Expand FeatureList to support FieldTrial association.

base/feature_list.cc

Index: base/feature_list.cc
diff --git a/base/feature_list.cc b/base/feature_list.cc
index 16eba67e99fd1ecd467cc1b0f20fde6c7f9b9149..93cda6d184a5afe2edbe188647998ab813c0335c 100644
--- a/base/feature_list.cc
+++ b/base/feature_list.cc
@@ -7,6 +7,7 @@
 #include <vector>
 
 #include "base/logging.h"
+#include "base/metrics/field_trial.h"
 #include "base/strings/string_split.h"
 
 namespace base {
@@ -37,13 +38,43 @@ 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);
   }
 }
 
+void FeatureList::RegisterFieldTrialOverride(const std::string& feature_name,
+                                             OverrideState override_state,
+                                             FieldTrial* field_trial) {
+  RegisterOverride(feature_name, override_state, field_trial);
+}
+
+FieldTrial* FeatureList::AssociateReportingFieldTrial(
+    const std::string& feature_name,
+    OverrideState for_overridden_state,
+    const std::string& field_trial_name,
+    const std::string& group_name) {
+  auto it = overrides_.find(feature_name);
+  // No association if there's no override entry or its state doesn't match
+  // |for_forced_state|.

[rkaplow, 2015/09/14 21:20:12]

overriden

[Alexei Svitkine (slow), 2015/09/14 22:03:43]

Done.

+  if (it == overrides_.end() ||
+      it->second.overridden_state != for_overridden_state) {
+    return nullptr;
+  }
+
+  // Only one associated field trial is supported per feature. This is generally
+  // enforced server-side.
+  if (it->second.field_trial) {
+    NOTREACHED();
+    return nullptr;
+  }
+  it->second.field_trial =
+      FieldTrialList::CreateFieldTrial(field_trial_name, group_name);
+  return it->second.field_trial;
+}
+
 // static
 bool FeatureList::IsEnabled(const Feature& feature) {
   return GetInstance()->IsFeatureEnabled(feature);
@@ -81,7 +112,11 @@ bool FeatureList::IsFeatureEnabled(const Feature& feature) {
   auto it = overrides_.find(feature.name);
   if (it != overrides_.end()) {
     const OverrideEntry& entry = it->second;
-    // TODO(asvitkine) Expand this section as more support is added.
+
+    // Activate the corresponding field trial, if necessary.
+    if (entry.field_trial)
+      entry.field_trial->group();
+
     return entry.overridden_state == OVERRIDE_ENABLE_FEATURE;
   }
   // Otherwise, return the default state.
@@ -89,9 +124,11 @@ 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)));
+  overrides_.insert(
+      make_pair(feature_name, OverrideEntry(overridden_state, field_trial)));
 }
 
 bool FeatureList::CheckFeatureIdentity(const Feature& feature) {
@@ -107,7 +144,8 @@ 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) {}
 
 }  // namespace base