Store field trial parameters in shared memory

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 eb882da14f18c03b45682c29f6aa5501c100a0bf..662e635531ef72dcb454d78ea22c9c52a9b44954 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -13,6 +13,7 @@
 #include "base/debug/alias.h"
 #include "base/feature_list.h"
 #include "base/logging.h"
+#include "base/metrics/field_trial_param_associator.h"
 #include "base/pickle.h"
 #include "base/process/memory.h"
 #include "base/rand_util.h"
@@ -71,25 +72,81 @@ struct FieldTrialEntry {
   // Size of the pickled structure, NOT the total size of this entry.
   uint32_t size;
 
-  // Calling this is only valid when the entry is initialized. That is, it
-  // resides in shared memory and has a pickle containing the trial name and
-  // group name following it.
-  bool GetTrialAndGroupName(StringPiece* trial_name,
-                            StringPiece* group_name) const {
+  // Returns an iterator over the data containing names and params.
+  PickleIterator GetPickleIterator() const {
     char* src = reinterpret_cast<char*>(const_cast<FieldTrialEntry*>(this)) +
                 sizeof(FieldTrialEntry);
 
     Pickle pickle(src, size);
-    PickleIterator pickle_iter(pickle);
+    return PickleIterator(pickle);
+  }
 
-    if (!pickle_iter.ReadStringPiece(trial_name))
+  // Takes the iterator and writes out the first two items into |trial_name| and
+  // |group_name|.
+  bool ReadStringPair(PickleIterator* iter,
+                      StringPiece* trial_name,
+                      StringPiece* group_name) const {
+    if (!iter->ReadStringPiece(trial_name))
       return false;
-    if (!pickle_iter.ReadStringPiece(group_name))
+    if (!iter->ReadStringPiece(group_name))
       return false;
     return true;
   }
+
+  // Calling this is only valid when the entry is initialized. That is, it
+  // resides in shared memory and has a pickle containing the trial name and
+  // group name following it.
+  bool GetTrialAndGroupName(StringPiece* trial_name,
+                            StringPiece* group_name) const {
+    PickleIterator iter = GetPickleIterator();
+    return ReadStringPair(&iter, trial_name, group_name);
+  }
+
+  // Calling this is only valid when the entry is initialized as well. Reads the
+  // parameters following the trial and group name and stores them as key-value
+  // mappings in |params|.
+  bool GetParams(std::map<std::string, std::string>* params) const {
+    PickleIterator iter = GetPickleIterator();
+    StringPiece tmp;
+    if (!ReadStringPair(&iter, &tmp, &tmp))
+      return false;
+
+    while (true) {
+      StringPiece key;
+      StringPiece value;
+      if (!iter.ReadStringPiece(&key))
+        return true;  // No more params to read.
+      if (!iter.ReadStringPiece(&value))
+        return false;
+      (*params)[key.as_string()] = value.as_string();
+    }
+  }
 };
 
+// Writes out the field trial's contents (via trial_state) to the pickle. The
+// format of the pickle looks like:
+// TrialName, GroupName, ParamKey1, ParamValue1, ParamKey2, ParamValue2, ...
+// If there are no parameters, then it just ends at GroupName.
+bool PickleFieldTrial(const FieldTrial::State& trial_state, Pickle* pickle) {
+  if (!pickle->WriteString(trial_state.trial_name))
+    return false;
+  if (!pickle->WriteString(trial_state.group_name))
+    return false;
+
+  // Get field trial params.
+  std::map<std::string, std::string> params;
+  FieldTrialParamAssociator::GetInstance()->GetFieldTrialParamsWithoutFallback(
+      trial_state.trial_name.as_string(), trial_state.group_name.as_string(),
+      &params);
+
+  // Write params to pickle.
+  for (const auto& param : params) {
+    pickle->WriteString(StringPiece(param.first));
+    pickle->WriteString(StringPiece(param.second));
+  }
+  return true;
+}
+
 // Created a time value based on |year|, |month| and |day_of_month| parameters.
 Time CreateTimeFromParams(int year, int month, int day_of_month) {
   DCHECK_GT(year, 1970);
@@ -584,6 +641,79 @@ std::string FieldTrialList::FindFullName(const std::string& trial_name) {
 }
 
