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)
@@ -21,6 +21,9 @@
 const int FieldTrial::kDefaultGroupNumber = 0;
 
 // static
+const uint32 FieldTrial::kReservedHashValue = 0;
+
+// static
 bool FieldTrial::enable_benchmarking_ = false;
 
 // static
@@ -42,12 +45,14 @@
                        const int month,
                        const int day_of_month)
   : name_(name),
+    name_hash_(HashName(name)),
     divisor_(total_probability),
     default_group_name_(default_group_name),
     random_(static_cast<Probability>(divisor_ * RandDouble())),
     accumulated_group_probability_(0),
     next_group_number_(kDefaultGroupNumber + 1),
     group_(kNotFinalized),
+    group_name_hash_(kReservedHashValue),
     enable_field_trial_(true) {
   DCHECK_GT(total_probability, 0);
   DCHECK(!name_.empty());
@@ -96,6 +101,7 @@
   if (group_ != kNotFinalized) {
     group_ = kDefaultGroupNumber;
     group_name_ = default_group_name_;
+    group_name_hash_ = HashName(group_name_);
   }
 }
 
@@ -117,6 +123,7 @@
       StringAppendF(&group_name_, "%d", group_);
     else
       group_name_ = name;
+    group_name_hash_ = HashName(group_name_);
     FieldTrialList::NotifyFieldTrialGroupSelection(name_, group_name_);
   }
   return next_group_number_++;
@@ -127,6 +134,7 @@
     accumulated_group_probability_ = divisor_;
     group_ = kDefaultGroupNumber;
     group_name_ = default_group_name_;
+    group_name_hash_ = HashName(group_name_);
     FieldTrialList::NotifyFieldTrialGroupSelection(name_, group_name_);
   }
   return group_;
@@ -138,6 +146,14 @@
   return group_name_;
 }
 
+bool FieldTrial::GetNameGroupId(NameGroupId* name_group_id) {
+  if (group_name_hash_ == kReservedHashValue)
+    return false;
+  name_group_id->name = name_hash_;
+  name_group_id->group = group_name_hash_;
+  return true;
+}
+
 // static
 std::string FieldTrial::MakeName(const std::string& name_prefix,
                                  const std::string& trial_name) {
@@ -173,6 +189,21 @@
   return BitsToOpenEndedUnitInterval(*bits);
 }
 
+// static
+uint32 FieldTrial::HashName(const std::string& name) {
+  // SHA-1 is designed to produce a uniformly random spread in its output space,
+  // even for nearly-identical name.

[Alexei Svitkine (slow), 2012/01/25 17:40:16]

"nearly-identical name" -> "nearly-identical inputs" (more grammatical).

[MAD, 2012/01/25 18:48:53]

Done.

+  unsigned char sha1_hash[kSHA1Length];
+  SHA1HashBytes(reinterpret_cast<const unsigned char*>(name.c_str()),

[Alexei Svitkine (slow), 2012/01/25 15:25:19]

I think you can use |SHA1HashString()| instead of |SHA1HashBytes()| here.

[MAD, 2012/01/25 17:02:33]

I copied the behavior from HashClientId... Should we change both?

[Alexei Svitkine (slow), 2012/01/25 17:40:16]

Looking closer at |SHA1HashString()|, it returns its output as a string - which
is not a problem since we can still cast its data() to bits for the same effect,
but is less efficient because it involves an extra temporary string and probably
an allocation.

So actually, I'm okay with keeping it as it is, since it is slightly more
efficient.

[MAD, 2012/01/25 18:48:53]

OK, Thanks for checking... ;-)

+                name.size(),
+                sha1_hash);
+
+  COMPILE_ASSERT(sizeof(uint32) < sizeof(sha1_hash), need_more_data);
+  uint32* bits = reinterpret_cast<uint32*>(&sha1_hash[0]);
+  DCHECK(*bits != kReservedHashValue) << "Bad luck, that name can't be used!";
+  return *bits;
+}
+
 //------------------------------------------------------------------------------
 // FieldTrialList methods and members.
 
@@ -272,6 +303,22 @@
 }
 
 // static
+void FieldTrialList::GetFieldTrialNameGroupIds(
+    std::vector<FieldTrial::NameGroupId>* name_group_ids) {
+  if (!global_)

[Alexei Svitkine (slow), 2012/01/25 15:25:19]

Should you clear |name_group_ids| in this case (and also before appending)?

[MAD, 2012/01/25 17:02:33]

Again, I copied the behavior from StatesToString which DCHECK that the output
argument is empty... Should we change both?

[Alexei Svitkine (slow), 2012/01/25 17:40:16]

I'm okay with the DCHECK(), though then it should be before if (!global_) and
the header comment should mention that |name_group_ids| must be empty initially.

Can you make those changes both here and StatesToString()?

[MAD, 2012/01/25 18:48:53]

Done.

+    return;
+  DCHECK(name_group_ids->empty());
+  AutoLock auto_lock(global_->lock_);
+
+  for (RegistrationList::iterator it = global_->registered_.begin();
+       it != global_->registered_.end(); ++it) {
+    FieldTrial::NameGroupId name_group_id;
+    if (it->second->GetNameGroupId(&name_group_id))
+      name_group_ids->push_back(name_group_id);
+  }
+}
+
+// static
 bool FieldTrialList::CreateTrialsInChildProcess(
     const std::string& parent_trials) {
   DCHECK(global_);