Expand FeatureList to support FieldTrial association.

This CL adds the following:
  - Two new APIs on FeatureList to be used during initialization. One to
associate a field trial for reporting purposes when the feature is forced
from the command line and the other to override the feature state via a
field trial.
  - Passing the FeatureList instance to VariationsService during browser
start up.
  - Extension of VariationsService (and associated proto changes) to
invoke the two above APIs, when processing variations with the new
proto fields.
  - A new API on FieldTrial to get the group name of a field trial without
activating it, used by VariationsService when association a field trial.

BUG=526169

Review URL: https://codereview.chromium.org/1306653004

Cr-Commit-Position: refs/heads/master@{#351199}

base/feature_list.cc

@@ -4,9 +4,11 @@
 
 #include "base/feature_list.h"
 
+#include <utility>
 #include <vector>
 
 #include "base/logging.h"
+#include "base/metrics/field_trial.h"
 #include "base/strings/string_split.h"
 
 namespace base {
@@ -37,13 +39,55 @@ void FeatureList::InitializeFromCommandLine(
   // Process disabled features first, so that disabled ones take precedence over
   // enabled ones (since RegisterOverride() uses insert()).
   for (const auto& feature_name : SplitFeatureListString(disable_features)) {
-    RegisterOverride(feature_name, OVERRIDE_DISABLE_FEATURE);
+    RegisterOverride(feature_name, OVERRIDE_DISABLE_FEATURE, nullptr);
   }
   for (const auto& feature_name : SplitFeatureListString(enable_features)) {
-    RegisterOverride(feature_name, OVERRIDE_ENABLE_FEATURE);
+    RegisterOverride(feature_name, OVERRIDE_ENABLE_FEATURE, nullptr);
   }
 }
 
+bool FeatureList::IsFeatureOverriddenFromCommandLine(
+    const std::string& feature_name,
+    OverrideState state) const {
+  auto it = overrides_.find(feature_name);
+  return it != overrides_.end() && it->second.overridden_state == state &&
+         !it->second.overridden_by_field_trial;
+}
+
+void FeatureList::RegisterFieldTrialOverride(const std::string& feature_name,
+                                             OverrideState override_state,
+                                             FieldTrial* field_trial) {
+  DCHECK(field_trial);
+  DCHECK(!ContainsKey(overrides_, feature_name) ||
+         !overrides_.find(feature_name)->second.field_trial)
+      << "Feature " << feature_name
+      << " has conflicting field trial overrides: "
+      << overrides_.find(feature_name)->second.field_trial->trial_name()
+      << " / " << field_trial->trial_name();
+
+  RegisterOverride(feature_name, override_state, field_trial);
+}
+
+void FeatureList::AssociateReportingFieldTrial(
+    const std::string& feature_name,
+    OverrideState for_overridden_state,
+    FieldTrial* field_trial) {
+  DCHECK(
+      IsFeatureOverriddenFromCommandLine(feature_name, for_overridden_state));
+
+  // Only one associated field trial is supported per feature. This is generally
+  // enforced server-side.
+  OverrideEntry* entry = &overrides_.find(feature_name)->second;
+  if (entry->field_trial) {
+    NOTREACHED() << "Feature " << feature_name
+                 << " already has trial: " << entry->field_trial->trial_name()
+                 << ", associating trial: " << field_trial->trial_name();
+    return;
+  }
+
+  entry->field_trial = field_trial;
+}
+
 // static
 bool FeatureList::IsEnabled(const Feature& feature) {
   return GetInstance()->IsFeatureEnabled(feature);
@@ -81,6 +125,11 @@ bool FeatureList::IsFeatureEnabled(const Feature& feature) {
   auto it = overrides_.find(feature.name);
   if (it != overrides_.end()) {
     const OverrideEntry& entry = it->second;
+
+    // Activate the corresponding field trial, if necessary.
+    if (entry.field_trial)
+      entry.field_trial->group();
+
     // TODO(asvitkine) Expand this section as more support is added.
     return entry.overridden_state == OVERRIDE_ENABLE_FEATURE;
   }
@@ -89,9 +138,14 @@ bool FeatureList::IsFeatureEnabled(const Feature& feature) {
 }
 
 void FeatureList::RegisterOverride(const std::string& feature_name,
-                                   OverrideState overridden_state) {
+                                   OverrideState overridden_state,
+                                   FieldTrial* field_trial) {
   DCHECK(!initialized_);
-  overrides_.insert(make_pair(feature_name, OverrideEntry(overridden_state)));
+  // Note: The semantics of insert() is that it does not overwrite the entry if
+  // one already exists for the key. Thus, only the first override for a given
+  // feature name takes effect.
+  overrides_.insert(std::make_pair(
+      feature_name, OverrideEntry(overridden_state, field_trial)));
 }
 
 bool FeatureList::CheckFeatureIdentity(const Feature& feature) {
@@ -107,7 +161,10 @@ bool FeatureList::CheckFeatureIdentity(const Feature& feature) {
   return it->second == &feature;
 }
 
