const Probability total_probability,
                        const std::string& default_group_name)
-  : name_(name),
+  : entropy_provider_(entropy_provider),
+    name_(name),
     divisor_(total_probability),
     default_group_name_(default_group_name),
     random_(static_cast<Probability>(divisor_ * RandDouble())),
@@ -83,7 +85,7 @@
   }
 
   random_ = static_cast<Probability>(
-      divisor_ * HashClientId(FieldTrialList::client_id(), name_));
+      divisor_ * entropy_provider_->GetEntropyForTrial(name_));

[SteveT, 2012/08/16 20:55:42]

Could |entropy_provider_| be NULL?

[Alexei Svitkine (slow), 2012/08/16 21:27:11]

If it is, then FieldTrialList::IsOneTimeRandomizationEnabled() will be true and
there will be an early return above.

 }
 
 void FieldTrial::Disable() {
@@ -198,27 +200,6 @@
   DVLOG(1) << "Field trial: " << name_ << " Group choice:" << group_name_;
 }
 
-// static
-double FieldTrial::HashClientId(const std::string& client_id,
-                                const std::string& trial_name) {
-  // SHA-1 is designed to produce a uniformly random spread in its output space,
-  // even for nearly-identical inputs, so it helps massage whatever client_id
-  // and trial_name we get into something with a uniform distribution, which
-  // is desirable so that we don't skew any part of the 0-100% spectrum.
-  std::string input(client_id + trial_name);
-  unsigned char sha1_hash[kSHA1Length];
-  SHA1HashBytes(reinterpret_cast<const unsigned char*>(input.c_str()),
-                input.size(),
-                sha1_hash);
-
-  COMPILE_ASSERT(sizeof(uint64) < sizeof(sha1_hash), need_more_data);
-  uint64 bits;
-  memcpy(&bits, sha1_hash, sizeof(bits));
-  bits = base::ByteSwapToLE64(bits);
-
-  return BitsToOpenEndedUnitInterval(bits);
-}
-
 //------------------------------------------------------------------------------
 // FieldTrialList methods and members.
 
@@ -228,9 +209,9 @@
 // static
 bool FieldTrialList::used_without_global_ = false;
 
-FieldTrialList::FieldTrialList(const std::string& client_id)
+FieldTrialList::FieldTrialList(FieldTrial::EntropyProvider* entropy_provider)
     : application_start_time_(TimeTicks::Now()),
-      client_id_(client_id),
+      entropy_provider_(entropy_provider),
       observer_list_(new ObserverListThreadSafe<FieldTrialList::Observer>(
           ObserverListBase<FieldTrialList::Observer>::NOTIFY_EXISTING_ONLY)) {
   DCHECK(!global_);
@@ -279,8 +260,9 @@
     return existing_trial;
   }
 
-  FieldTrial* field_trial =
-      new FieldTrial(name, total_probability, default_group_name);
+  FieldTrial* field_trial = new FieldTrial(global_->entropy_provider_.get(),
+                                           name, total_probability,
+                                           default_group_name);
   if (GetBuildTime() > CreateTimeFromParams(year, month, day_of_month))
     field_trial->Disable();
   FieldTrialList::Register(field_trial);
@@ -399,7 +381,8 @@
     return field_trial;
   }
   const int kTotalProbability = 100;
-  field_trial = new FieldTrial(name, kTotalProbability, group_name);
+  field_trial = new FieldTrial(global_->entropy_provider_.get(),
+                               name, kTotalProbability, group_name);
   // This is where we may assign a group number different from
   // kDefaultGroupNumber to the default group.
   field_trial->AppendGroup(group_name, kTotalProbability);
@@ -452,18 +435,9 @@
     return false;
   }
 
-  return !global_->client_id_.empty();
+  return global_->entropy_provider_.get() != NULL;
 }
 
-// static
-const std::string& FieldTrialList::client_id() {
-  DCHECK(global_);
-  if (!global_)
-    return EmptyString();
-
-  return global_->client_id_;
-}
-
 FieldTrial* FieldTrialList::PreLockedFind(const std::string& name) {
   RegistrationList::iterator it = registered_.find(name);
   if (registered_.end() == it)