create: improve interface when attempting to create docker driver

commands/create.go

@@ -135,8 +135,8 @@ func runCreate(dockerCli command.Cli, in createOptions, args []string) error {
 		}
 	}
 
-	if driver.GetFactory(driverName, true) == nil {
-		return errors.Errorf("failed to find driver %q", driverName)
+	if _, err := driver.GetFactory(driverName, true); err != nil {
+		return err
 	}
 
 	ngOriginal := ng
@@ -282,7 +282,7 @@ func createCmd(dockerCli command.Cli) *cobra.Command {
 	var options createOptions
 
 	var drivers bytes.Buffer
-	for _, d := range driver.GetFactories() {
+	for _, d := range driver.GetFactories(true) {
 		if len(drivers.String()) > 0 {
 			drivers.WriteString(", ")
 		}

commands/util.go

@@ -60,8 +60,9 @@ func driversForNodeGroup(ctx context.Context, dockerCli command.Cli, ng *store.N
 
 	var f driver.Factory
 	if ng.Driver != "" {
-		f = driver.GetFactory(ng.Driver, true)
-		if f == nil {
+		var err error
+		f, err = driver.GetFactory(ng.Driver, true)
+		if err != nil {
 			return nil, errors.Errorf("failed to find driver %q", f)
 		}
 	} else {

docs/reference/buildx_create.md

@@ -15,7 +15,7 @@ Create a new builder instance
 | `--bootstrap` |  |  | Boot builder after creation |
 | [`--buildkitd-flags`](#buildkitd-flags) | `string` |  | Flags for buildkitd daemon |
 | [`--config`](#config) | `string` |  | BuildKit config file |
-| [`--driver`](#driver) | `string` |  | Driver to use (available: `docker`, `docker-container`, `kubernetes`, `remote`) |
+| [`--driver`](#driver) | `string` |  | Driver to use (available: `docker-container`, `kubernetes`, `remote`) |
 | [`--driver-opt`](#driver-opt) | `stringArray` |  | Options for the driver |
 | [`--leave`](#leave) |  |  | Remove a node from builder instead of changing it |
 | [`--name`](#name) | `string` |  | Builder instance name |

driver/manager.go

@@ -92,16 +92,16 @@ func GetDefaultFactory(ctx context.Context, ep string, c dockerclient.APIClient,
 	return dd[0].f, nil
 }
 
-func GetFactory(name string, instanceRequired bool) Factory {
+func GetFactory(name string, instanceRequired bool) (Factory, error) {
 	for _, f := range drivers {
-		if instanceRequired && !f.AllowsInstances() {
-			continue
-		}
 		if f.Name() == name {
-			return f
+			if instanceRequired && !f.AllowsInstances() {
+				return nil, errors.Errorf("additional instances of driver %q cannot be created", name)
+			}
+			return f, nil
 		}
 	}
-	return nil
+	return nil, errors.Errorf("failed to find driver %q", name)
 }
 
 func GetDriver(ctx context.Context, name string, f Factory, endpointAddr string, api dockerclient.APIClient, auth Auth, kcc KubeClientConfig, flags []string, files map[string][]byte, do map[string]string, platforms []specs.Platform, contextPathHash string) (Driver, error) {
@@ -131,9 +131,12 @@ func GetDriver(ctx context.Context, name string, f Factory, endpointAddr string,
 	return &cachedDriver{Driver: d}, nil
 }
 
-func GetFactories() []Factory {
+func GetFactories(instanceRequired bool) []Factory {
 	ds := make([]Factory, 0, len(drivers))
 	for _, d := range drivers {
+		if instanceRequired && !d.AllowsInstances() {
+			continue
+		}
 		ds = append(ds, d)
 	}
 	sort.Slice(ds, func(i, j int) bool {