Store field trial parameters in shared memory

base/metrics/field_trial_param_associator.cc

 // Copyright 2016 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.
 
 #include "base/metrics/field_trial_param_associator.h"
 
+#include "base/debug/stack_trace.h"
 #include "base/metrics/field_trial.h"
 
 namespace base {
 
 FieldTrialParamAssociator::FieldTrialParamAssociator() {}
 FieldTrialParamAssociator::~FieldTrialParamAssociator() {}
 
 // static
 FieldTrialParamAssociator* FieldTrialParamAssociator::GetInstance() {
   return Singleton<FieldTrialParamAssociator,
@@ skipping 13 matching lines @@
   if (ContainsKey(field_trial_params_, key))
     return false;
 
   field_trial_params_[key] = params;
   return true;
 }
 
 bool FieldTrialParamAssociator::GetFieldTrialParams(
     const std::string& trial_name,
     FieldTrialParams* params) {
+  FieldTrial* field_trial = FieldTrialList::Find(trial_name);
+  if (!field_trial)
+    return false;
+  const FieldTrialKey key(trial_name, field_trial->group_name());
+
+  // We have to scope down here, since calling group_name() might result in a
+  // call to AddToAllocatorWhileLocked(), which might call

[Alexei Svitkine (slow), 2016/11/22 17:46:37]

So one consequence of this set up is that all field trial parameters must be
registered before any calls to group()/group_name(). I think this is OK with the
current code - but let's document this requirement - at least at the call site
where this matters - which is in variations_seed_processor.cc where it calls
group() and group_name() - add comments above those that say it's important that
this happens after params have been registered for the given trial.

[lawrencewu, 2016/11/22 20:25:44]

Done.

+  // GetFieldTrialParamsWithoutFallback() and cause a deadlock.
   AutoLock scoped_lock(lock_);
-
-  const std::string group_name = FieldTrialList::FindFullName(trial_name);
-  const FieldTrialKey key(trial_name, group_name);
   if (!ContainsKey(field_trial_params_, key))
-    return false;
+    return FieldTrialList::GetParamsFromSharedMemory(field_trial, params);

[Alexei Svitkine (slow), 2016/11/22 17:46:37]

Please add a comment explaining the flow for future readers.

Also, how about making this use GetFieldTrialParamsWithoutFallback()?

i.e. get the group name outside the lock and then call that function and if it
returns false, fallback to shared mem.

[lawrencewu, 2016/11/22 20:25:44]

Done.

 
   *params = field_trial_params_[key];
   return true;
 }
 
+void FieldTrialParamAssociator::GetFieldTrialParamsWithoutFallback(
+    const std::string& trial_name,
+    const std::string& group_name,
+    FieldTrialParams* params) {
+  AutoLock scoped_lock(lock_);
+
+  const FieldTrialKey key(trial_name, group_name);
+  if (!ContainsKey(field_trial_params_, key))
+    return;
+
+  *params = field_trial_params_[key];
+}
+
 void FieldTrialParamAssociator::ClearAllParamsForTesting() {
   AutoLock scoped_lock(lock_);
   field_trial_params_.clear();
+  FieldTrialList::ClearParamsFromSharedMemoryForTesting();
+}
+
+void FieldTrialParamAssociator::ClearAllCachedParamsForTesting() {
+  AutoLock scoped_lock(lock_);
+  field_trial_params_.clear();
 }
 
 }  // namespace base