Store field trial parameters in shared memory

base/metrics/field_trial_param_associator.cc

Index: base/metrics/field_trial_param_associator.cc
diff --git a/base/metrics/field_trial_param_associator.cc b/base/metrics/field_trial_param_associator.cc
index b96cc396df874689af034691d9479a780ae78417..3c35c1887fabc59992a406c77820a6a2f60445e1 100644
--- a/base/metrics/field_trial_param_associator.cc
+++ b/base/metrics/field_trial_param_associator.cc
@@ -4,6 +4,7 @@
 
 #include "base/metrics/field_trial_param_associator.h"
 
+#include "base/debug/stack_trace.h"
 #include "base/metrics/field_trial.h"
 
 namespace base {
@@ -37,20 +38,44 @@ bool FieldTrialParamAssociator::AssociateFieldTrialParams(
 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_);
+  if (!ContainsKey(field_trial_params_, key))
+    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 std::string group_name = FieldTrialList::FindFullName(trial_name);
   const FieldTrialKey key(trial_name, group_name);
   if (!ContainsKey(field_trial_params_, key))
-    return false;
+    return;
 
   *params = field_trial_params_[key];
-  return true;
 }
 
 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