hi: add list-versions subcommand

Prints the tailscale versions integration tests use, with optional
filter and format flags. Source of truth stays in hscontrol/capver;
CI shells out to this instead of duplicating the version list in
YAML.

Examples:
  hi list-versions
  hi list-versions --set=must --exclude=head
  hi list-versions --set=all --format=json

cmd/hi/listversions.go

@@ -0,0 +1,89 @@
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/creachadair/command"
+	"github.com/juanfont/headscale/hscontrol/capver"
+)
+
+var (
+	errUnknownSet    = errors.New("unknown --set value (want must|all)")
+	errUnknownFormat = errors.New("unknown --format value (want space|newline|json)")
+)
+
+// ListVersionsConfig holds flags for the list-versions subcommand.
+type ListVersionsConfig struct {
+	Set     string `flag:"set,default=must,Version set: must|all"`
+	Exclude string `flag:"exclude,Comma-separated versions to exclude (e.g. head,unstable)"`
+	Format  string `flag:"format,default=space,Output format: space|newline|json"`
+}
+
+var listVersionsConfig ListVersionsConfig
+
+// listVersions prints the Tailscale versions used by integration tests
+// in a format CI can shell out to. Mirrors integration/scenario.go
+// AllVersions and MustTestVersions: "head" and "unstable" are bare
+// tags, releases get a "v" prefix so each entry can be appended to
+// "ghcr.io/tailscale/tailscale:" directly.
+func listVersions(env *command.Env) error {
+	release := capver.TailscaleLatestMajorMinor(capver.SupportedMajorMinorVersions, true)
+	all := append([]string{"head", "unstable"}, release...)
+	must := append(append([]string{}, all[0:4]...), all[len(all)-2:]...)
+
+	var versions []string
+
+	switch listVersionsConfig.Set {
+	case "must":
+		versions = must
+	case "all":
+		versions = all
+	default:
+		return fmt.Errorf("%w: %q", errUnknownSet, listVersionsConfig.Set)
+	}
+
+	excluded := make(map[string]bool)
+
+	if listVersionsConfig.Exclude != "" {
+		for v := range strings.SplitSeq(listVersionsConfig.Exclude, ",") {
+			excluded[strings.TrimSpace(v)] = true
+		}
+	}
+
+	out := make([]string, 0, len(versions))
+
+	for _, v := range versions {
+		if excluded[v] {
+			continue
+		}
+
+		if v != "head" && v != "unstable" {
+			v = "v" + v
+		}
+
+		out = append(out, v)
+	}
+
+	switch listVersionsConfig.Format {
+	case "space":
+		fmt.Println(strings.Join(out, " "))
+	case "newline":
+		for _, v := range out {
+			fmt.Println(v)
+		}
+	case "json":
+		b, err := json.Marshal(out)
+		if err != nil {
+			return err
+		}
+
+		fmt.Println(string(b))
+	default:
+		return fmt.Errorf("%w: %q", errUnknownFormat, listVersionsConfig.Format)
+	}
+
+	return nil
+}

cmd/hi/main.go

@@ -29,6 +29,13 @@ func main() {
 					return runDoctorCheck(env.Context())
 				},
 			},
+			{
+				Name:     "list-versions",
+				Help:     "Print Tailscale versions used by integration tests",
+				Usage:    "list-versions [flags]",
+				SetFlags: command.Flags(flax.MustBind, &listVersionsConfig),
+				Run:      listVersions,
+			},
 			{
 				Name: "clean",
 				Help: "Clean Docker resources",