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 7dc5cb454e55877d7e345bbe13c1a4acca4d2711..7803ab69ab30261618d2f9521046a2747af69588 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 +72,88 @@ 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 (!ReadStringPair(&iter, &key, &value))
+        return key.empty();  // Non-empty is bad: got one of a pair.
+      (*params)[key.as_string()] = value.as_string();
+    }
+  }
 };
 
+// Writes out string1 and then string2 to pickle.
+bool WriteStringPair(Pickle* pickle,
+                     const StringPiece& string1,
+                     const StringPiece& string2) {
+  if (!pickle->WriteString(string1))
+    return false;
+  if (!pickle->WriteString(string2))
+    return false;
+  return true;
+}
+
+// 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 (!WriteStringPair(pickle, trial_state.trial_name, 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) {
+    if (!WriteStringPair(pickle, param.first, param.second))
+      return false;
+  }
+  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);
@@ -893,6 +957,102 @@ size_t FieldTrialList::GetFieldTrialCount() {
   return global_->registered_.size();
 }
 
+// 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_)

[Alexei Svitkine (slow), 2016/11/24 21:58:48]

field_trial_allocator_ shouldn't be used outside the lock. Can you use a lock
around this function?

[lawrencewu, 2016/11/24 22:07:03]

Done.

+    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);
+
+  size_t allocated_size =
+      global_->field_trial_allocator_->GetAllocSize(field_trial->ref_);
+  size_t actual_size = sizeof(FieldTrialEntry) + entry->size;
+  if (allocated_size < actual_size)
+    return false;
+
+  return entry->GetParams(params);
+}
+
+// static
+void FieldTrialList::ClearParamsFromSharedMemoryForTesting() {
+  if (!global_ || !global_->field_trial_allocator_)

[Alexei Svitkine (slow), 2016/11/24 21:58:48]

field_trial_allocator_ shouldn't be checked outside the lock. Split the if.

[lawrencewu, 2016/11/24 22:07:03]

Done.

+    return;
+  AutoLock auto_lock(global_->lock_);
+
+  // 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.
+  FieldTrialAllocator* allocator = global_->field_trial_allocator_.get();
+  FieldTrialAllocator::Iterator mem_iter(allocator);
+
+  // List of refs to eventually be made iterable. We can't make it in the loop,

[bcwhite, 2016/11/28 21:57:41]

It shouldn't.  You're iterating for kFieldTrialType but the last thing you do is
reset the type to zero (0).  Also, iteration only moves forward so you can
change current/past without affecting what is returned in the past.

But I think you can omit the MakeIterable completely.  You don't care to be able
to iterate over cleared entries... but even if you do, it was already made
iterable on line #1222 after it was first allocated.  Once done it's never
undone.

[lawrencewu, 2016/11/28 22:51:26]

I'm trying to make iterable the newly-allocated entries, not the cleared ones,
though. If I don't make it iterable then we can't access it anymore, and if I do
it in here, then we'll end up in a loop of allocating and then iterating over
that new piece until we run out of space.

[bcwhite, 2016/11/30 15:05:33]

Ah, of course.  Good, then.

+  // since it would go on forever.
+  std::vector<FieldTrial::FieldTrialRef> new_refs;
+
+  FieldTrial::FieldTrialRef prev_ref;
+  while ((prev_ref = mem_iter.GetNextOfType(kFieldTrialType)) !=
+         FieldTrialAllocator::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;
+    if (!prev_entry->GetTrialAndGroupName(&trial_name, &group_name))
+      continue;
+
+    // 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();
+    FieldTrial::FieldTrialRef 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();
+
+    // TODO(lawrencewu): Modify base::Pickle to be able to write over a section
+    // in memory, so we can avoid this memcpy.
+    char* dst = reinterpret_cast<char*>(new_entry) + sizeof(FieldTrialEntry);
+    memcpy(dst, pickle.data(), pickle.size());
+
+    // Update the ref on the field trial and add it to the list to be made
+    // iterable.
+    FieldTrial* trial = global_->PreLockedFind(trial_name.as_string());
+    trial->ref_ = new_ref;
+    new_refs.push_back(new_ref);
+
+    // Mark the existing entry as unused.
+    allocator->ChangeType(prev_ref, 0, kFieldTrialType);
+  }
+
+  for (const auto& ref : new_refs) {
+    allocator->MakeIterable(ref);
+  }
+}
+
 #if defined(OS_WIN)
 // static
 bool FieldTrialList::CreateTrialsFromWindowsHandle(HANDLE handle) {
@@ -1036,14 +1196,18 @@ 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)) {
+    NOTREACHED();
+    return;
+  }
 
   size_t total_size = sizeof(FieldTrialEntry) + pickle.size();
   FieldTrial::FieldTrialRef ref =
       allocator->Allocate(total_size, kFieldTrialType);
-  if (ref == FieldTrialAllocator::kReferenceNull)
+  if (ref == FieldTrialAllocator::kReferenceNull) {
+    NOTREACHED();
     return;
+  }
 
   FieldTrialEntry* entry =
       allocator->GetAsObject<FieldTrialEntry>(ref, kFieldTrialType);