Add one-time randomization support for FieldTrial, and the ability to

disable field trials. I am going to have a need for both soon.

Update some documentation about empty trial names, add TrialExists() method and update many call-sites to use this (it simplifies the previous logic which checked for existence and then for non-empty name, which can no longer happen).

Refactor a bit in browser_main.

While I'm in there and needing base/OWNERS approval, add an OWNERS
file for base/metrics that adds jar@chromium.org as an owner for that
directory.

Initially committed as r84197.
Rolled back due to DCHECK in official builds, r84373.
Will re-submit with fix.

BUG=81750
TEST=base_unittests

Review URL: http://codereview.chromium.org/6883102

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@84801 0039d316-1c4b-4281-b951-d872f2087c98

6 files changed, 298 insertions, 51 deletions

diff --git a/base/metrics/OWNERS b/base/metrics/OWNERS
new file mode 100644
index 0000000..8b39a54
--- /dev/null
+++ b/base/metrics/OWNERS
@@ -0,0 +1 @@
+jar@chromium.org
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index 03e5809..b86b9cc 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -6,6 +6,8 @@
 
 #include "base/logging.h"
 #include "base/rand_util.h"
+#include "base/sha1.h"
+#include "base/string_util.h"
 #include "base/stringprintf.h"
 #include "base/utf_string_conversions.h"
 
@@ -41,11 +43,13 @@ FieldTrial::FieldTrial(const std::string& name,
   : name_(name),
     divisor_(total_probability),
     default_group_name_(default_group_name),
-    random_(static_cast<Probability>(divisor_ * base::RandDouble())),
+    random_(static_cast<Probability>(divisor_ * RandDouble())),
     accumulated_group_probability_(0),
-    next_group_number_(kDefaultGroupNumber+1),
-    group_(kNotFinalized) {
+    next_group_number_(kDefaultGroupNumber + 1),
+    group_(kNotFinalized),
+    enable_field_trial_(true) {
   DCHECK_GT(total_probability, 0);
+  DCHECK(!name_.empty());
   DCHECK(!default_group_name_.empty());
   FieldTrialList::Register(this);
 
@@ -55,7 +59,7 @@ FieldTrial::FieldTrial(const std::string& name,
   DCHECK_GT(day_of_month, 0);
   DCHECK_LT(day_of_month, 32);
 
-  base::Time::Exploded exploded;
+  Time::Exploded exploded;
   exploded.year = year;
   exploded.month = month;
   exploded.day_of_week = 0;  // Should be unused.
@@ -65,8 +69,33 @@ FieldTrial::FieldTrial(const std::string& name,
   exploded.second = 0;
   exploded.millisecond = 0;
 
-  base::Time expiration_time = Time::FromLocalExploded(exploded);
-  disable_field_trial_ = (GetBuildTime() > expiration_time) ? true : false;
+  Time expiration_time = Time::FromLocalExploded(exploded);
+  if (GetBuildTime() > expiration_time)
+    Disable();
+}
+
+void FieldTrial::UseOneTimeRandomization() {
+  DCHECK_EQ(group_, kNotFinalized);
+  DCHECK_EQ(kDefaultGroupNumber + 1, next_group_number_);
+  if (!FieldTrialList::IsOneTimeRandomizationEnabled()) {
+    NOTREACHED();
+    Disable();
+    return;
+  }
+
+  random_ = static_cast<Probability>(
+      divisor_ * HashClientId(FieldTrialList::client_id(), name_));
+}
+
+void FieldTrial::Disable() {
+  enable_field_trial_ = false;
+
+  // In case we are disabled after initialization, we need to switch
+  // the trial to the default group.
+  if (group_ != kNotFinalized) {
+    group_ = kDefaultGroupNumber;
+    group_name_ = default_group_name_;
+  }
 }
 
 int FieldTrial::AppendGroup(const std::string& name,
@@ -74,7 +103,7 @@ int FieldTrial::AppendGroup(const std::string& name,
   DCHECK_LE(group_probability, divisor_);
   DCHECK_GE(group_probability, 0);
 