-FeatureList::OverrideEntry::OverrideEntry(OverrideState overridden_state)
-    : overridden_state(overridden_state) {}
+FeatureList::OverrideEntry::OverrideEntry(OverrideState overridden_state,
+                                          FieldTrial* field_trial)
+    : overridden_state(overridden_state),
+      field_trial(field_trial),
+      overridden_by_field_trial(field_trial != nullptr) {}
 
 }  // namespace base

base/feature_list.h

@@ -16,6 +16,8 @@
 
 namespace base {
 
+class FieldTrial;
+
 // Specifies whether a given feature is enabled or disabled by default.
 enum FeatureState {
   FEATURE_DISABLED_BY_DEFAULT,
@@ -83,6 +85,37 @@ class BASE_EXPORT FeatureList {
   void InitializeFromCommandLine(const std::string& enable_features,
                                  const std::string& disable_features);
 
+  // Specifies whether a feature override enables or disables the feature.
+  enum OverrideState {
+    OVERRIDE_DISABLE_FEATURE,
+    OVERRIDE_ENABLE_FEATURE,
+  };
+
+  // Returns true if the state of |feature_name| has been overridden via
+  // |InitializeFromCommandLine()|.
+  bool IsFeatureOverriddenFromCommandLine(const std::string& feature_name,
+                                          OverrideState state) const;
+
+  // Associates a field trial for reporting purposes corresponding to the
+  // command-line setting the feature state to |for_overridden_state|. The trial
+  // will be activated when the state of the feature is first queried. This
+  // should be called during registration, after InitializeFromCommandLine() has
+  // been called but before the instance is registered via SetInstance().
+  void AssociateReportingFieldTrial(const std::string& feature_name,
+                                    OverrideState for_overridden_state,
+                                    FieldTrial* field_trial);
+
+  // Registers a field trial to override the enabled state of the specified
+  // feature to |override_state|. Command-line overrides still take precedence
+  // over field trials, so this will have no effect if the feature is being
+  // overridden from the command-line. The associated field trial will be
+  // activated when the feature state for this feature is queried. This should
+  // be called during registration, after InitializeFromCommandLine() has been
+  // called but before the instance is registered via SetInstance().
+  void RegisterFieldTrialOverride(const std::string& feature_name,
+                                  OverrideState override_state,
+                                  FieldTrial* field_trial);
+
   // Returns whether the given |feature| is enabled. Must only be called after
   // the singleton instance has been registered via SetInstance(). Additionally,
   // a feature with a given name must only have a single corresponding Feature
@@ -103,10 +136,27 @@ class BASE_EXPORT FeatureList {
  private:
   FRIEND_TEST_ALL_PREFIXES(FeatureListTest, CheckFeatureIdentity);
 
-  // Specifies whether a feature override enables or disables the feature.
-  enum OverrideState {
-    OVERRIDE_DISABLE_FEATURE,
-    OVERRIDE_ENABLE_FEATURE,
+  struct OverrideEntry {
+    // The overridden enable (on/off) state of the feature.
+    const OverrideState overridden_state;
+
+    // An optional associated field trial, which will be activated when the
+    // state of the feature is queried for the first time. Weak pointer to the
+    // FieldTrial object that is owned by the FieldTrialList singleton.
+    base::FieldTrial* field_trial;
+
+    // Specifies whether the feature's state is overridden by |field_trial|.
+    // If it's not, and |field_trial| is not null, it means it is simply an
+    // associated field trial for reporting purposes (and |overridden_state|
+    // came from the command-line).
+    const bool overridden_by_field_trial;
+
+    // TODO(asvitkine): Expand this as more support is added.
+
+    // Constructs an OverrideEntry for the given |overridden_state|. If
+    // |field_trial| is not null, it implies that |overridden_state| comes from
+    // the trial, so |overridden_by_field_trial| will be set to true.
+    OverrideEntry(OverrideState overridden_state, FieldTrial* field_trial);
   };
 
   // Finalizes the initialization state of the FeatureList, so that no further
@@ -121,9 +171,14 @@ class BASE_EXPORT FeatureList {
 
   // Registers an override for feature |feature_name|. The override specifies
   // whether the feature should be on or off (via |overridden_state|), which
-  // will take precedence over the feature's default state.
+  // will take precedence over the feature's default state. If |field_trial| is
+  // not null, registers the specified field trial object to be associated with
+  // the feature, which will activate the field trial when the feature state is
+  // queried. If an override is already registered for the given feature, it
+  // will not be changed.
   void RegisterOverride(const std::string& feature_name,
-                        OverrideState overridden_state);
+                        OverrideState overridden_state,
+                        FieldTrial* field_trial);
 
