Is the "cant duplicate this flag" error (v3) makes sense?

[Pаramtamtām]

Hi there, team of the best CLI package 😄

I have a question that might be a bit silly, but I was confused when I encountered the error can't duplicate this flag while using only one flag with the OnlyOnce: true property for a cli.Command.

cli/flag_impl.go

Lines 166 to 168 in c4cd0a5

 if f.count == 1 && f.OnlyOnce { 
 	return fmt.Errorf("cant duplicate this flag") 
 } 

Is this a mistake or not? If not, what is the purpose of this guard? Could someone explain it to me?

[Pаramtamtām]

Ouch, I could provide a bit more context on how exactly I encountered this error.

Usually, if the app I develop has more than one sub-command that requires the same flags, I prefer to define every flag that I could reuse in a separate package named shared:

package shared

import (
	// ...
	"github.com/urfave/cli/v3"
)

var ListenPortFlag = &cli.UintFlag{
	Name:     "port",
	Aliases:  []string{"p"},
	Usage:    "TCP port number",
	Value:    8080, // default port number
	Sources:  cli.EnvVars("LISTEN_PORT"),
	OnlyOnce: true,
	Required: true,
	Validator: func(port uint64) error {
		if port == 0 || port > 65535 {
			return fmt.Errorf("invalid TCP port number [%d]", port)
		}
		return nil
	},
}

// ...

And use it in my commands in this manner:

import (
	// ...
	"github.com/urfave/cli/v3"
)

func NewCommand() *cli.Command {
	return &cli.Command{
		Name:    "some_command",
		Aliases: []string{"some", "command"},
		Action: func(ctx context.Context, c *cli.Command) error {
			runtime.Gosched() // <-- important, can be replaced with something else

			return nil // some code goes here
		},
		Flags: []cli.Flag{
			shared.ListenPortFlag, // <-- important
		},
	}
}

Everything works fine until I write a unit test to test the command execution:

func TestCommand_RunSuccess(t *testing.T) {
	t.Parallel()

	var cmd = healthcheck.NewCommand()

	require.NoError(t, cmd.Run(context.Background(), []string{"", "--port", "1234"}))
}

func TestCommand_RunFail(t *testing.T) {
	t.Parallel()

	cmd := healthcheck.NewCommand()

	require.NoError(t, cmd.Run(context.Background(), []string{"", "--port", "4321"}))
}

If you run each test one by one (not together - that's important), each of them will pass. But, if you run them in parallel (go test ./...), the test will fail with the error:

=== RUN   TestCommand_RunFail
=== PAUSE TestCommand_RunFail
=== CONT  TestCommand_RunFail
    command_test.go:47: 
        	Error Trace:	/path/to/command_test.go:666
        	Error:      	Received unexpected error:
        	            	invalid value "4321" for flag -port: cant duplicate this flag
        	Test:       	TestCommand_RunFail
--- FAIL: TestCommand_RunFail (0.00s)

To fix this, all I need to do is replace

// file: shared/flags.go

var ListenPortFlag = &cli.UintFlag{
//                   ^

with

// file: shared/flags.go

var ListenPortFlag = cli.UintFlag{

// file: command.go

func NewCommand() *cli.Command {
	var portFlag = shared.ListenPortFlag // make a copy
//      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

	return &cli.Command{
		Name:    "some_command",
		Aliases: []string{"some", "command"},
		Action: func(ctx context.Context, c *cli.Command) error {
			return nil // some code goes here
		},
		Flags: []cli.Flag{
			&portFlag, // use the copy
//                      ^^^^^^^^^
		},
	}
}

This was so strange that I decided to create this issue to highlight this workaround.

Note

TL;DR - flags have their own state, so don't reuse them using pointers.

[dearchap]

@tarampampam The OnlyOnce field is used to tell the cli library to expect this flag on the cmd line only once. So if it is specified twice , for example

--port 1234 --port 4566

then you will get that error. This is to protect users from themselves. Without this option in above example port would be set to 4566