-  if (enable_benchmarking_ || disable_field_trial_)
+  if (enable_benchmarking_ || !enable_field_trial_)
     group_probability = 0;
 
   accumulated_group_probability_ += group_probability;
@@ -84,7 +113,7 @@ int FieldTrial::AppendGroup(const std::string& name,
     // This is the group that crossed the random line, so we do the assignment.
     group_ = next_group_number_;
     if (name.empty())
-      base::StringAppendF(&group_name_, "%d", group_);
+      StringAppendF(&group_name_, "%d", group_);
     else
       group_name_ = name;
     FieldTrialList::NotifyFieldTrialGroupSelection(name_, group_name_);
@@ -103,7 +132,8 @@ int FieldTrial::group() {
 }
 
 std::string FieldTrial::group_name() {
-  group();  // call group() to make group assignment was done.
+  group();  // call group() to make sure group assignment was done.
+  DCHECK(!group_name_.empty());
   return group_name_;
 }
 
@@ -133,6 +163,25 @@ Time FieldTrial::GetBuildTime() {
   return integral_build_time;
 }
 
+// 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[SHA1_LENGTH];
+  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 = reinterpret_cast<uint64*>(&sha1_hash[0]);
+
+  return BitsToOpenEndedUnitInterval(*bits);
+}
+
 //------------------------------------------------------------------------------
 // FieldTrialList methods and members.
 
@@ -142,8 +191,9 @@ FieldTrialList* FieldTrialList::global_ = NULL;
 // static
 bool FieldTrialList::register_without_global_ = false;
 
-FieldTrialList::FieldTrialList()
+FieldTrialList::FieldTrialList(const std::string& client_id)
     : application_start_time_(TimeTicks::Now()),
+      client_id_(client_id),
       observer_list_(ObserverList<Observer>::NOTIFY_EXISTING_ONLY) {
   DCHECK(!global_);
   DCHECK(!register_without_global_);
@@ -204,19 +254,23 @@ std::string FieldTrialList::FindFullName(const std::string& name) {
 }
 
 // static
+bool FieldTrialList::TrialExists(const std::string& name) {
+  return Find(name) != NULL;
+}
+
+// static
 void FieldTrialList::StatesToString(std::string* output) {
   if (!global_)
     return;
   DCHECK(output->empty());
   AutoLock auto_lock(global_->lock_);
+
   for (RegistrationList::iterator it = global_->registered_.begin();
        it != global_->registered_.end(); ++it) {
     const std::string name = it->first;
     std::string group_name = it->second->group_name_internal();
     if (group_name.empty())
-      // No definitive winner in this trial, use default_group_name as the
-      // group_name.
-      group_name = it->second->default_group_name();
+      continue;  // Should not include uninitialized trials.
     DCHECK_EQ(name.find(kPersistentStringSeparator), std::string::npos);
     DCHECK_EQ(group_name.find(kPersistentStringSeparator), std::string::npos);
     output->append(name);
@@ -313,6 +367,24 @@ size_t FieldTrialList::GetFieldTrialCount() {
   return global_->registered_.size();
 }
 
+// static
+bool FieldTrialList::IsOneTimeRandomizationEnabled() {
+  DCHECK(global_);
+  if (!global_)
+    return false;
+
+  return !global_->client_id_.empty();
+}
+
+// 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)
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index b8ab0c5..4e89ccf 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -13,12 +13,13 @@
 // pseudo-randomly selected).
 //
 // States are typically generated randomly, either based on a one time
-// randomization (generated randomly once, and then persistently reused in the
-// client during each future run of the program), or by a startup randomization
-// (generated each time the application starts up, but held constant during the
-// duration of the process), or by continuous randomization across a run (where
-// the state can be recalculated again and again, many times during a process).
-// Only startup randomization is implemented thus far.
+// randomization (which will yield the same results, in terms of selecting
+// the client for a field trial or not, for every run of the program on a
+// given machine), or by a startup randomization (generated each time the
+// application starts up, but held constant during the duration of the
+// process), or by continuous randomization across a run (where the state
+// can be recalculated again and again, many times during a process).
+// Continuous randomization is not yet implemented.
 