   // Verifies that there's only a single definition of a Feature struct for a
   // given feature name. Keeps track of the first seen Feature struct for each
@@ -132,14 +187,6 @@ class BASE_EXPORT FeatureList {
   // DCHECKs and tests.
   bool CheckFeatureIdentity(const Feature& feature);
 
-  struct OverrideEntry {
-    // The overridden enable (on/off) state of the feature.
-    const OverrideState overridden_state;
-
-    // TODO(asvitkine): Expand this as more support is added.
-
-    explicit OverrideEntry(OverrideState overridden_state);
-  };
   // Map from feature name to an OverrideEntry struct for the feature, if it
   // exists.
   std::map<std::string, OverrideEntry> overrides_;

base/feature_list_unittest.cc

@@ -4,6 +4,9 @@
 
 #include "base/feature_list.h"
 
+#include "base/format_macros.h"
+#include "base/metrics/field_trial.h"
+#include "base/strings/stringprintf.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
@@ -70,6 +73,9 @@ TEST_F(FeatureListTest, InitializeFromCommandLine) {
 
   for (size_t i = 0; i < arraysize(test_cases); ++i) {
     const auto& test_case = test_cases[i];
+    SCOPED_TRACE(base::StringPrintf("Test[%" PRIuS "]: [%s] [%s]", i,
+                                    test_case.enable_features,
+                                    test_case.disable_features));
 
     ClearFeatureListInstance();
     scoped_ptr<FeatureList> feature_list(new FeatureList);
@@ -105,4 +111,200 @@ TEST_F(FeatureListTest, CheckFeatureIdentity) {
   EXPECT_FALSE(feature_list()->CheckFeatureIdentity(kFeatureOnByDefault2));
 }
 
