core: Context-level test for stub EvalDiff

Added a new test that ensures that pre/post-diff hooks are not called
when EvalDiff is run with Stub set, tested through a full refresh run.
This helps test the expected behaviour of EvalDiff itself, versus the
end result of the diff being counted in a plan, which is what the
TestLocal_planScaleOutNoDupeCount test in backend/local checks.

terraform/context_refresh_test.go

@@ -1049,3 +1049,63 @@ func TestContext2Validate(t *testing.T) {
 		t.Fatalf("bad: %s", e)
 	}
 }
+
+// TestContext2Refresh_noDiffHookOnScaleOut tests to make sure that
+// pre/post-diff hooks are not called when running EvalDiff on scale-out nodes
+// (nodes with no state). The effect here is to make sure that the diffs -
+// which only exist for interpolation of parallel resources or data sources -
+// do not end up being counted in the UI.
+func TestContext2Refresh_noDiffHookOnScaleOut(t *testing.T) {
+	h := new(MockHook)
+	p := testProvider("aws")
+	m := testModule(t, "refresh-resource-scale-inout")
+	p.RefreshFn = nil
+
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_instance.foo.0": &ResourceState{
+						Type: "aws_instance",
+						Deposed: []*InstanceState{
+							&InstanceState{
+								ID: "foo",
+							},
+						},
+					},
+					"aws_instance.foo.1": &ResourceState{
+						Type: "aws_instance",
+						Deposed: []*InstanceState{
+							&InstanceState{
+								ID: "bar",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Hooks:  []Hook{h},
+		ProviderResolver: ResourceProviderResolverFixed(
+			map[string]ResourceProviderFactory{
+				"aws": testProviderFuncFixed(p),
+			},
+		),
+		State: state,
+	})
+
+	_, err := ctx.Refresh()
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	if h.PreDiffCalled {
+		t.Fatal("PreDiff should not have been called")
+	}
+	if h.PostDiffCalled {
+		t.Fatal("PostDiff should not have been called")
+	}
+}