command/init: Fail if -input=false but required

When running `terraform init` against a backend with multiple
workspaces, none of which are the currently indicated local workspace,
Terraform prompts the user to choose a workspace from the list. In
automation, using the `-input=false` argument should disable asking for
input, but previously would hang instead.

internal/command/meta_backend.go

@@ -241,6 +241,10 @@ func (m *Meta) selectWorkspace(b backend.Backend) error {
 		return m.SetWorkspace(workspaces[0])
 	}
 
+	if !m.input {
+		return fmt.Errorf("Currently selected workspace %q does not exist", workspace)
+	}
+
 	// Otherwise, ask the user to select a workspace from the list of existing workspaces.
 	v, err := m.UIInput().Input(context.Background(), &terraform.InputOpts{
 		Id: "select-workspace",

internal/command/meta_backend_test.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"sort"
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/terraform/internal/addrs"
@@ -867,6 +868,28 @@ func TestMetaBackend_initSelectedWorkspaceDoesNotExistAutoSelect(t *testing.T) {
 	}
 }
 
+// Initializing a backend which supports workspaces and does *not* have
+// the currently selected workspace with input=false should fail.
+func TestMetaBackend_initSelectedWorkspaceDoesNotExistInputFalse(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	testCopyDir(t, testFixturePath("init-backend-selected-workspace-doesnt-exist-multi"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+	m.input = false
+
+	// Get the backend
+	_, diags := m.Backend(&BackendOpts{Init: true})
+
+	// Should fail immediately
+	if got, want := diags.ErrWithWarnings().Error(), `Currently selected workspace "bar" does not exist`; !strings.Contains(got, want) {
+		t.Fatalf("wrong error\ngot:  %s\nwant: %s", got, want)
+	}
+}
+
 // Changing a configured backend, copying state
 func TestMetaBackend_configuredChangeCopy(t *testing.T) {
 	// Create a temporary working directory that is empty