core: fix targeting in destroy w/ provisioners

The `TargetTransform` was dropping provisioner nodes, which caused graph
validation to fail with messages about uninitialized provisioners when a
`terraform destroy` was attempted.

This was because `destroy` flops the dependency calculation to try and
address any nodes in the graph that "depend on" the target node. But we
still need to keep the provisioner node in the graph.

closes #1541

terraform/context_test.go

@@ -2688,6 +2688,48 @@ func TestContext2Validate_tainted(t *testing.T) {
 	}
 }
 
+func TestContext2Validate_targetedDestroy(t *testing.T) {
+	m := testModule(t, "validate-targeted")
+	p := testProvider("aws")
+	pr := testProvisioner()
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		Provisioners: map[string]ResourceProvisionerFactory{
+			"shell": testProvisionerFuncFixed(pr),
+		},
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: rootModulePath,
+					Resources: map[string]*ResourceState{
+						"aws_instance.foo": resourceState("aws_instance", "i-bcd345"),
+						"aws_instance.bar": resourceState("aws_instance", "i-abc123"),
+					},
+				},
+			},
+		},
+		Targets: []string{"aws_instance.foo"},
+		Destroy: true,
+	})
+
+	w, e := ctx.Validate()
+	if len(w) > 0 {
+		warnStr := ""
+		for _, v := range w {
+			warnStr = warnStr + " " + v
+		}
+		t.Fatalf("bad: %s", warnStr)
+	}
+	if len(e) > 0 {
+		t.Fatalf("bad: %s", e)
+	}
+}
+
 func TestContext2Validate_varRef(t *testing.T) {
 	m := testModule(t, "validate-variable-ref")
 	p := testProvider("aws")

terraform/test-fixtures/validate-targeted/main.tf

@@ -0,0 +1,13 @@
+resource "aws_instance" "foo" {
+    num = "2"
+    provisioner "shell" {
+       command = "echo hi"
+    }
+}
+
+resource "aws_instance" "bar" {
+    foo = "bar"
+    provisioner "shell" {
+       command = "echo hi"
+    }
+}

terraform/transform_targets.go

@@ -59,6 +59,12 @@ func (t *TargetsTransformer) selectTargetedNodes(
 			continue
 		}
 
+		// Keep all provisioners; they'll be pruned later if necessary
+		if r, ok := v.(GraphNodeProvisioner); ok {
+			targetedNodes.Add(r)
+			continue
+		}
+
 		// For the remaining filter, we only care about addressable nodes
 		r, ok := v.(GraphNodeAddressable)
 		if !ok {