+TEST_F(FeatureListTest, FieldTrialOverrides) {
+  struct {
+    FeatureList::OverrideState trial1_state;
+    FeatureList::OverrideState trial2_state;
+  } test_cases[] = {
+      {FeatureList::OVERRIDE_DISABLE_FEATURE,
+       FeatureList::OVERRIDE_DISABLE_FEATURE},
+      {FeatureList::OVERRIDE_DISABLE_FEATURE,
+       FeatureList::OVERRIDE_ENABLE_FEATURE},
+      {FeatureList::OVERRIDE_ENABLE_FEATURE,
+       FeatureList::OVERRIDE_DISABLE_FEATURE},
+      {FeatureList::OVERRIDE_ENABLE_FEATURE,
+       FeatureList::OVERRIDE_ENABLE_FEATURE},
+  };
+
+  FieldTrial::ActiveGroup active_group;
+  for (size_t i = 0; i < arraysize(test_cases); ++i) {
+    const auto& test_case = test_cases[i];
+    SCOPED_TRACE(base::StringPrintf("Test[%" PRIuS "]", i));
+
+    ClearFeatureListInstance();
+
+    FieldTrialList field_trial_list(nullptr);
+    scoped_ptr<FeatureList> feature_list(new FeatureList);
+
+    FieldTrial* trial1 = FieldTrialList::CreateFieldTrial("TrialExample1", "A");
+    FieldTrial* trial2 = FieldTrialList::CreateFieldTrial("TrialExample2", "B");
+    feature_list->RegisterFieldTrialOverride(kFeatureOnByDefaultName,
+                                             test_case.trial1_state, trial1);
+    feature_list->RegisterFieldTrialOverride(kFeatureOffByDefaultName,
+                                             test_case.trial2_state, trial2);
+    RegisterFeatureListInstance(feature_list.Pass());
+
+    // Initially, neither trial should be active.
+    EXPECT_FALSE(FieldTrialList::IsTrialActive(trial1->trial_name()));
+    EXPECT_FALSE(FieldTrialList::IsTrialActive(trial2->trial_name()));
+
+    const bool expected_enabled_1 =
+        (test_case.trial1_state == FeatureList::OVERRIDE_ENABLE_FEATURE);
+    EXPECT_EQ(expected_enabled_1, FeatureList::IsEnabled(kFeatureOnByDefault));
+    // The above should have activated |trial1|.
+    EXPECT_TRUE(FieldTrialList::IsTrialActive(trial1->trial_name()));
+    EXPECT_FALSE(FieldTrialList::IsTrialActive(trial2->trial_name()));
+
+    const bool expected_enabled_2 =
+        (test_case.trial2_state == FeatureList::OVERRIDE_ENABLE_FEATURE);
+    EXPECT_EQ(expected_enabled_2, FeatureList::IsEnabled(kFeatureOffByDefault));
+    // The above should have activated |trial2|.
+    EXPECT_TRUE(FieldTrialList::IsTrialActive(trial1->trial_name()));
+    EXPECT_TRUE(FieldTrialList::IsTrialActive(trial2->trial_name()));
+  }
+}
+
+TEST_F(FeatureListTest, CommandLineTakesPrecedenceOverFieldTrial) {
+  ClearFeatureListInstance();
+
+  FieldTrialList field_trial_list(nullptr);
+  scoped_ptr<FeatureList> feature_list(new FeatureList);
+
+  // The feature is explicitly enabled on the command-line.
+  feature_list->InitializeFromCommandLine(kFeatureOffByDefaultName, "");
+
+  // But the FieldTrial would set the feature to disabled.
+  FieldTrial* trial = FieldTrialList::CreateFieldTrial("TrialExample2", "A");
+  feature_list->RegisterFieldTrialOverride(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE, trial);
+  RegisterFeatureListInstance(feature_list.Pass());
+
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(trial->trial_name()));
+  // Command-line should take precedence.
+  EXPECT_TRUE(FeatureList::IsEnabled(kFeatureOffByDefault));
+  // Since the feature is on due to the command-line, and not as a result of the
+  // field trial, the field trial should not be activated (since the Associate*
+  // API wasn't used.)
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(trial->trial_name()));
+}
+
+TEST_F(FeatureListTest, IsFeatureOverriddenFromCommandLine) {
+  ClearFeatureListInstance();
+
+  FieldTrialList field_trial_list(nullptr);
+  scoped_ptr<FeatureList> feature_list(new FeatureList);
+
+  // No features are overridden from the command line yet
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOnByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE));
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOnByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE));
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE));
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE));
+
+  // Now, enable |kFeatureOffByDefaultName| via the command-line.
+  feature_list->InitializeFromCommandLine(kFeatureOffByDefaultName, "");
+
+  // It should now be overridden for the enabled group.
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE));
+  EXPECT_TRUE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE));
+
+  // Register a field trial to associate with the feature and ensure that the
+  // results are still the same.
+  feature_list->AssociateReportingFieldTrial(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE,
+      FieldTrialList::CreateFieldTrial("Trial1", "A"));
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE));
+  EXPECT_TRUE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOffByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE));
+
+  // Now, register a field trial to override |kFeatureOnByDefaultName| state
+  // and check that the function still returns false for that feature.
+  feature_list->RegisterFieldTrialOverride(
+      kFeatureOnByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE,
+      FieldTrialList::CreateFieldTrial("Trial2", "A"));
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOnByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE));
+  EXPECT_FALSE(feature_list->IsFeatureOverriddenFromCommandLine(
+      kFeatureOnByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE));
+  RegisterFeatureListInstance(feature_list.Pass());
+
+  // Check the expected feature states for good measure.
+  EXPECT_TRUE(FeatureList::IsEnabled(kFeatureOffByDefault));
+  EXPECT_FALSE(FeatureList::IsEnabled(kFeatureOnByDefault));
+}
+
+TEST_F(FeatureListTest, AssociateReportingFieldTrial) {
+  struct {
+    const char* enable_features;
+    const char* disable_features;
+    bool expected_enable_trial_created;
+    bool expected_disable_trial_created;
+  } test_cases[] = {
+      // If no enable/disable flags are specified, no trials should be created.
+      {"", "", false, false},
+      // Enabling the feature should result in the enable trial created.
+      {kFeatureOffByDefaultName, "", true, false},
+      // Disabling the feature should result in the disable trial created.
+      {"", kFeatureOffByDefaultName, false, true},
+  };
+
+  const char kTrialName[] = "ForcingTrial";
+  const char kForcedOnGroupName[] = "ForcedOn";
+  const char kForcedOffGroupName[] = "ForcedOff";
+
+  for (size_t i = 0; i < arraysize(test_cases); ++i) {
+    const auto& test_case = test_cases[i];
+    SCOPED_TRACE(base::StringPrintf("Test[%" PRIuS "]: [%s] [%s]", i,
+                                    test_case.enable_features,
+                                    test_case.disable_features));
+
+    ClearFeatureListInstance();
+
+    FieldTrialList field_trial_list(nullptr);
+    scoped_ptr<FeatureList> feature_list(new FeatureList);
+    feature_list->InitializeFromCommandLine(test_case.enable_features,
+                                            test_case.disable_features);
+
+    FieldTrial* enable_trial = nullptr;
+    if (feature_list->IsFeatureOverriddenFromCommandLine(
+            kFeatureOffByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE)) {
+      enable_trial = base::FieldTrialList::CreateFieldTrial(kTrialName,
+                                                            kForcedOnGroupName);
+      feature_list->AssociateReportingFieldTrial(
+          kFeatureOffByDefaultName, FeatureList::OVERRIDE_ENABLE_FEATURE,
+          enable_trial);
+    }
+    FieldTrial* disable_trial = nullptr;
+    if (feature_list->IsFeatureOverriddenFromCommandLine(
+            kFeatureOffByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE)) {
+      disable_trial = base::FieldTrialList::CreateFieldTrial(
+          kTrialName, kForcedOffGroupName);
+      feature_list->AssociateReportingFieldTrial(
+          kFeatureOffByDefaultName, FeatureList::OVERRIDE_DISABLE_FEATURE,
+          disable_trial);
+    }
+    EXPECT_EQ(test_case.expected_enable_trial_created, enable_trial != nullptr);
+    EXPECT_EQ(test_case.expected_disable_trial_created,
+              disable_trial != nullptr);
+    RegisterFeatureListInstance(feature_list.Pass());
+
+    EXPECT_FALSE(FieldTrialList::IsTrialActive(kTrialName));
+    if (disable_trial) {
+      EXPECT_FALSE(FeatureList::IsEnabled(kFeatureOffByDefault));
+      EXPECT_TRUE(FieldTrialList::IsTrialActive(kTrialName));
+      EXPECT_EQ(kForcedOffGroupName, disable_trial->group_name());
+    } else if (enable_trial) {
+      EXPECT_TRUE(FeatureList::IsEnabled(kFeatureOffByDefault));
+      EXPECT_TRUE(FieldTrialList::IsTrialActive(kTrialName));
+      EXPECT_EQ(kForcedOnGroupName, enable_trial->group_name());
+    }
+  }
+}
+
 }  // namespace base