 //------------------------------------------------------------------------------
 // Example:  Suppose we have an experiment involving memory, such as determining
@@ -52,10 +53,9 @@
 // to randomly be assigned:
 
 // HISTOGRAM_COUNTS("Memory.RendererTotal", count);  // The original histogram.
-// static bool use_memoryexperiment_histogram(
-//     base::FieldTrialList::Find("MemoryExperiment") &&
-//     !base::FieldTrialList::Find("MemoryExperiment")->group_name().empty());
-// if (use_memoryexperiment_histogram) {
+// static const bool memory_renderer_total_trial_exists =
+//     FieldTrialList::TrialExists("Memory.RendererTotal");
+// if (memory_renderer_total_trial_exists) {
 //   HISTOGRAM_COUNTS(FieldTrial::MakeName("Memory.RendererTotal",
 //                                         "MemoryExperiment"), count);
 // }
@@ -103,15 +103,35 @@ class BASE_API FieldTrial : public RefCounted<FieldTrial> {
 
   // The name is used to register the instance with the FieldTrialList class,
   // and can be used to find the trial (only one trial can be present for each
-  // name).
+  // name).  |name| and |default_group_name| may not be empty.
+  //
   // Group probabilities that are later supplied must sum to less than or equal
   // to the total_probability. Arguments year, month and day_of_month specify
   // the expiration time. If the build time is after the expiration time then
   // the field trial reverts to the 'default' group.
+  //
+  // Using this constructor creates a startup-randomized FieldTrial. If you
+  // want a one-time randomized trial, call UseOneTimeRandomization() right
+  // after construction.
   FieldTrial(const std::string& name, Probability total_probability,
              const std::string& default_group_name, const int year,
              const int month, const int day_of_month);
 
+  // Changes the field trial to use one-time randomization, i.e. produce the
+  // same result for the current trial on every run of this client. Must be
+  // called right after construction.
+  //
+  // Before using this method, |FieldTrialList::EnableOneTimeRandomization()|
+  // must be called exactly once.
+  void UseOneTimeRandomization();
+
+  // Disables this trial, meaning it always determines the default group
+  // has been selected. May be called immediately after construction, or
+  // at any time after initialization (should not be interleaved with
+  // AppendGroup calls). Once disabled, there is no way to re-enable a
+  // trial.
+  void Disable();
+
   // Establish the name and probability of the next group in this trial.
   // Sometimes, based on construction randomization, this call may cause the
   // provided group to be *THE* group selected for use in this instance.
@@ -124,18 +144,17 @@ class BASE_API FieldTrial : public RefCounted<FieldTrial> {
   // Return the randomly selected group number that was assigned.
   // Return kDefaultGroupNumber if the instance is in the 'default' group.
   // Note that this will force an instance to participate, and make it illegal
-  // to attempt to probabalistically add any other groups to the trial.
+  // to attempt to probabilistically add any other groups to the trial.
   int group();
 
-  // If the field trial is not in an experiment, this returns the empty string.
-  // if the group's name is empty, a name of "_" concatenated with the group
+  // If the group's name is empty, a string version containing the group
   // number is used as the group name.
   std::string group_name();
 
   // Return the default group name of the FieldTrial.
   std::string default_group_name() const { return default_group_name_; }
 
