terraform: support create-before-destroy

terraform/graph.go

@@ -482,6 +482,7 @@ func graphAddConfigResources(
 // these nodes for you.
 func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
 	var nlist []*depgraph.Noun
+	injected := make(map[*depgraph.Dependency]struct{})
 	for _, n := range g.Nouns {
 		rn, ok := n.Meta.(*GraphNodeResource)
 		if !ok {
@@ -530,13 +531,43 @@ func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
 			newDiff.Destroy = false
 			rd = newDiff
 
-			// Add to the new noun to our dependencies so that the destroy
-			// happens before the apply.
-			n.Deps = append(n.Deps, &depgraph.Dependency{
+			// The dependency ordering depends on if the CreateBeforeDestroy
+			// flag is enabled. If so, we must create the replacement first,
+			// and then destroy the old instance.
+			if rn.Config != nil && rn.Config.CreateBeforeDestroy && !rd.Empty() {
+				dep := &depgraph.Dependency{
+					Name:   n.Name,
+					Source: newN,
+					Target: n,
+				}
+
+				// Add the old noun to the new noun dependencies so that
+				// the create happens before the destroy.
+				newN.Deps = append(newN.Deps, dep)
+
+				// Mark that this dependency has been injected so that
+				// we do not invert the direction below.
+				injected[dep] = struct{}{}
+
+				// Add a depedency from the root, since the create node
+				// does not depend on us
+				g.Root.Deps = append(g.Root.Deps, &depgraph.Dependency{
+					Name:   newN.Name,
+					Source: g.Root,
+					Target: newN,
+				})
+
+			} else {
+				dep := &depgraph.Dependency{
 					Name:   newN.Name,
 					Source: n,
 					Target: newN,
-			})
+				}
+
+				// Add the new noun to our dependencies so that
+				// the destroy happens before the apply.
+				n.Deps = append(n.Deps, dep)
+			}
 		}
 
 		rn.Resource.Diff = rd
@@ -544,7 +575,6 @@ func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
 
 	// Go through each noun and make sure we calculate all the dependencies
 	// properly.
-	injected := make(map[*depgraph.Dependency]struct{})
 	for _, n := range nlist {
 		deps := n.Deps
 		num := len(deps)
@@ -948,6 +978,7 @@ func graphAddRoot(g *depgraph.Graph) {
 		})
 	}
 	g.Nouns = append(g.Nouns, root)
+	g.Root = root
 }
 
 // graphAddVariableDeps inspects all the nouns and adds any dependencies

terraform/graph_test.go

@@ -652,6 +652,92 @@ func TestGraphAddDiff_module(t *testing.T) {
 	}
 }
 
+func TestGraphAddDiff_createBeforeDestroy(t *testing.T) {
+	config := testConfig(t, "graph-diff-create-before")
+	diff := &Diff{
+		Resources: map[string]*InstanceDiff{
+			"aws_instance.bar": &InstanceDiff{
+				Destroy: true,
+				Attributes: map[string]*ResourceAttrDiff{
+					"ami": &ResourceAttrDiff{
+						Old:         "abc",
+						New:         "xyz",
+						RequiresNew: true,
+					},
+				},
+			},
+		},
+	}
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_instance.bar": &ResourceState{
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "bar",
+							Attributes: map[string]string{
+								"ami": "abc",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	diffHash := checksumStruct(t, diff)
+
+	g, err := Graph(&GraphOpts{
+		Config: config,
+		Diff:   diff,
+		State:  state,
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(g.String())
+	expected := strings.TrimSpace(testTerraformGraphDiffCreateBeforeDestroyStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s\n\nexpected:\n\n%s", actual, expected)
+	}
+
+	// Verify that our original structure has not been modified
+	diffHash2 := checksumStruct(t, diff)
+	if diffHash != diffHash2 {
+		t.Fatal("diff has been modified")
+	}
+}
+
+func TestGraphInitState(t *testing.T) {
+	config := testConfig(t, "graph-basic")
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*InstanceDiff{
+					"aws_instance.foo": &InstanceDiff{
+						Destroy: true,
+					},
+				},
+			},
+		},
+	}
+
+	g, err := Graph(&GraphOpts{Module: m, Diff: diff})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(g.String())
+	expected := strings.TrimSpace(testTerraformGraphDiffModuleStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s", actual)
+	}
+}
+
 func TestGraphAddDiff_moduleDestroy(t *testing.T) {
 	m := testModule(t, "graph-diff-module")
 	diff := &Diff{
@@ -1044,8 +1130,19 @@ aws_load_balancer.weblb
   aws_load_balancer.weblb -> provider.aws
 provider.aws
 root
-  root -> aws_load_balancer.weblb
-`
+  root -> aws_load_balancer.weblb`
+
+const testTerraformGraphDiffCreateBeforeDestroyStr = `
+root: root
+aws_instance.bar
+  aws_instance.bar -> provider.aws
+aws_instance.bar (destroy)
+  aws_instance.bar (destroy) -> aws_instance.bar
+  aws_instance.bar (destroy) -> provider.aws
+provider.aws
+root
+  root -> aws_instance.bar
+  root -> aws_instance.bar (destroy)`
 
 const testTerraformGraphStateStr = `
 root: root

terraform/test-fixtures/graph-diff-create-before/main.tf

@@ -0,0 +1,6 @@
+provider "aws" {}
+
+resource "aws_instance" "bar" {
+    ami = "abc"
+    create_before_destroy = true
+}