backend/local: refresh with no config should not crash on input

Fixes #12174

You're allowed to refresh with a nil module (no configs) as long as you
have state. However, if `-input=true` (default) then this would crash
since the input attempts to read the configs.

The API contract with `terraform.Context` says that the module tree must
be non-nil and loaded. To do this for other commands we create an empty
module tree. We do that here now.

backend/local/backend_refresh.go

@@ -9,6 +9,7 @@ import (
 	"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"
 )
 
@@ -42,6 +43,12 @@ func (b *Local) opRefresh(
 		}
 	}
 
+	// If we have no config module given to use, create an empty tree to
+	// avoid crashes when Terraform.Context is initialized.
+	if op.Module == nil {
+		op.Module = module.NewEmptyTree()
+	}
+
 	// Get our context
 	tfCtx, opState, err := b.context(op)
 	if err != nil {

backend/local/backend_refresh_test.go

@@ -40,6 +40,63 @@ test_instance.foo:
 	`)
 }
 
+func TestLocal_refreshNilModule(t *testing.T) {
+	b := TestLocal(t)
+	p := TestLocalProvider(t, b, "test")
+	terraform.TestStateFile(t, b.StatePath, testRefreshState())
+
+	p.RefreshFn = nil
+	p.RefreshReturn = &terraform.InstanceState{ID: "yes"}
+
+	op := testOperationRefresh()
+	op.Module = nil
+
+	run, err := b.Operation(context.Background(), op)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	<-run.Done()
+
+	if !p.RefreshCalled {
+		t.Fatal("refresh should be called")
+	}
+
+	checkState(t, b.StateOutPath, `
+test_instance.foo:
+  ID = yes
+	`)
+}
+
+// GH-12174
+func TestLocal_refreshNilModuleWithInput(t *testing.T) {
+	b := TestLocal(t)
+	p := TestLocalProvider(t, b, "test")
+	terraform.TestStateFile(t, b.StatePath, testRefreshState())
+
+	p.RefreshFn = nil
+	p.RefreshReturn = &terraform.InstanceState{ID: "yes"}
+
+	b.OpInput = true
+
+	op := testOperationRefresh()
+	op.Module = nil
+
+	run, err := b.Operation(context.Background(), op)
+	if err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+	<-run.Done()
+
+	if !p.RefreshCalled {
+		t.Fatal("refresh should be called")
+	}
+
+	checkState(t, b.StateOutPath, `
+test_instance.foo:
+  ID = yes
+	`)
+}
+
 func TestLocal_refreshInput(t *testing.T) {
 	b := TestLocal(t)
 	p := TestLocalProvider(t, b, "test")