inspect: improve (flag) validation

Produce an error if the `--type` flag was set, but an empty value
was passed.

Before this patch:

    docker inspect --type "" foo
    # json output

    docker inspect --type unknown foo
    "unknown" is not a valid value for --type

With this patch:

    docker inspect --type "" foo
    type is empty: must be one of "config", "container", "image", "network", "node", "plugin", "secret", "service", "task", "volume"

    docker inspect --type unknown foo
    unknown type: "unknown": must be one of "config", "container", "image", "network", "node", "plugin", "secret", "service", "task", "volume"

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/inspect.go

@@ -69,6 +69,9 @@ func NewInspectCommand(dockerCli command.Cli) *cobra.Command {
 		Args:  cli.RequiresMinArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			opts.ids = args
+			if cmd.Flags().Changed("type") && opts.objectType == "" {
+				return fmt.Errorf(`type is empty: must be one of "%s"`, strings.Join(allTypes, `", "`))
+			}
 			return runInspect(cmd.Context(), dockerCli, opts)
 		},
 		// TODO(thaJeztah): should we consider adding completion for common object-types? (images, containers?)
@@ -97,7 +100,7 @@ func runInspect(ctx context.Context, dockerCli command.Cli, opts inspectOptions)
 		typePlugin, typeSecret, typeService, typeTask, typeVolume:
 		elementSearcher = inspectAll(ctx, dockerCli, opts.size, opts.objectType)
 	default:
-		return errors.Errorf("%q is not a valid value for --type", opts.objectType)
+		return errors.Errorf(`unknown type: %q: must be one of "%s"`, opts.objectType, strings.Join(allTypes, `", "`))
 	}
 	return inspect.Inspect(dockerCli.Out(), opts.ids, opts.format, elementSearcher)
 }

cli/command/system/inspect_test.go

@@ -0,0 +1,48 @@
+package system
+
+import (
+	"io"
+	"testing"
+
+	"github.com/docker/cli/internal/test"
+	"gotest.tools/v3/assert"
+	is "gotest.tools/v3/assert/cmp"
+)
+
+func TestInspectValidateFlagsAndArgs(t *testing.T) {
+	for _, tc := range []struct {
+		name        string
+		args        []string
+		expectedErr string
+	}{
+		{
+			name:        "empty type",
+			args:        []string{"--type", "", "something"},
+			expectedErr: `type is empty: must be one of "config", "container", "image", "network", "node", "plugin", "secret", "service", "task", "volume"`,
+		},
+		{
+			name:        "unknown type",
+			args:        []string{"--type", "unknown", "something"},
+			expectedErr: `unknown type: "unknown": must be one of "config", "container", "image", "network", "node", "plugin", "secret", "service", "task", "volume"`,
+		},
+		{
+			name:        "no arg",
+			args:        []string{},
+			expectedErr: `inspect: 'inspect' requires at least 1 argument`,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := NewInspectCommand(test.NewFakeCli(&fakeClient{}))
+			cmd.SetOut(io.Discard)
+			cmd.SetErr(io.Discard)
+			cmd.SetArgs(tc.args)
+
+			err := cmd.Execute()
+			if tc.expectedErr != "" {
+				assert.Check(t, is.ErrorContains(err, tc.expectedErr))
+			} else {
+				assert.Check(t, is.Nil(err))
+			}
+		})
+	}
+}