base/metrics/field_trial.cc

@@ -139,6 +139,11 @@ const std::string& FieldTrial::group_name() {
   return group_name_;
 }
 
+const std::string& FieldTrial::GetGroupNameWithoutActivation() {
+  FinalizeGroupChoice();
+  return group_name_;
+}
+
 void FieldTrial::SetForced() {
   // We might have been forced before (e.g., by CreateFieldTrial) and it's
   // first come first served, e.g., command line switch has precedence.

base/metrics/field_trial.h

@@ -149,6 +149,11 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
   // is used as the group name. This causes a winner to be chosen if none was.
   const std::string& group_name();
 
+  // Finalizes the group choice and returns the chosen group, but does not mark
+  // the trial as active - so its state will not be reported until group_name()
+  // or similar is called.
+  const std::string& GetGroupNameWithoutActivation();
+
   // Set the field trial as forced, meaning that it was setup earlier than
   // the hard coded registration of the field trial to override it.
   // This allows the code that was hard coded to register the field trial to

base/metrics/field_trial_unittest.cc

@@ -72,6 +72,8 @@ class FieldTrialTest : public testing::Test {
  private:
   MessageLoop message_loop_;
   FieldTrialList trial_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(FieldTrialTest);
 };
 
 // Test registration, and also check that destructors are called for trials
@@ -376,6 +378,28 @@ TEST_F(FieldTrialTest, ActiveGroupsNotFinalized) {
   EXPECT_EQ(active_group.group_name, active_groups[0].group_name);
 }
 
+TEST_F(FieldTrialTest, GetGroupNameWithoutActivation) {
+  const char kTrialName[] = "TestTrial";
+  const char kSecondaryGroupName[] = "SecondaryGroup";
+
+  int default_group = -1;
+  scoped_refptr<FieldTrial> trial =
+      CreateFieldTrial(kTrialName, 100, kDefaultGroupName, &default_group);
+  trial->AppendGroup(kSecondaryGroupName, 50);
+
+  // The trial should start inactive.
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(kTrialName));
+
+  // Calling |GetGroupNameWithoutActivation()| should not activate the trial.
+  std::string group_name = trial->GetGroupNameWithoutActivation();
+  EXPECT_FALSE(group_name.empty());
+  EXPECT_FALSE(FieldTrialList::IsTrialActive(kTrialName));
+
+  // Calling |group_name()| should activate it and return the same group name.
+  EXPECT_EQ(group_name, trial->group_name());
+  EXPECT_TRUE(FieldTrialList::IsTrialActive(kTrialName));
+}
+
 TEST_F(FieldTrialTest, Save) {
   std::string save_string;
 

chrome/browser/chrome_browser_main.cc

@@ -678,9 +678,8 @@ void ChromeBrowserMainParts::SetupMetricsAndFieldTrials() {
   variations::VariationsService* variations_service =
       browser_process_->variations_service();
   if (variations_service)
-    variations_service->CreateTrialsFromSeed();
+    variations_service->CreateTrialsFromSeed(feature_list.get());
 
