Refactor distributor API so that methods always get the Quest_Desc too.

dm/appengine/distributor/test_registry.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package distributor
 
 import (
         "fmt"
 
         "github.com/luci/luci-go/dm/api/service/v1"
         "github.com/luci/luci-go/tumble"
         "golang.org/x/net/context"
 )
 
+type TestFactoryFn func(context.Context, *Config) D
+
+type TestFactoryMap map[string]TestFactoryFn
+
 type testRegistry struct {
         finishExecutionImpl FinishExecutionFn
-        data                map[string]D
+        data                TestFactoryMap
 }
 
 var _ Registry = (*testRegistry)(nil)
 
 // NewTestingRegistry returns a new testing registry.
 //
 // The mocks dictionary maps from cfgName to a mock implementation of the
 // distributor.
-func NewTestingRegistry(mocks map[string]D, fFn FinishExecutionFn) Registry {
+func NewTestingRegistry(mocks TestFactoryMap, fFn FinishExecutionFn) Registry {
         return &testRegistry{fFn, mocks}
 }
 
 func (t *testRegistry) FinishExecution(c context.Context, eid *dm.Execution_ID, rslt *dm.Result) ([]tumble.Mutation, error) {
         return t.finishExecutionImpl(c, eid, rslt)
 }
 
-func (t *testRegistry) MakeDistributor(_ context.Context, cfgName string) (D, string, error) {
+func (t *testRegistry) MakeDistributor(c context.Context, cfgName string) (D, string, error) {
         ret, ok := t.data[cfgName]
         if !ok {
                 return nil, "", fmt.Errorf("unknown distributor configuration: %q", cfgName)
         }
-        return ret, "testing", nil
+        return ret(c, &Config{
+                DMHost:  "test-dm-host.example.com",
+                Version: "test-version",
+                Name:    cfgName,
+        }), "testing", nil
 }