v1 bug: Panic when printing completions

[VirrageS]

my urfave/cli version is

v1.22.2, v1.22.3

Checklist

• Are you running the latest v1 release? The list of releases is here.
• Did you check the manual for your release? The v1 manual is here
• Did you perform a search about this problem? Here's the Github guide about searching.

Dependency Management

• My project is using go modules.
• My project is using vendoring.
• My project is automatically downloading the latest version.
• I am unsure of what my dependency management setup is.

Describe the bug

I'm experiencing panic when trying to print bash completions for partially completed flag cmd subcmd --abc[TAB].

To reproduce

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/urfave/cli"
)

func main() {
	app := cli.NewApp()
	app.Name = "example"
	app.EnableBashCompletion = true
	app.Commands = []cli.Command{{
		Name:   "cmd",
		Action: func(c *cli.Context) error { return nil },
		Flags:  []cli.Flag{cli.StringFlag{Name: "abc1"}, cli.StringFlag{Name: "abc2"}},
		BashComplete: func(c *cli.Context) {
			fmt.Println(c.NArg())
			fmt.Println("--abc1")
			fmt.Println("--abc2")
		},
	}}
	app.Action = func(c *cli.Context) error {
		fmt.Println("Hello friend!")
		return nil
	}

	err := app.Run(os.Args)
	if err != nil {
		log.Fatal(err)
	}
}

1. Build code above
2. Load completions
3. example cmd --abc[TAB]

Observed behavior

Panic

Expected behavior

No panic. I also would expect that I will get completions for the flag:

$ example cmd --abc[TAB]
--abc1 --abc2

Additional context

cli/command.go

Lines 117 to 125 in c354cec

	set, err := c.parseFlags(ctx.Args().Tail())
	context := NewContext(ctx.App, set, ctx)
	context.Command = c
	if checkCommandCompletions(context, c.Name) {
	return nil
	}
	if err != nil {

Code here looks very suspicious. If we have err returned in c.parseFlags then set will be nil and checkCommandCompletions will access ctx which has ctx.flagSet == nil. This means that any calls inside the checkCommandCompletions accessing flagSet will panic (like .NArg()).

There was also similar issue for the v2: #944. It would be nice to have that fix backported to the v1 as well.

Run go version and paste its output here

go version go1.14 darwin/amd64

[coilysiren]

It would be nice to have that fix backported to the v1 as well.

@VirrageS PRs are welcome, if you want to take care of this yourself! The v1 code lives here => https://github.com/urfave/cli/tree/v1

[coilysiren]

released in 1.22.4! ✅ https://github.com/urfave/cli/releases/tag/v1.22.4