-  // TODO(asvitkine): Pass |feature_list| to CreateTrialsFromSeed() above.
   base::FeatureList::SetInstance(feature_list.Pass());
 
   // This must be called after |local_state_| is initialized.

components/variations/proto/study.proto

@@ -44,7 +44,7 @@ message Study {
 
   // An experiment within the study.
   //
-  // Next tag: 11
+  // Next tag: 12
   message Experiment {
     // A named parameter value for this experiment.
     //
@@ -79,10 +79,44 @@ message Study {
     // Optional id used to uniquely identify this experiment for Chrome Sync.
     optional uint64 chrome_sync_experiment_id = 10;
 
+    // Specifies the feature association parameters for this experiment group.
+    //
+    // Next tag: 5
+    message FeatureAssociation {
+      // Optional list of features to enable when this experiment is selected.
+      // Command-line overrides take precedence over this setting. No feature
+      // listed here should exist in the |disable_feature| list.
+      repeated string enable_feature = 1;
+
+      // Optional list of features to disable when this experiment is selected.
+      // Command-line overrides take precedence over this setting. No feature
+      // listed here should exist in the |enable_feature| list.
+      repeated string disable_feature = 2;
+
+      // Similar to |forcing_flag|, this is an optional name of a feature which
+      // will cause this experiment to be activated, if that feature is enabled
+      // from the command-line. Experiment with this set are not eligible for
+      // selection via a random dice roll.
+      // Mutually exclusive with |forcing_flag|, |forcing_feature_off| or
+      // having a non-zero |probability_weight|.
+      optional string forcing_feature_on = 3;
+
+      // Similar to |forcing_flag|, this is an optional name of a feature which
+      // will cause this experiment to be activated, if that feature is disabled
+      // from the command-line. Experiment with this set are not eligible for
+      // selection via a random dice roll.
+      // Mutually exclusive with |forcing_flag|, |forcing_feature_on| or having
+      // a non-zero |probability_weight|.
+      optional string forcing_feature_off = 4;
+    }
+    optional FeatureAssociation feature_association = 11;
+
     // Optional name of a Chrome flag that, when present, causes this experiment
     // to be forced. If the forcing_flag field is set, users will not be
     // assigned to this experiment unless that flag is present in Chrome's
     // command line.
+    // Mutually exclusive with |forcing_feature_on|, |forcing_feature_off| or
+    // having a non-zero |probability_weight|.
     optional string forcing_flag = 5;
 
     // Parameter values for this experiment.

components/variations/service/variations_service.cc

@@ -223,7 +223,7 @@ VariationsService::VariationsService(
 VariationsService::~VariationsService() {
 }
 
-bool VariationsService::CreateTrialsFromSeed() {
+bool VariationsService::CreateTrialsFromSeed(base::FeatureList* feature_list) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
   create_trials_from_seed_called_ = true;
@@ -252,7 +252,8 @@ bool VariationsService::CreateTrialsFromSeed() {
       GetCurrentFormFactor(), GetHardwareClass(), latest_country,
       LoadPermanentConsistencyCountry(current_version, latest_country),
       base::Bind(&UIStringOverrider::OverrideUIString,
-                 base::Unretained(&ui_string_overrider_)));
+                 base::Unretained(&ui_string_overrider_)),
+      feature_list);
 
   const base::Time now = base::Time::Now();
 

components/variations/service/variations_service.h

@@ -28,6 +28,7 @@ class PrefService;
 class PrefRegistrySimple;
 
 namespace base {
+class FeatureList;
 class Version;
 }
 
@@ -72,10 +73,12 @@ class VariationsService
 
   ~VariationsService() override;
 
-  // Creates field trials based on Variations Seed loaded from local prefs. If
-  // there is a problem loading the seed data, all trials specified by the seed
-  // may not be created.
-  bool CreateTrialsFromSeed();
+  // Creates field trials based on the variations seed loaded from local state.
+  // If there is a problem loading the seed data, all trials specified by the
+  // seed may not be created. Some field trials are configured to override or
+  // associate with (for reporting) specific features. These associations are
+  // registered with |feature_list|.
+  bool CreateTrialsFromSeed(base::FeatureList* feature_list);
 
   // Should be called before startup of the main message loop.
   void PerformPreMainMessageLoopStartup();

components/variations/variations_seed_processor.cc

@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/metrics/field_trial.h"
 #include "base/strings/utf_string_conversions.h"
 #include "components/variations/processed_study.h"
@@ -79,6 +80,72 @@ void ApplyUIStringOverrides(
   }
 }
 