-  // Helper function for the most common use: as an argument to specifiy the
+  // Helper function for the most common use: as an argument to specify the
   // name of a HISTOGRAM.  Use the original histogram name as the name_prefix.
   static std::string MakeName(const std::string& name_prefix,
                               const std::string& trial_name);
@@ -155,6 +174,9 @@ class BASE_API FieldTrial : public RefCounted<FieldTrial> {
   FRIEND_TEST(FieldTrialTest, Save);
   FRIEND_TEST(FieldTrialTest, DuplicateRestore);
   FRIEND_TEST(FieldTrialTest, MakeName);
+  FRIEND_TEST(FieldTrialTest, HashClientId);
+  FRIEND_TEST(FieldTrialTest, HashClientIdIsUniform);
+  FRIEND_TEST(FieldTrialTest, UseOneTimeRandomization);
 
   friend class base::FieldTrialList;
 
@@ -168,8 +190,13 @@ class BASE_API FieldTrial : public RefCounted<FieldTrial> {
   // Get build time.
   static Time GetBuildTime();
 
+  // Calculates a uniformly-distributed double between [0.0, 1.0) given
+  // a |client_id| and a |trial_name| (the latter is used as salt to avoid
+  // separate one-time randomized trials from all having the same results).
+  static double HashClientId(const std::string& client_id,
+                             const std::string& trial_name);
+
   // The name of the field trial, as can be found via the FieldTrialList.
-  // This is empty of the trial is not in the experiment.
   const std::string name_;
 
   // The maximum sum of all probabilities supplied, which corresponds to 100%.
@@ -182,7 +209,7 @@ class BASE_API FieldTrial : public RefCounted<FieldTrial> {
   // The randomly selected probability that is used to select a group (or have
   // the instance not participate).  It is the product of divisor_ and a random
   // number between [0, 1).
-  const Probability random_;
+  Probability random_;
 
   // Sum of the probabilities of all appended groups.
   Probability accumulated_group_probability_;
@@ -193,13 +220,13 @@ class BASE_API FieldTrial : public RefCounted<FieldTrial> {
   // This is kNotFinalized if no group has been assigned.
   int group_;
 
-  // A textual name for the randomly selected group. If this Trial is not a
-  // member of an group, this string is empty.
+  // A textual name for the randomly selected group. Valid after |group()|
+  // has been called.
   std::string group_name_;
 
-  // When disable_field_trial_ is true, field trial reverts to the 'default'
+  // When enable_field_trial_ is false, field trial reverts to the 'default'
   // group.
-  bool disable_field_trial_;
+  bool enable_field_trial_;
 
   // When benchmarking is enabled, field trials all revert to the 'default'
   // group.
@@ -234,7 +261,12 @@ class BASE_API FieldTrialList {
   };
 
   // This singleton holds the global list of registered FieldTrials.
-  FieldTrialList();
+  //
+  // |client_id| should be an opaque, diverse ID for this client that does not
+  // change between sessions, to enable one-time randomized trials. The empty
+  // string may be provided, in which case one-time randomized trials will
+  // not be available.
+  explicit FieldTrialList(const std::string& client_id);
   // Destructor Release()'s references to all registered FieldTrial instances.
   ~FieldTrialList();
 
@@ -246,15 +278,22 @@ class BASE_API FieldTrialList {
   // registered, or to retrieve a pointer to it from the global map.
   static FieldTrial* Find(const std::string& name);
 
+  // Returns the group number chosen for the named trial, or
+  // FieldTrial::kNotFinalized if the trial does not exist.
   static int FindValue(const std::string& name);
 
+  // Returns the group name chosen for the named trial, or the
+  // empty string if the trial does not exist.
   static std::string FindFullName(const std::string& name);
 
-  // Create a persistent representation of all FieldTrial instances for
-  // resurrection in another process.  This allows randomization to be done in
-  // one process, and secondary processes can by synchronized on the result.
-  // The resulting string contains only the names, the trial name, and a "/"
-  // separator.
+  // Returns true if the named trial has been registered.
+  static bool TrialExists(const std::string& name);
+
+  // Create a persistent representation of all FieldTrial instances and the
+  // |client_id()| state for resurrection in another process.  This allows
+  // randomization to be done in one process, and secondary processes can by
+  // synchronized on the result. The resulting string contains the
+  // |client_id()|, the names, the trial name, and a "/" separator.
   static void StatesToString(std::string* output);
 
