terraform: nil diffs should not be written to the diff

terraform/context_test.go

@@ -442,12 +442,11 @@ func TestContext2Plan_moduleVarComputed(t *testing.T) {
 	}
 }
 
-/*
-func TestContextPlan_nil(t *testing.T) {
+func TestContext2Plan_nil(t *testing.T) {
 	m := testModule(t, "plan-nil")
 	p := testProvider("aws")
 	p.DiffFn = testDiffFn
-	ctx := testContext(t, &ContextOpts{
+	ctx := testContext2(t, &ContextOpts{
 		Module: m,
 		Providers: map[string]ResourceProviderFactory{
 			"aws": testProviderFuncFixed(p),
@@ -478,6 +477,7 @@ func TestContextPlan_nil(t *testing.T) {
 	}
 }
 
+/*
 func TestContextPlan_computed(t *testing.T) {
 	m := testModule(t, "plan-computed")
 	p := testProvider("aws")

terraform/diff.go

@@ -333,6 +333,10 @@ func (d *InstanceDiff) Empty() bool {
 	return !d.Destroy && len(d.Attributes) == 0
 }
 
+func (d *InstanceDiff) GoString() string {
+	return fmt.Sprintf("*%#v", *d)
+}
+
 // RequiresNew returns true if the diff requires the creation of a new
 // resource (implying the destruction of the old).
 func (d *InstanceDiff) RequiresNew() bool {

terraform/eval_diff.go

@@ -162,6 +162,12 @@ func (n *EvalWriteDiff) Eval(
 	ctx EvalContext, args []interface{}) (interface{}, error) {
 	diff, lock := ctx.Diff()
 
+	// The diff to write, if its empty it should write nil
+	diffVal := n.Diff
+	if diffVal.Empty() {
+		diffVal = nil
+	}
+
 	// Acquire the lock so that we can do this safely concurrently
 	lock.Lock()
 	defer lock.Unlock()
@@ -171,7 +177,11 @@ func (n *EvalWriteDiff) Eval(
 	if modDiff == nil {
 		modDiff = diff.AddModule(ctx.Path())
 	}
-	modDiff.Resources[n.Name] = n.Diff
+	if diffVal != nil {
+		modDiff.Resources[n.Name] = diffVal
+	} else {
+		delete(modDiff.Resources, n.Name)
+	}
 
 	return nil, nil
 }