backend/local: validate module exists for plan

Fixes #11504

The local backend should error if `terraform plan` is called in a
directory with no Terraform config files (same behavior as 0.8.x).
**New behavior:** We now allow `terraform plan -destroy` with no
configuration files since that seems reasonable.

backend/local/backend_plan.go

@@ -10,6 +10,7 @@ import (
 	"github.com/hashicorp/errwrap"
 	"github.com/hashicorp/terraform/backend"
 	"github.com/hashicorp/terraform/command/format"
+	"github.com/hashicorp/terraform/config/module"
 	"github.com/hashicorp/terraform/terraform"
 )
 
@@ -28,6 +29,18 @@ func (b *Local) opPlan(
 				"directory as an argument.\n\n"))
 	}
 
+	// A local plan requires either a plan or a module
+	if op.Plan == nil && op.Module == nil && !op.Destroy {
+		runningOp.Err = fmt.Errorf(strings.TrimSpace(planErrNoConfig))
+		return
+	}
+
+	// If we have a nil module at this point, then set it to an empty tree
+	// to avoid any potential crashes.
+	if op.Module == nil {
+		op.Module = module.NewEmptyTree()
+	}
+
 	// Setup our count hook that keeps track of resource changes
 	countHook := new(CountHook)
 	if b.ContextOpts == nil {
@@ -120,6 +133,16 @@ func (b *Local) opPlan(
 	}
 }
 
+const planErrNoConfig = `
+No configuration files found!
+
+Plan requires configuration to be present. Planning without a configuration
+would mark everything for destruction, which is normally not what is desired.
+If you would like to destroy everything, please run plan with the "-destroy"
+flag or create a single empty configuration file. Otherwise, please create
+a Terraform configuration file in the path being executed and try again.
+`
+
 const planHeaderNoOutput = `
 The Terraform execution plan has been generated and is shown below.
 Resources are shown in alphabetical order for quick scanning. Green resources

backend/local/backend_plan_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/terraform/backend"
@@ -36,6 +37,29 @@ func TestLocal_planBasic(t *testing.T) {
 	}
 }
 
+func TestLocal_planNoConfig(t *testing.T) {
+	b := TestLocal(t)
+	TestLocalProvider(t, b, "test")
+
+	op := testOperationPlan()
+	op.Module = nil
+	op.PlanRefresh = true
+
+	run, err := b.Operation(context.Background(), op)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	<-run.Done()
+
+	err = run.Err
+	if err == nil {
+		t.Fatal("should error")
+	}
+	if !strings.Contains(err.Error(), "configuration") {
+		t.Fatalf("bad: %s", err)
+	}
+}
+
 func TestLocal_planRefreshFalse(t *testing.T) {
 	b := TestLocal(t)
 	p := TestLocalProvider(t, b, "test")
@@ -110,6 +134,48 @@ func TestLocal_planDestroy(t *testing.T) {
 	}
 }
 
+func TestLocal_planDestroyNoConfig(t *testing.T) {
+	b := TestLocal(t)
+	p := TestLocalProvider(t, b, "test")
+	terraform.TestStateFile(t, b.StatePath, testPlanState())
+
+	outDir := testTempDir(t)
+	defer os.RemoveAll(outDir)
+	planPath := filepath.Join(outDir, "plan.tfplan")
+
+	op := testOperationPlan()
+	op.Destroy = true
+	op.PlanRefresh = true
+	op.Module = nil
+	op.PlanOutPath = planPath
+
+	run, err := b.Operation(context.Background(), op)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	<-run.Done()
+	if run.Err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if !p.RefreshCalled {
+		t.Fatal("refresh should be called")
+	}
+
+	if run.PlanEmpty {
+		t.Fatal("plan should not be empty")
+	}
+
+	plan := testReadPlan(t, planPath)
+	for _, m := range plan.Diff.Modules {
+		for _, r := range m.Resources {
+			if !r.Destroy {
+				t.Fatalf("bad: %#v", r)
+			}
+		}
+	}
+}
+
 func TestLocal_planOutPathNoChange(t *testing.T) {
 	b := TestLocal(t)
 	TestLocalProvider(t, b, "test")