terraform: copy before modifying struct in graphAddDiff

/cc @pearkes - Fixes the destroy bug

terraform/context.go

@@ -412,6 +412,7 @@ func (c *Context) applyWalkFn() depgraph.WalkFunc {
 	cb := func(r *Resource) error {
 		diff := r.Diff
 		if diff.Empty() {
+			log.Printf("[DEBUG] %s: Diff is empty. Will not apply.", r.Id)
 			return nil
 		}
 

terraform/graph.go

@@ -295,7 +295,10 @@ func graphAddDiff(g *depgraph.Graph, d *Diff) error {
 
 			// Mark the old diff to not destroy since we handle that in
 			// the dedicated node.
-			rd.Destroy = false
+			newDiff := new(ResourceDiff)
+			*newDiff = *rd
+			newDiff.Destroy = false
+			rd = newDiff
 
 			// Add to the new noun to our dependencies so that the destroy
 			// happens before the apply.

terraform/graph_test.go

@@ -195,6 +195,8 @@ func TestGraphAddDiff_destroy(t *testing.T) {
 		},
 	}
 
+	diffHash := checksumStruct(t, diff)
+
 	g, err := Graph(&GraphOpts{
 		Config: config,
 		Diff:   diff,
@@ -211,14 +213,20 @@ func TestGraphAddDiff_destroy(t *testing.T) {
 	}
 
 	// Verify that the state has been added
-	n := g.Noun("aws_instance.foo")
+	n := g.Noun("aws_instance.foo (destroy)")
 	rn := n.Meta.(*GraphNodeResource)
 
-	expected2 := diff.Resources["aws_instance.foo"]
+	expected2 := &ResourceDiff{Destroy: true}
 	actual2 := rn.Resource.Diff
 	if !reflect.DeepEqual(actual2, expected2) {
 		t.Fatalf("bad: %#v", actual2)
 	}
+
+	// Verify that our original structure has not been modified
+	diffHash2 := checksumStruct(t, diff)
+	if diffHash != diffHash2 {
+		t.Fatal("diff has been modified")
+	}
 }
 
 const testTerraformGraphStr = `

terraform/terraform_test.go

@@ -1,6 +1,10 @@
 package terraform
 
 import (
+	"bytes"
+	"crypto/sha1"
+	"encoding/gob"
+	"encoding/hex"
 	"path/filepath"
 	"sync"
 	"testing"
@@ -11,6 +15,17 @@ import (
 // This is the directory where our test fixtures are.
 const fixtureDir = "./test-fixtures"
 
+func checksumStruct(t *testing.T, i interface{}) string {
+	buf := new(bytes.Buffer)
+	enc := gob.NewEncoder(buf)
+	if err := enc.Encode(i); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	sum := sha1.Sum(buf.Bytes())
+	return hex.EncodeToString(sum[:])
+}
+
 func testConfig(t *testing.T, name string) *config.Config {
 	c, err := config.Load(filepath.Join(fixtureDir, name, "main.tf"))
 	if err != nil {