   // Use a previously generated state string (re: StatesToString()) augment the
@@ -263,7 +302,9 @@ class BASE_API FieldTrialList {
   // is commonly used in a non-browser process, to carry randomly selected state
   // in a browser process into this non-browser process. This method calls
   // CreateFieldTrial to create the FieldTrial in the non-browser process.
-  // Currently only the group_name_ and name_ are restored.
+  // Currently only the group_name_ and name_ are restored, as well as the
+  // |client_id()| state, that could be used for one-time randomized trials
+  // set up only in child processes.
   static bool CreateTrialsInChildProcess(const std::string& prior_trials);
 
   // Create a FieldTrial with the given |name| and using 100% probability for
@@ -303,6 +344,17 @@ class BASE_API FieldTrialList {
   // Return the number of active field trials.
   static size_t GetFieldTrialCount();
 
+  // Returns true if you can call |FieldTrial::UseOneTimeRandomization()|
+  // without error, i.e. if a non-empty string was provided as the client_id
+  // when constructing the FieldTrialList singleton.
+  static bool IsOneTimeRandomizationEnabled();
+
+  // Returns an opaque, diverse ID for this client that does not change
+  // between sessions.
+  //
+  // Returns the empty string if one-time randomization is not enabled.
+  static const std::string& client_id();
+
  private:
   // A map from FieldTrial names to the actual instances.
   typedef std::map<std::string, FieldTrial*> RegistrationList;
@@ -317,7 +369,7 @@ class BASE_API FieldTrialList {
   // is created after that.
   static bool register_without_global_;
 
-  // A helper value made availabel to users, that shows when the FieldTrialList
+  // A helper value made available to users, that shows when the FieldTrialList
   // was initialized.  Note that this is a singleton instance, and hence is a
   // good approximation to the start of the process.
   TimeTicks application_start_time_;
@@ -326,6 +378,10 @@ class BASE_API FieldTrialList {
   base::Lock lock_;
   RegistrationList registered_;
 
+  // An opaque, diverse ID for this client that does not change
+  // between sessions, or the empty string if not initialized.
+  std::string client_id_;
+
   // List of observers to be notified when a group is selected for a FieldTrial.
   ObserverList<Observer> observer_list_;
 
@@ -335,4 +391,3 @@ class BASE_API FieldTrialList {
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_
-
diff --git a/base/metrics/field_trial_unittest.cc b/base/metrics/field_trial_unittest.cc
index 3357085..a790add 100644
--- a/base/metrics/field_trial_unittest.cc
+++ b/base/metrics/field_trial_unittest.cc
@@ -6,14 +6,18 @@
 
 #include "base/metrics/field_trial.h"
 
+#include "base/rand_util.h"
 #include "base/stringprintf.h"
+#include "base/string_number_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
+#include <limits>
+
 namespace base {
 
 class FieldTrialTest : public testing::Test {
  public:
-  FieldTrialTest() : trial_list_() {
+  FieldTrialTest() : trial_list_("client_id") {
     Time now = Time::NowFromSystemTime();
     TimeDelta oneYear = TimeDelta::FromDays(365);
     Time::Exploded exploded;
@@ -224,9 +228,10 @@ TEST_F(FieldTrialTest, Save) {
       new FieldTrial(
           "Some name", 10, "Default some name", next_year_, 12, 31);
   // There is no winner yet, so no textual group name is associated with trial.
+  // In this case, the trial should not be included.
   EXPECT_EQ("", trial->group_name_internal());
   FieldTrialList::StatesToString(&save_string);
-  EXPECT_EQ("Some name/Default some name/", save_string);
+  EXPECT_EQ("", save_string);
   save_string.clear();
 
