terraform: prevent_destroy works for decreasing count

Fixes #5826

The `prevent_destroy` lifecycle configuration was not being checked when
the count was decreased for a resource with a count. It was only
checking when attributes changed on pre-existing resources.

This fixes that.

terraform/context_plan_test.go

@@ -805,6 +805,127 @@ func TestContext2Plan_preventDestroy_good(t *testing.T) {
 	}
 }
 
+func TestContext2Plan_preventDestroy_countBad(t *testing.T) {
+	m := testModule(t, "plan-prevent-destroy-count-bad")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: rootModulePath,
+					Resources: map[string]*ResourceState{
+						"aws_instance.foo.0": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-abc123",
+							},
+						},
+						"aws_instance.foo.1": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-abc345",
+							},
+						},
+					},
+				},
+			},
+		},
+	})
+
+	plan, err := ctx.Plan()
+
+	expectedErr := "aws_instance.foo.1: the plan would destroy"
+	if !strings.Contains(fmt.Sprintf("%s", err), expectedErr) {
+		t.Fatalf("expected err would contain %q\nerr: %s\nplan: %s",
+			expectedErr, err, plan)
+	}
+}
+
+func TestContext2Plan_preventDestroy_countGood(t *testing.T) {
+	m := testModule(t, "plan-prevent-destroy-count-good")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: rootModulePath,
+					Resources: map[string]*ResourceState{
+						"aws_instance.foo.0": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-abc123",
+							},
+						},
+						"aws_instance.foo.1": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-abc345",
+							},
+						},
+					},
+				},
+			},
+		},
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if plan.Diff.Empty() {
+		t.Fatalf("Expected non-empty plan, got %s", plan.String())
+	}
+}
+
+func TestContext2Plan_preventDestroy_countGoodNoChange(t *testing.T) {
+	m := testModule(t, "plan-prevent-destroy-count-good")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: rootModulePath,
+					Resources: map[string]*ResourceState{
+						"aws_instance.foo.0": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-abc123",
+								Attributes: map[string]string{
+									"current": "0",
+									"type":    "aws_instance",
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if !plan.Diff.Empty() {
+		t.Fatalf("Expected empty plan, got %s", plan.String())
+	}
+}
+
 func TestContext2Plan_preventDestroy_destroyPlan(t *testing.T) {
 	m := testModule(t, "plan-prevent-destroy-good")
 	p := testProvider("aws")

terraform/eval_check_prevent_destroy.go

@@ -10,8 +10,9 @@ import (
 // error if a resource has PreventDestroy configured and the diff
 // would destroy the resource.
 type EvalCheckPreventDestroy struct {
-	Resource *config.Resource
-	Diff     **InstanceDiff
+	Resource   *config.Resource
+	ResourceId string
+	Diff       **InstanceDiff
 }
 
 func (n *EvalCheckPreventDestroy) Eval(ctx EvalContext) (interface{}, error) {
@@ -23,7 +24,12 @@ func (n *EvalCheckPreventDestroy) Eval(ctx EvalContext) (interface{}, error) {
 	preventDestroy := n.Resource.Lifecycle.PreventDestroy
 
 	if diff.GetDestroy() && preventDestroy {
-		return nil, fmt.Errorf(preventDestroyErrStr, n.Resource.Id())
+		resourceId := n.ResourceId
+		if resourceId == "" {
+			resourceId = n.Resource.Id()
+		}
+
+		return nil, fmt.Errorf(preventDestroyErrStr, resourceId)
 	}
 
 	return nil, nil

terraform/graph_config_node_resource.go

@@ -168,8 +168,9 @@ func (n *GraphNodeConfigResource) DynamicExpand(ctx EvalContext) (*Graph, error)
 		// expand orphans, which have all the same semantics in a destroy
 		// as a primary or tainted resource.
 		steps = append(steps, &OrphanTransformer{
-			State: state,
-			View:  n.Resource.Id(),
+			Resource: n.Resource,
+			State:    state,
+			View:     n.Resource.Id(),
 		})
 
 		steps = append(steps, &DeposedTransformer{

terraform/test-fixtures/plan-prevent-destroy-count-bad/main.tf

@@ -0,0 +1,8 @@
+resource "aws_instance" "foo" {
+  count = "1"
+  current = "${count.index}"
+
+  lifecycle {
+    prevent_destroy = true
+  }
+}

terraform/test-fixtures/plan-prevent-destroy-count-good/main.tf

@@ -0,0 +1,4 @@
+resource "aws_instance" "foo" {
+  count = "1"
+  current = "${count.index}"
+}

terraform/transform_orphan.go

@@ -17,6 +17,11 @@ type GraphNodeStateRepresentative interface {
 // OrphanTransformer is a GraphTransformer that adds orphans to the
 // graph. This transformer adds both resource and module orphans.
 type OrphanTransformer struct {
+	// Resource is resource configuration. This is only non-nil when
+	// expanding a resource that is in the configuration. It can't be
+	// dependend on.
+	Resource *config.Resource
+
 	// State is the global state. We require the global state to
 	// properly find module orphans at our path.
 	State *State
@@ -80,6 +85,7 @@ func (t *OrphanTransformer) Transform(g *Graph) error {
 			resourceVertexes[i] = g.Add(&graphNodeOrphanResource{
 				Path:        g.Path,
 				ResourceKey: rsk,
+				Resource:    t.Resource,
 				Provider:    rs.Provider,
 				dependentOn: rs.Dependencies,
 			})
@@ -159,6 +165,7 @@ func (n *graphNodeOrphanModule) Expand(b GraphBuilder) (GraphNodeSubgraph, error
 type graphNodeOrphanResource struct {
 	Path        []string
 	ResourceKey *ResourceStateKey
+	Resource    *config.Resource
 	Provider    string
 
 	dependentOn []string
@@ -283,6 +290,11 @@ func (n *graphNodeOrphanResource) managedResourceEvalNodes(info *InstanceInfo) [
 					State:  &state,
 					Output: &diff,
 				},
+				&EvalCheckPreventDestroy{
+					Resource:   n.Resource,
+					ResourceId: n.ResourceKey.String(),
+					Diff:       &diff,
+				},
 				&EvalWriteDiff{
 					Name: n.ResourceKey.String(),
 					Diff: &diff,