 // static
+bool FieldTrialList::GetParamsFromSharedMemory(
+    FieldTrial* field_trial,
+    std::map<std::string, std::string>* params) {
+  DCHECK(global_);
+  // If the field trial allocator is not set up yet, then there are several
+  // cases:
+  //   - We are in the browser process and the allocator has not been set up
+  //   yet. If we got here, then we couldn't find the params in
+  //   FieldTrialParamAssociator, so it's definitely not here. Return false.
+  //   - Using shared memory for field trials is not enabled. If we got here,
+  //   then there's nothing in shared memory. Return false.
+  //   - We are in the child process and the allocator has not been set up yet.
+  //   If this is the case, then you are calling this too early. The field trial
+  //   allocator should get set up very early in the lifecycle. Try to see if
+  //   you can call it after it's been set up.
+  if (!global_->field_trial_allocator_)
+    return false;
+
+  // If ref_ isn't set, then the field trial data can't be in shared memory.
+  if (!field_trial->ref_)
+    return false;
+
+  const FieldTrialEntry* entry =
+      global_->field_trial_allocator_->GetAsObject<const FieldTrialEntry>(
+          field_trial->ref_, kFieldTrialType);
+  return entry->GetParams(params);
+}
+
+// static
+void FieldTrialList::ClearParamsFromSharedMemoryForTesting() {

[Alexei Svitkine (slow), 2016/11/23 17:23:39]

Use the lock for this.

[lawrencewu, 2016/11/23 19:07:33]

Done.

+  if (!global_ || !global_->field_trial_allocator_)
+    return;
+  // To clear the params, we iterate through every item in the allocator, copy
+  // just the trial and group name into a newly-allocated segment and then clear
+  // the existing item.
+  SharedPersistentMemoryAllocator* allocator =

[Alexei Svitkine (slow), 2016/11/23 17:23:39]

FieldTrialAllocator

Change the other types below to your new typedefs too.

[lawrencewu, 2016/11/23 19:07:33]

Done.

+      global_->field_trial_allocator_.get();
+  PersistentMemoryAllocator::Iterator mem_iter(allocator);
+
+  SharedPersistentMemoryAllocator::Reference prev_ref;
+  while ((prev_ref = mem_iter.GetNextOfType(kFieldTrialType)) !=
+         SharedPersistentMemoryAllocator::kReferenceNull) {
+    // Get the existing field trial entry in shared memory.
+    const FieldTrialEntry* prev_entry =
+        allocator->GetAsObject<const FieldTrialEntry>(prev_ref,
+                                                      kFieldTrialType);
+    StringPiece trial_name;
+    StringPiece group_name;
+    prev_entry->GetTrialAndGroupName(&trial_name, &group_name);

[Alexei Svitkine (slow), 2016/11/23 17:23:39]

Check return value?

[lawrencewu, 2016/11/23 19:07:33]

Done by continuing if we failed to get the names.

+
+    // Write a new entry, minus the params.
+    Pickle pickle;
+    pickle.WriteString(trial_name);
+    pickle.WriteString(group_name);
+    size_t total_size = sizeof(FieldTrialEntry) + pickle.size();
+    SharedPersistentMemoryAllocator::Reference new_ref =
+        allocator->Allocate(total_size, kFieldTrialType);
+    FieldTrialEntry* new_entry =
+        allocator->GetAsObject<FieldTrialEntry>(new_ref, kFieldTrialType);
+    new_entry->activated = prev_entry->activated;
+    new_entry->size = pickle.size();
+
+    // Update the ref on the field trial.
+    FieldTrial* trial = Find(trial_name.as_string());
+    trial->ref_ = new_ref;
+
+    // Mark the existing entry as unused.
+    uint32_t oldType = allocator->GetType(prev_ref);

[Alexei Svitkine (slow), 2016/11/23 17:23:39]

old_type

[lawrencewu, 2016/11/23 19:07:33]

Fixed.

+    allocator->ChangeType(prev_ref, 0, oldType);
+  }
+}
+
+// static
 bool FieldTrialList::TrialExists(const std::string& trial_name) {
   return Find(trial_name) != NULL;
 }
@@ -1003,8 +1133,8 @@ void FieldTrialList::AddToAllocatorWhileLocked(FieldTrial* field_trial) {
     return;
 
   Pickle pickle;
-  pickle.WriteString(trial_state.trial_name);
-  pickle.WriteString(trial_state.group_name);
+  if (!PickleFieldTrial(trial_state, &pickle))
+    return;
 
   size_t total_size = sizeof(FieldTrialEntry) + pickle.size();
   FieldTrial::FieldTrialRef ref =