+// Forces the specified |experiment| to be enabled in |study|.
+void ForceExperimentState(
+    const Study& study,
+    const Study_Experiment& experiment,
+    const VariationsSeedProcessor::UIStringOverrideCallback& override_callback,
+    base::FieldTrial* trial) {
+  RegisterExperimentParams(study, experiment);
+  RegisterVariationIds(experiment, study.name());
+  if (study.activation_type() == Study_ActivationType_ACTIVATION_AUTO) {
+    trial->group();
+    // UI Strings can only be overridden from ACTIVATION_AUTO experiments.
+    ApplyUIStringOverrides(experiment, override_callback);
+  }
+}
+
+// Registers feature overrides for the chosen experiment in the specified study.
+void RegisterFeatureOverrides(const ProcessedStudy& processed_study,
+                              base::FieldTrial* trial,
+                              base::FeatureList* feature_list) {
+  const std::string& group_name = trial->GetGroupNameWithoutActivation();
+  int experiment_index = processed_study.GetExperimentIndexByName(group_name);
+  // The field trial was defined from |study|, so the active experiment's name
+  // must be in the |study|.
+  DCHECK_NE(-1, experiment_index);
+
+  const Study_Experiment& experiment =
+      processed_study.study()->experiment(experiment_index);
+
+  // Process all the features to enable.
+  int feature_count = experiment.feature_association().enable_feature_size();
+  for (int i = 0; i < feature_count; ++i) {
+    feature_list->RegisterFieldTrialOverride(
+        experiment.feature_association().enable_feature(i),
+        base::FeatureList::OVERRIDE_ENABLE_FEATURE, trial);
+  }
+
+  // Process all the features to disable.
+  feature_count = experiment.feature_association().disable_feature_size();
+  for (int i = 0; i < feature_count; ++i) {
+    feature_list->RegisterFieldTrialOverride(
+        experiment.feature_association().disable_feature(i),
+        base::FeatureList::OVERRIDE_DISABLE_FEATURE, trial);
+  }
+}
+
+// Checks if |experiment| is associated with a forcing flag or feature and if it
+// is, returns whether it should be forced enabled based on the |command_line|
+// or |feature_list| state.
+bool ShouldForceExperiment(const Study_Experiment& experiment,
+                           const base::CommandLine& command_line,
+                           const base::FeatureList& feature_list) {
+  if (experiment.feature_association().has_forcing_feature_on()) {
+    return feature_list.IsFeatureOverriddenFromCommandLine(
+        experiment.feature_association().forcing_feature_on(),
+        base::FeatureList::OVERRIDE_ENABLE_FEATURE);
+  }
+  if (experiment.feature_association().has_forcing_feature_off()) {
+    return feature_list.IsFeatureOverriddenFromCommandLine(
+        experiment.feature_association().forcing_feature_off(),
+        base::FeatureList::OVERRIDE_DISABLE_FEATURE);
+  }
+  if (experiment.has_forcing_flag())
+    return command_line.HasSwitch(experiment.forcing_flag());
+  return false;
+}
+
 }  // namespace
 
 VariationsSeedProcessor::VariationsSeedProcessor() {
@@ -97,7 +164,8 @@ void VariationsSeedProcessor::CreateTrialsFromSeed(
     const std::string& hardware_class,
     const std::string& session_consistency_country,
     const std::string& permanent_consistency_country,
-    const UIStringOverrideCallback& override_callback) {
+    const UIStringOverrideCallback& override_callback,
+    base::FeatureList* feature_list) {
   std::vector<ProcessedStudy> filtered_studies;
   FilterAndValidateStudies(seed, locale, reference_date, version, channel,
                            form_factor, hardware_class,
@@ -105,12 +173,13 @@ void VariationsSeedProcessor::CreateTrialsFromSeed(
                            permanent_consistency_country, &filtered_studies);
 
   for (size_t i = 0; i < filtered_studies.size(); ++i)
-    CreateTrialFromStudy(filtered_studies[i], override_callback);
+    CreateTrialFromStudy(filtered_studies[i], override_callback, feature_list);
 }
 
 void VariationsSeedProcessor::CreateTrialFromStudy(
     const ProcessedStudy& processed_study,
-    const UIStringOverrideCallback& override_callback) {
+    const UIStringOverrideCallback& override_callback,
+    base::FeatureList* feature_list) {
   const Study& study = *processed_study.study();
 