   // Create a winning group.
@@ -265,10 +270,10 @@ TEST_F(FieldTrialTest, Restore) {
 TEST_F(FieldTrialTest, BogusRestore) {
   EXPECT_FALSE(FieldTrialList::CreateTrialsInChildProcess("MissingSlash"));
   EXPECT_FALSE(FieldTrialList::CreateTrialsInChildProcess("MissingGroupName/"));
-  EXPECT_FALSE(
-      FieldTrialList::CreateTrialsInChildProcess("MissingFinalSlash/gname"));
-  EXPECT_FALSE(
-      FieldTrialList::CreateTrialsInChildProcess("/noname, only group/"));
+  EXPECT_FALSE(FieldTrialList::CreateTrialsInChildProcess(
+      "MissingFinalSlash/gname"));
+  EXPECT_FALSE(FieldTrialList::CreateTrialsInChildProcess(
+      "noname, only group/"));
 }
 
 TEST_F(FieldTrialTest, DuplicateRestore) {
@@ -284,7 +289,8 @@ TEST_F(FieldTrialTest, DuplicateRestore) {
   EXPECT_TRUE(FieldTrialList::CreateTrialsInChildProcess(save_string));
 
   // But it is an error to try to change to a different winner.
-  EXPECT_FALSE(FieldTrialList::CreateTrialsInChildProcess("Some name/Loser/"));
+  EXPECT_FALSE(FieldTrialList::CreateTrialsInChildProcess(
+      "Some name/Loser/"));
 }
 
 TEST_F(FieldTrialTest, CreateFieldTrial) {
@@ -321,4 +327,109 @@ TEST_F(FieldTrialTest, MakeName) {
             FieldTrial::MakeName("Histogram", "Field Trial"));
 }
 
+TEST_F(FieldTrialTest, HashClientId) {
+  double results[] = {
+      FieldTrial::HashClientId("hi", "1"),
+      FieldTrial::HashClientId("there", "1"),
+  };
+  ASSERT_NE(results[0], results[1]);
+  for (size_t i = 0; i < arraysize(results); ++i) {
+    ASSERT_LE(0.0, results[i]);
+    ASSERT_GT(1.0, results[i]);
+  }
+
+  ASSERT_EQ(FieldTrial::HashClientId("yo", "1"),
+            FieldTrial::HashClientId("yo", "1"));
+  ASSERT_NE(FieldTrial::HashClientId("yo", "something"),
+            FieldTrial::HashClientId("yo", "else"));
+}
+
+TEST_F(FieldTrialTest, HashClientIdIsUniform) {
+  // Choose a random start number but go sequentially from there, so
+  // that each test tries a different range but we never provide uniformly
+  // distributed input data.
+  int current_number = RandInt(0, std::numeric_limits<int>::max());
+
+  // The expected value of a random distribution is the average over all
+  // samples as the number of samples approaches infinity.  For a uniform
+  // distribution from [0.0, 1.0) this would be 0.5.
+  //
+  // We do kSamplesBetweenChecks at a time and check if the value has converged
+  // to a narrow interval around 0.5.  A non-uniform distribution would likely
+  // converge at something different, or not converge consistently within this
+  // range (i.e. the test would start timing out occasionally).
+  int kSamplesBetweenChecks = 300;
+  int num_samples = 0;
+  double total_value = 0.0;
+  while (true) {
+    for (int i = 0; i < kSamplesBetweenChecks; ++i) {
+      total_value += FieldTrial::HashClientId(
+          IntToString(current_number++), "salt");
+      num_samples++;
+    }
+
+    double average = total_value / num_samples;
+    double kExpectedMin = 0.48;
+    double kExpectedMax = 0.52;
+
+    if (num_samples > 1000 &&
+        (average < kExpectedMin || average > kExpectedMax)) {
+      // Only printed once we have enough samples that it's very unlikely
+      // things haven't converged.
+      printf("After %d samples, the average was %f, outside the expected\n"
+             "range (%f, %f). We will add more samples and check after every\n"
+             "%d samples. If the average does not converge, something\n"
+             "is broken. If it does converge, the test will pass.\n",
+             num_samples, average,
+             kExpectedMin, kExpectedMax, kSamplesBetweenChecks);
+    } else {
+      // Success.
+      break;
+    }
+  }
+}
+
+TEST_F(FieldTrialTest, UseOneTimeRandomization) {
+  // Simply asserts that two trials using one-time randomization
+  // that have different names, normally generate different results.
+  //
+  // Note that depending on the one-time random initialization, they
+  // _might_ actually give the same result, but we know that given
+  // the particular client_id we use for unit tests they won't.
+  scoped_refptr<FieldTrial> trials[] = {
+    new FieldTrial("one", 100, "default", next_year_, 1, 1),
+    new FieldTrial("two", 100, "default", next_year_, 1, 1),
+  };
+
+  for (size_t i = 0; i < arraysize(trials); ++i) {
+    trials[i]->UseOneTimeRandomization();
+
+    for (int j = 0; j < 100; ++j) {
+      trials[i]->AppendGroup("", 1);
+    }
+  }
+
+  // The trials are most likely to give different results since they have
+  // different names.
+  ASSERT_NE(trials[0]->group(), trials[1]->group());
+  ASSERT_NE(trials[0]->group_name(), trials[1]->group_name());
+}
+
+TEST_F(FieldTrialTest, DisableImmediately) {
+  FieldTrial* trial =
+      new FieldTrial("trial", 100, "default", next_year_, 12, 31);
+  trial->Disable();
+  ASSERT_EQ("default", trial->group_name());
+  ASSERT_EQ(FieldTrial::kDefaultGroupNumber, trial->group());
+}
+
+TEST_F(FieldTrialTest, DisableAfterInitialization) {
+  FieldTrial* trial =
+      new FieldTrial("trial", 100, "default", next_year_, 12, 31);
+  trial->AppendGroup("non_default", 100);
+  ASSERT_EQ("non_default", trial->group_name());
+  trial->Disable();
+  ASSERT_EQ("default", trial->group_name());
+}
+
 }  // namespace base
diff --git a/base/rand_util.cc b/base/rand_util.cc
index fc41ce8..4140e9a 100644
--- a/base/rand_util.cc
+++ b/base/rand_util.cc
@@ -25,6 +25,10 @@ int RandInt(int min, int max) {
 }
 
