v2: if custom subcommand completion is provided, completing after partial flag input panics about cli

[dearchap]

@bobheadxi Can you provide an example of what you are doing in the completion func ?

[bobheadxi]

Can you provide an example of what you are doing in the completion func ?

The bug occurs even if you don't do anything in the completion func directly (see description - it seems to happen in urfave/cli before the handler actually executes), but sourcegraph/sourcegraph#58569 has a concrete example

[dearchap]

The reason I ask is that I dont see the issue when the completion func is not set. Let me investigate further. I have specified an invalid flag on the command line and then tried completion of a valid flag and it works just fine

$ issue_1822 command subcommand --stss --
--help        --stringflag  
$ issue_1822 command subcommand --stss --stringflag

[bobheadxi]

Hm, now that I'm taking a closer look it seems I can't reproduce it either:

	if err := (&cli.App{
		Name:                 "sg",
		EnableBashCompletion: true,
		Commands: []*cli.Command{{
			Name: "bar",
			Flags: []cli.Flag{
				&cli.StringFlag{
					Name:  "stringflag",
					Value: "vcs",
				},
			},
// internal helper used in my panic scenario
			BashComplete: completions.CompletePositionalArgs(func(args cli.Args) (options []string) {
				return []string{args.First()}
			}),
			Action: func(ctx *cli.Context) error {
				println("action!")
				return nil
			},
		}},
	}).RunContext(context.Background(), os.Args); err != nil {
		println(err.Error())
		os.Exit(1)
	}

However, in our full CLI app, the panic continues to point to the same place in the issue description, i.e.

cli/help.go

Line 467 in c023d9b

if args := cCtx.Args(); args.Present() {

, and the stacktrace looks like the one I shared previously. We do some pretty extensive setup so there might be an issue deep in there (though we skip most setup if we detect the bash completion flag), but the stacktrace doesn't seem to indicate that - let me dig into this some more