backend/local: check for empty config on apply

This prevents Terraform from crashing on apply/destroy with a directory
with no Terraform configuration files. We allow a destroy with no files
but not an apply.

backend/local/backend_apply.go

@@ -4,11 +4,13 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"strings"
 
 	"github.com/hashicorp/errwrap"
 	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/backend"
 	clistate "github.com/hashicorp/terraform/command/state"
+	"github.com/hashicorp/terraform/config/module"
 	"github.com/hashicorp/terraform/state"
 	"github.com/hashicorp/terraform/terraform"
 )
@@ -19,6 +21,19 @@ func (b *Local) opApply(
 	runningOp *backend.RunningOperation) {
 	log.Printf("[INFO] backend/local: starting Apply operation")
 
+	// 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.Destroy {
+		runningOp.Err = fmt.Errorf(strings.TrimSpace(applyErrNoConfig))
+		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)
 	stateHook := new(StateHook)
@@ -165,3 +180,12 @@ func (b *Local) opApply(
 		}
 	}
 }
+
+const applyErrNoConfig = `
+No configuration files found!
+
+Apply requires configuration to be present. Applying without a configuration
+would mark everything for destruction, which is normally not what is desired.
+If you would like to destroy everything, please run 'terraform destroy' instead
+which does not require any configuration files.
+`

backend/local/backend_apply_test.go

@@ -3,6 +3,7 @@ package local
 import (
 	"context"
 	"fmt"
+	"os"
 	"sync"
 	"testing"
 
@@ -50,6 +51,59 @@ test_instance.foo:
 	`)
 }
 
+func TestLocal_applyEmptyDir(t *testing.T) {
+	b := TestLocal(t)
+	p := TestLocalProvider(t, b, "test")
+
+	p.ApplyReturn = &terraform.InstanceState{ID: "yes"}
+
+	op := testOperationApply()
+	op.Module = nil
+
+	run, err := b.Operation(context.Background(), op)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	<-run.Done()
+	if run.Err == nil {
+		t.Fatal("should error")
+	}
+
+	if p.ApplyCalled {
+		t.Fatal("apply should not be called")
+	}
+
+	if _, err := os.Stat(b.StateOutPath); err == nil {
+		t.Fatal("should not exist")
+	}
+}
+
+func TestLocal_applyEmptyDirDestroy(t *testing.T) {
+	b := TestLocal(t)
+	p := TestLocalProvider(t, b, "test")
+
+	p.ApplyReturn = nil
+
+	op := testOperationApply()
+	op.Module = nil
+	op.Destroy = true
+
+	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.ApplyCalled {
+		t.Fatal("apply should not be called")
+	}
+
+	checkState(t, b.StateOutPath, `<no state>`)
+}
+
 func TestLocal_applyError(t *testing.T) {
 	b := TestLocal(t)
 	p := TestLocalProvider(t, b, "test")

backend/local/test-fixtures/apply-empty/hello.txt

@@ -0,0 +1 @@
+This is an empty dir