   // Check if any experiments need to be forced due to a command line
@@ -118,28 +187,26 @@ void VariationsSeedProcessor::CreateTrialFromStudy(
   base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
   for (int i = 0; i < study.experiment_size(); ++i) {
     const Study_Experiment& experiment = study.experiment(i);
-    if (experiment.has_forcing_flag() &&
-        command_line->HasSwitch(experiment.forcing_flag())) {
-      scoped_refptr<base::FieldTrial> trial(
-          base::FieldTrialList::CreateFieldTrial(study.name(),
-                                                 experiment.name()));
-      // If |trial| is NULL, then there might already be a trial forced to a
+    if (ShouldForceExperiment(experiment, *command_line, *feature_list)) {
+      base::FieldTrial* trial = base::FieldTrialList::CreateFieldTrial(
+          study.name(), experiment.name());
+      // If |trial| is null, then there might already be a trial forced to a
       // different group (e.g. via --force-fieldtrials). Break out of the loop,
       // but don't return, so that variation ids and params for the selected
       // group will still be picked up.
-      if (!trial.get())
+      if (!trial)
         break;
 
-      RegisterExperimentParams(study, experiment);
-      RegisterVariationIds(experiment, study.name());
-      if (study.activation_type() == Study_ActivationType_ACTIVATION_AUTO) {
-        trial->group();
-        // UI Strings can only be overridden from ACTIVATION_AUTO experiments.
-        ApplyUIStringOverrides(experiment, override_callback);
+      if (experiment.feature_association().has_forcing_feature_on()) {
+        feature_list->AssociateReportingFieldTrial(
+            experiment.feature_association().forcing_feature_on(),
+            base::FeatureList::OVERRIDE_ENABLE_FEATURE, trial);
+      } else if (experiment.feature_association().has_forcing_feature_off()) {
+        feature_list->AssociateReportingFieldTrial(
+            experiment.feature_association().forcing_feature_off(),
+            base::FeatureList::OVERRIDE_DISABLE_FEATURE, trial);
       }
-
-      DVLOG(1) << "Trial " << study.name() << " forced by flag: "
-               << experiment.forcing_flag();
+      ForceExperimentState(study, experiment, override_callback, trial);
       return;
     }
   }
@@ -169,14 +236,18 @@ void VariationsSeedProcessor::CreateTrialFromStudy(
           randomization_seed, NULL));
 
   bool has_overrides = false;
+  bool enables_or_disables_features = false;
   for (int i = 0; i < study.experiment_size(); ++i) {
     const Study_Experiment& experiment = study.experiment(i);
     RegisterExperimentParams(study, experiment);
 
     // Groups with forcing flags have probability 0 and will never be selected.
     // Therefore, there's no need to add them to the field trial.
-    if (experiment.has_forcing_flag())
+    if (experiment.has_forcing_flag() ||
+        experiment.feature_association().has_forcing_feature_on() ||
+        experiment.feature_association().has_forcing_feature_off()) {
       continue;
+    }
 
     if (experiment.name() != study.default_experiment_name())
       trial->AppendGroup(experiment.name(), experiment.probability_weight());
@@ -184,9 +255,17 @@ void VariationsSeedProcessor::CreateTrialFromStudy(
     RegisterVariationIds(experiment, study.name());
 
     has_overrides = has_overrides || experiment.override_ui_string_size() > 0;
+    if (experiment.feature_association().enable_feature_size() != 0 ||
+        experiment.feature_association().disable_feature_size() != 0) {
+      enables_or_disables_features = true;
+    }
   }
 
   trial->SetForced();
+
+  if (enables_or_disables_features)
+    RegisterFeatureOverrides(processed_study, trial.get(), feature_list);
+
   if (processed_study.is_expired()) {
     trial->Disable();
   } else if (study.activation_type() == Study_ActivationType_ACTIVATION_AUTO) {

components/variations/variations_seed_processor.h

@@ -18,6 +18,10 @@
 #include "components/variations/proto/study.pb.h"
 #include "components/variations/proto/variations_seed.pb.h"
 
+namespace base {
+class FeatureList;
+}
+
 namespace variations {
 
 class ProcessedStudy;
@@ -42,7 +46,8 @@ class VariationsSeedProcessor {
                             const std::string& hardware_class,
                             const std::string& session_consistency_country,
                             const std::string& permanent_consistency_country,
-                            const UIStringOverrideCallback& override_callback);
+                            const UIStringOverrideCallback& override_callback,
+                            base::FeatureList* feature_list);
 
  private:
   friend class VariationsSeedProcessorTest;
@@ -71,7 +76,8 @@ class VariationsSeedProcessor {
   // Creates and registers a field trial from the |processed_study| data.
   // Disables the trial if |processed_study.is_expired| is true.
   void CreateTrialFromStudy(const ProcessedStudy& processed_study,
-                            const UIStringOverrideCallback& override_callback);
+                            const UIStringOverrideCallback& override_callback,
+                            base::FeatureList* feature_list);
 
   DISALLOW_COPY_AND_ASSIGN(VariationsSeedProcessor);
 };