Added a Unique ID for a Field Trial containing it's hashed name and the selected group ID.

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
===================================================================
--- base/metrics/field_trial.cc	(revision 118842)
+++ base/metrics/field_trial.cc	(working copy)
@@ -5,6 +5,7 @@
 #include "base/metrics/field_trial.h"
 
 #include "base/build_time.h"
+#include "base/hash_tables.h"
 #include "base/logging.h"
 #include "base/rand_util.h"
 #include "base/sha1.h"
@@ -138,6 +139,13 @@
   return group_name_;
 }
 
+bool FieldTrial::GetUID(UID* uid) {
+  if (group_ == kNotFinalized)
+    return false;
+  *uid = std::make_pair(BASE_HASH_NAMESPACE::hash_value(name_), group_);

[jar (doing other things), 2012/01/24 19:19:45]

IMO, you should consider generating the hash once and for all when the field
trial is created, and leaving it in a slot for the field trial.

[I was also mistakenly worried about races here... but since you're recording
group, and not group name, there is no problem. If you transition to
group_name_, and hashing it, it will be critical to put the hash in a slot so
that you can atomically access it without a race concern vs group_, which is set
asynchronously compared to the group_name_.]

[MAD, 2012/01/24 20:30:24]

Yes, good point, I agree...
Done...

+  return true;
+}
+
 // static
 std::string FieldTrial::MakeName(const std::string& name_prefix,
                                  const std::string& trial_name) {
@@ -272,6 +280,21 @@
 }
 
 // static
+void FieldTrialList::GetFieldTrialUIDs(std::vector<FieldTrial::UID>* uids) {
+  if (!global_)
+    return;
+  DCHECK(uids->empty());
+  AutoLock auto_lock(global_->lock_);

[jar (doing other things), 2012/01/24 19:19:45]

This is probably ok.  I'm always careful to do as little as possible under a
lock.  Generating the hash does take a bit of time... so the temptation would be
to read the list of FieldTrial instances into a vector, release the lock, and
then process those instances.

Alternatively, if you moved to having a slot for the hashes, then the getter
would be more clearly insignificant.

[MAD, 2012/01/24 20:30:24]

Done...

+
+  for (RegistrationList::iterator it = global_->registered_.begin();
+       it != global_->registered_.end(); ++it) {
+    FieldTrial::UID uid;
+    if (it->second->GetUID(&uid))
+      uids->push_back(uid);
+  }
+}
+
+// static
 bool FieldTrialList::CreateTrialsInChildProcess(
     const std::string& parent_trials) {
   DCHECK(global_);