 double RandDouble() {
+  return BitsToOpenEndedUnitInterval(base::RandUint64());
+}
+
+double BitsToOpenEndedUnitInterval(uint64 bits) {
   // We try to get maximum precision by masking out as many bits as will fit
   // in the target type's mantissa, and raising it to an appropriate power to
   // produce output in the range [0, 1).  For IEEE 754 doubles, the mantissa
@@ -32,7 +36,7 @@ double RandDouble() {
 
   COMPILE_ASSERT(std::numeric_limits<double>::radix == 2, otherwise_use_scalbn);
   static const int kBits = std::numeric_limits<double>::digits;
-  uint64 random_bits = base::RandUint64() & ((GG_UINT64_C(1) << kBits) - 1);
+  uint64 random_bits = bits & ((GG_UINT64_C(1) << kBits) - 1);
   double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
   DCHECK_GE(result, 0.0);
   DCHECK_LT(result, 1.0);
diff --git a/base/rand_util.h b/base/rand_util.h
index 541fd77..6bfbcb4 100644
--- a/base/rand_util.h
+++ b/base/rand_util.h
@@ -29,6 +29,10 @@ BASE_API uint64 RandGenerator(uint64 max);
 // Returns a random double in range [0, 1). Thread-safe.
 BASE_API double RandDouble();
 
+// Given input |bits|, convert with maximum precision to a double in
+// the range [0, 1). Thread-safe.
+BASE_API double BitsToOpenEndedUnitInterval(uint64 bits);
+
 // Fills |output_length| bytes of |output| with cryptographically strong random
 // data.
 BASE_API void RandBytes(void* output, size_t output_length);