Make clock sync latency metric go between each pair of clock domains.

tracing/tracing/model/clock_sync_manager.html

Index: tracing/tracing/model/clock_sync_manager.html
diff --git a/tracing/tracing/model/clock_sync_manager.html b/tracing/tracing/model/clock_sync_manager.html
index 82531ce46a3b8b41ea444bfaba3890340f22ef74..724a9675f59a08901f86424fd9f3020df32fba32 100644
--- a/tracing/tracing/model/clock_sync_manager.html
+++ b/tracing/tracing/model/clock_sync_manager.html
@@ -147,30 +147,33 @@ tr.exportTo('tr.model', function() {
      * domains are viable candidates, the one with the clock domain ID that is
      * the first alphabetically is selected.
      */
-    getModelTimeTransformer: function(domainId) {
-      return this.getModelTimeTransformerRaw_(domainId).fn;
+    getModelTimeTransformer: function(domainId, targetDomainId) {

[charliea (OOO until 10-5), 2016/09/21 18:05:33]

We don't need this function to accept a targetDomainId, do we? It doesn't seem
like it's used anywhere, and it just provides people with a footgun.

[alexandermont, 2016/09/21 19:26:40]

Done

+      return this.getModelTimeTransformerRaw_(domainId, targetDomainId).fn;
     },
 
     /**
      * Returns the error associated with the transformation given by
      * |getModelTimeTransformer(domainId)|.
      */
-    getModelTimeTransformerError: function(domainId) {
-      return this.getModelTimeTransformerRaw_(domainId).error;
+    getModelTimeTransformerError: function(domainId, targetDomainId) {

[charliea (OOO until 10-5), 2016/09/21 18:05:33]

Hmmm, this doesn't make any sense: getModelTimeTransformerError indicates that
the transformer is *between* the *model* domain and domainId. Providing a second
parameter without changing the function name doesn't make sense.

My vote is to change it to 

getTimeTransformerError: function(fromDomainId, toDomainId) {
  ...
}

When you do this, don't forget to update the jsdoc to something reasonable for
the new func name.

[alexandermont, 2016/09/21 19:26:40]

Done

+      return this.getModelTimeTransformerRaw_(domainId, targetDomainId).error;
     },
 
-    getModelTimeTransformerRaw_: function(domainId) {
+    getModelTimeTransformerRaw_: function(domainId, targetDomainId) {

[charliea (OOO until 10-5), 2016/09/21 18:05:33]

As mentioned above, having a "modelTimeTransformer..." function that accepts two
domains doesn't make any sense name-wise.

My vote is that we have two functions, getModelTimeTransformerRaw_ and
getTimeTransformerRaw_. getTransformerRaw_ accepts two domains, fromDomainId and
toDomainId. getModelTimeTransformerRaw_ only accepts one, and defers to
getTimeTransformerRaw_ after doing any model-domain-specific things.

I'm thinking something like:

getModelTimeTransformerRaw_: function(domainId) {
  this.onDomainSeen_(domainId);
  
  if (!this.modelDomainId_)
    this.selectModelDomainId_();

  return this.getTimeTransformerRaw_(domainId, this.modelDomainId_);
},

getTimeTransformerRaw_: function(fromDomainId, toDomainId) {
  this.onDomainSeen_(fromDomainId);
  this.onDomainSeen_(toDomainId);

  var transformer = 
    this.getTransformerBetween_(fromDomainId, toDomainId);
  if (!transformer) {
    ...
  }

  return transformer;
}

Note that, we have to put this.onDomainSeen_(domainId) before the model
selection, even though it happens again in getTimeTransformerRaw_, because which
domains have been seen might have an impact on model clock domain selection.

[alexandermont, 2016/09/21 19:26:40]

Done

       this.onDomainSeen_(domainId);
 
       if (!this.modelDomainId_)
         this.selectModelDomainId_();
 
+      if (targetDomainId === undefined)
+        targetDomainId = this.modelDomainId_;
+
       var transformer =
-          this.getTransformerBetween_(domainId, this.modelDomainId_);
+          this.getTransformerBetween_(domainId, targetDomainId);
       if (!transformer) {
         throw new Error('No clock sync markers exist pairing clock domain "' +
-            domainId + '" ' + 'with model clock domain "' +
-            this.modelDomainId_ + '".');
+            domainId + '" ' + 'with target clock domain "' +
+            targetDomainId + '".');
       }
 
       return transformer;