Add testable interface for datastore.

service/datastore/interface.go

Index: service/datastore/interface.go
diff --git a/service/datastore/interface.go b/service/datastore/interface.go
index f767a81261489630e957770586b4d24c8f92b799..9eef7e7b6cd66c939dfc56204964f3550e492f3d 100644
--- a/service/datastore/interface.go
+++ b/service/datastore/interface.go
@@ -140,4 +140,8 @@ type Interface interface {
 	// be used interchangably; there's no danger of interleaving access to the
 	// datastore via the two.
 	Raw() RawInterface
+
+	// Testable returns the Testable interface for the implementation, or nil if
+	// there is none.
+	Testable() Testable

[dnj (Google), 2015/08/14 22:53:54]

Since most implementations return nil here, should this really be a
datastore.Interface method? It feels like it's bloating the interface.

I think that "Testable" and all supporting functions should be moved into
impl/memory, since:
(a) it's the only thing that's going to implement Testable, and
(b) different implementations down the road may have different controls or
testable primitives.

[iannucci, 2015/08/15 01:59:25]

There's no way to cast to it, which is why I'm doing this in the first place.
memory doesn't have the necessary access to the types to get a Testable from a
RawInterface.

I've moved this down to RawInterface only

[dnj (Google), 2015/08/15 02:07:53]

I was thinking something like:

testable := memory.Testable(ctx)

func Testable(ctx context.Context) {
  d := ds.getUnfiltered(ctx)
  //          ^ make me public
  if d != nil {
    if t, ok := d.(*dsImpl); ok {
      return t
    }
  }
}

 }