Introduce dynamic ablation of AutocompleteProviders using AC dynamic trials.

chrome/browser/autocomplete/autocomplete_field_trial.cc

Index: chrome/browser/autocomplete/autocomplete_field_trial.cc
diff --git a/chrome/browser/autocomplete/autocomplete_field_trial.cc b/chrome/browser/autocomplete/autocomplete_field_trial.cc
index 412936e77793066d4798d957b025974020113f7c..cfed6bbd37d03650128fbc3479b31c47108c351d 100644
--- a/chrome/browser/autocomplete/autocomplete_field_trial.cc
+++ b/chrome/browser/autocomplete/autocomplete_field_trial.cc
@@ -96,12 +96,16 @@ const base::FieldTrial::Probability
 
 // Field trial IDs.
 // Though they are not literally "const", they are set only once, in
-// Activate() below.
+// ActivateStaticTrials() below.
 
 // Whether the static field trials have been initialized by
-// ActivateStaticTrials method.
+// ActivateStaticTrials() method.
 bool static_field_trials_initialized = false;
 
+// Whether the dynamic field trials have been initialized by
+// ActivateDynamicTrials() method.
+bool dynamic_field_trials_initialized = false;
+
 // Field trial ID for the disallow-inline History Quick Provider
 // experiment group.
 int disallow_inline_hqp_experiment_group = 0;
@@ -124,8 +128,12 @@ int hqp_replace_hup_scoring_experiment_group = 0;
 // word boundaries experiment group.
 int hqp_only_count_matches_at_word_boundaries_experiment_group = 0;
 
+std::string DynamicFieldTrialName(int id) {

[Mark P, 2013/02/11 21:13:34]

please comment about expected use.

[Bart N., 2013/02/11 21:42:12]

Done.

+  return base::StringPrintf("%s%d", kAutocompleteDynamicFieldTrialPrefix, id);
 }
 
+}  // namespace
+
 
 void AutocompleteFieldTrial::ActivateStaticTrials() {
   DCHECK(!static_field_trials_initialized);
@@ -237,11 +245,36 @@ void AutocompleteFieldTrial::ActivateStaticTrials() {
 }
 
 void AutocompleteFieldTrial::ActivateDynamicTrials() {
-  // Initialize all autocomplete dynamic field trials.
+  // Initialize all autocomplete dynamic field trials.  This method may be
+  // called multiple times.
+  for (int i = 0; i < kMaxAutocompleteDynamicFieldTrials; ++i)
+    base::FieldTrialList::FindValue(DynamicFieldTrialName(i));
+
+  dynamic_field_trials_initialized = true;
+}
+
+int AutocompleteFieldTrial::GetDisabledProviderTypes() {
+  DCHECK(dynamic_field_trials_initialized);
+
+  // We assume that each group may contain a substring
+  // "DisabledProviders_<mask>" where "mask" is a bitmap of disabled provider
+  // types (AutocompleteProvider::Type).
+  int provider_types = 0;
   for (int i = 0; i < kMaxAutocompleteDynamicFieldTrials; ++i) {
-    base::FieldTrialList::FindValue(
-        base::StringPrintf("%s%d", kAutocompleteDynamicFieldTrialPrefix, i));
+    std::string group_name = base::FieldTrialList::FindFullName(
+        DynamicFieldTrialName(i));
+    if (!group_name.empty()) {
+      const char kDisabledProviders[] = "DisabledProviders_";
+      size_t pos = group_name.find(kDisabledProviders);

[Mark P, 2013/02/11 21:13:34]

const

[Bart N., 2013/02/11 21:42:12]

Done.

+      if (pos == std::string::npos)
+        continue;
+      int types = 0;

[Mark P, 2013/02/11 21:13:34]

do you expect pos to ever not be 0?
perhaps dcheck this

[Bart N., 2013/02/11 21:42:12]

I don't think it's a good idea to DCHECK against something that is not entirely
in your control. In this case, if someone makes a mistake on the server side, we
would start DCHECKing in Chrome.

Instead, I've added a warning message. Is that OK with you?

[Mark P, 2013/02/11 22:04:18]

I don't see a warning message.  Can you please add it?

[Bart N., 2013/02/12 01:18:19]

As discussed offline, I made now a stronger requirement and group name must
match "DisabledProviders_XXX".

+      base::StringToInt(base::StringPiece(

[Mark P, 2013/02/11 21:13:34]

consider dchecking the return type of this

[Bart N., 2013/02/11 21:42:12]

Not really; clarified in the comment.

[Mark P, 2013/02/11 22:04:18]

You really do want perfect conversions.  Don't try to convert
DisabledProviders_12i  ("i" typo instead of 8) to 12 and blindly continue.

Please do some sort of error checking.  (The one test you added doesn't catch
this.)

[Bart N., 2013/02/12 01:18:19]

As discussed offline, it was intended. However, I made it strict now.

+          group_name.substr(pos + strlen(kDisabledProviders))), &types);
+      provider_types |= types;
+    }
   }
+  return provider_types;
 }
 
 bool AutocompleteFieldTrial::InDisallowInlineHQPFieldTrial() {