completion: let nerdctl build --target <TAB> show Dockerfile stage names

[AkihiroSuda]

https://github.com/docker/cli/blob/9bc104eff0798097954f5d9bc25ca93f892e63f5/contrib/completion/bash/docker#L2929-L2941

Docker uses sed, but we can probably use real Dockerfile parser package
https://pkg.go.dev/github.com/moby/buildkit/frontend/dockerfile/parser#Parse

[djdongjin]

I did a try on this but encountered a wired go mod tidy issue while importing buildkit 🤔 (specifically, "github.com/moby/buildkit/frontend/dockerfile/instructions"

➜  nerdctl git:(build-stage-complete) ✗ go mod tidy
go: finding module for package github.com/docker/libnetwork/ipamutils
go: found github.com/docker/libnetwork/ipamutils in github.com/docker/libnetwork v0.5.6
go: finding module for package github.com/Sirupsen/logrus
go: found github.com/Sirupsen/logrus in github.com/Sirupsen/logrus v1.9.0
go: github.com/containerd/nerdctl/cmd/nerdctl imports
        github.com/moby/buildkit/frontend/dockerfile/instructions imports
        github.com/docker/docker/opts imports
        github.com/docker/libnetwork/ipamutils imports
        github.com/docker/libnetwork/osl imports
        github.com/Sirupsen/logrus: github.com/Sirupsen/[email protected]: parsing go.mod:
        module declares its path as: github.com/sirupsen/logrus
                but was required as: github.com/Sirupsen/logrus

The change is not much

// https://github.com/containerd/nerdctl/blob/7dfbaa2122628921febeb097e7a8a86074dc931d/cmd/nerdctl/completion.go
func shellCompleteDockerfile(cmd *cobra.Command) ([]string, cobra.ShellCompDirective) {
	filename := buildkitutil.DefaultDockerfileName
	if df, err := cmd.Flags().GetString("file"); err == nil && df != "" {
		filename = df
	}

	df, err := os.ReadFile(filename)
	if err != nil {
		return nil, cobra.ShellCompDirectiveError
	}
	ast, err := parser.Parse(bytes.NewBuffer(df))
	stages, _, err := instructions.Parse(ast.AST)

	candidates := []string{}
	for _, stage := range stages {
		candidates = append(candidates, stage.Name)
	}

	return nil, cobra.ShellCompDirectiveNoFileComp
}

Will visit this later.

[yuchanns]

I did a try on this but encountered a wired go mod tidy issue while importing buildkit 🤔 (specifically, "github.com/moby/buildkit/frontend/dockerfile/instructions"

FYI sirupsen/logrus#1041