Supporting command line argument to force field trials

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index b211326ac37c61e7c8130107a669fdbfc36d2b22..0ba4b02e95ff12a856abd94f7752c964357b2c1c 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -28,6 +28,22 @@ int FieldTrialList::kExpirationYearInFuture = 0;
 //------------------------------------------------------------------------------
 // FieldTrial methods and members.
 
+FieldTrial* FieldTrial::CreateInstance(
+    const std::string& name, Probability total_probability,
+    const std::string& default_group_name, const int year, const int month,
+    const int day_of_month) {
+  // First check if the field trial has already been created in some other ways.

[Alexei Svitkine (slow), 2012/03/23 18:41:55]

Nit: "some other ways" -> "some other way"

[MAD, 2012/03/23 20:20:12]

Done.

+  FieldTrial* existing_trial = FieldTrialList::Find(name);
+  if (existing_trial) {
+    return existing_trial;
+  } else {

[Alexei Svitkine (slow), 2012/03/23 18:41:55]

Nit: Since you're early returning, don't need an else block.

[MAD, 2012/03/23 20:20:12]

Done.

+    FieldTrial* new_trial = new FieldTrial(
+        name, total_probability, default_group_name, year, month, day_of_month);
+    FieldTrialList::Register(new_trial);
+    return new_trial;
+  }
+}
+
 FieldTrial::FieldTrial(const std::string& name,
                        const Probability total_probability,
                        const std::string& default_group_name,
@@ -43,11 +59,11 @@ FieldTrial::FieldTrial(const std::string& name,
     next_group_number_(kDefaultGroupNumber + 1),
     group_(kNotFinalized),
     group_name_hash_(kReservedHashValue),
-    enable_field_trial_(true) {
+    enable_field_trial_(true),
+    forced_(false) {
   DCHECK_GT(total_probability, 0);
   DCHECK(!name_.empty());
   DCHECK(!default_group_name_.empty());
-  FieldTrialList::Register(this);
 
   DCHECK_GT(year, 1970);
   DCHECK_GT(month, 0);
@@ -71,6 +87,8 @@ FieldTrial::FieldTrial(const std::string& name,
 }
 
 void FieldTrial::UseOneTimeRandomization() {
+  if (forced_)
+    return;
   DCHECK_EQ(group_, kNotFinalized);
   DCHECK_EQ(kDefaultGroupNumber + 1, next_group_number_);
   if (!FieldTrialList::IsOneTimeRandomizationEnabled()) {
@@ -97,6 +115,13 @@ void FieldTrial::Disable() {
 
 int FieldTrial::AppendGroup(const std::string& name,
                             Probability group_probability) {
+  if (forced_) {
+    if (name == group_name_)
+      return group_;
+    else
+      return group_ + 1;
+  }
+
   DCHECK_LE(group_probability, divisor_);
   DCHECK_GE(group_probability, 0);
 
@@ -106,7 +131,7 @@ int FieldTrial::AppendGroup(const std::string& name,
   accumulated_group_probability_ += group_probability;
 
   DCHECK_LE(accumulated_group_probability_, divisor_);
-  if (group_ == kNotFinalized && accumulated_group_probability_ > random_) {
+  if (group_ == kNotFinalized && (accumulated_group_probability_ > random_)) {
     // This is the group that crossed the random line, so we do the assignment.
     group_ = next_group_number_;
     if (name.empty())
@@ -234,18 +259,6 @@ FieldTrialList::~FieldTrialList() {
 }
 
 // static
-void FieldTrialList::Register(FieldTrial* trial) {
-  if (!global_) {
-    used_without_global_ = true;
-    return;
-  }
-  AutoLock auto_lock(global_->lock_);
-  DCHECK(!global_->PreLockedFind(trial->name()));
-  trial->AddRef();
-  global_->registered_[trial->name()] = trial;
-}
-
-// static
 FieldTrial* FieldTrialList::Find(const std::string& name) {
   if (!global_)
     return NULL;
@@ -283,7 +296,7 @@ void FieldTrialList::StatesToString(std::string* output) {
 
   for (RegistrationList::iterator it = global_->registered_.begin();
        it != global_->registered_.end(); ++it) {
-    const std::string name = it->first;
+    const std::string& name = it->first;
     std::string group_name = it->second->group_name_internal();
     if (group_name.empty())
       continue;  // Should not include uninitialized trials.
@@ -313,23 +326,22 @@ void FieldTrialList::GetFieldTrialNameGroupIds(
 }
 
 // static
-bool FieldTrialList::CreateTrialsInChildProcess(
-    const std::string& parent_trials) {
+bool FieldTrialList::CreateTrialsFromString(const std::string& trials_string) {
   DCHECK(global_);
-  if (parent_trials.empty() || !global_)
+  if (trials_string.empty() || !global_)
     return true;
 
   size_t next_item = 0;
-  while (next_item < parent_trials.length()) {
-    size_t name_end = parent_trials.find(kPersistentStringSeparator, next_item);
-    if (name_end == parent_trials.npos || next_item == name_end)
+  while (next_item < trials_string.length()) {
+    size_t name_end = trials_string.find(kPersistentStringSeparator, next_item);
+    if (name_end == trials_string.npos || next_item == name_end)
       return false;
-    size_t group_name_end = parent_trials.find(kPersistentStringSeparator,
+    size_t group_name_end = trials_string.find(kPersistentStringSeparator,
                                                name_end + 1);
-    if (group_name_end == parent_trials.npos || name_end + 1 == group_name_end)
+    if (group_name_end == trials_string.npos || name_end + 1 == group_name_end)
       return false;
-    std::string name(parent_trials, next_item, name_end - next_item);
-    std::string group_name(parent_trials, name_end + 1,
+    std::string name(trials_string, next_item, name_end - next_item);
+    std::string group_name(trials_string, name_end + 1,
                            group_name_end - name_end - 1);
     next_item = group_name_end + 1;
 
@@ -349,9 +361,10 @@ FieldTrial* FieldTrialList::CreateFieldTrial(
   if (name.empty() || group_name.empty() || !global_)
     return NULL;
 
-  FieldTrial *field_trial(FieldTrialList::Find(name));
+  FieldTrial* field_trial(FieldTrialList::Find(name));
   if (field_trial) {
-    // In single process mode, we may have already created the field trial.
+    // In single process mode, or when we force them from the command line,
+    // we may have already created the field trial.
     if (field_trial->group_name_internal() != group_name)
       return NULL;
     return field_trial;
@@ -360,6 +373,8 @@ FieldTrial* FieldTrialList::CreateFieldTrial(
   field_trial = new FieldTrial(name, kTotalProbability, group_name,
                                kExpirationYearInFuture, 1, 1);
   field_trial->AppendGroup(group_name, kTotalProbability);
+  field_trial->forced_ = true;
+  FieldTrialList::Register(field_trial);
   return field_trial;
 }
 
@@ -425,4 +440,16 @@ FieldTrial* FieldTrialList::PreLockedFind(const std::string& name) {
   return it->second;
 }
 
+// static
+void FieldTrialList::Register(FieldTrial* trial) {
+  if (!global_) {
+    used_without_global_ = true;
+    return;
+  }
+  AutoLock auto_lock(global_->lock_);
+  DCHECK(!global_->PreLockedFind(trial->name()));
+  trial->AddRef();
+  global_->registered_[trial->name()] = trial;
+}
+
 }  // namespace base