feat: show RootlessKit version in nerdctl version output

When running in rootless mode, nerdctl version now includes the
RootlessKit version as a server component, matching the format used
by docker version. This helps users quickly verify which RootlessKit
version is in use without running rootlesskit --version separately.

The version is only shown when rootless mode is active (i.e. when
rootlessutil.IsRootless() is true). If rootlesskit is not found on
PATH or its output cannot be parsed, a warning is logged and the
component is shown without a version string.

Fixes #4936

Signed-off-by: Aaron <aaroniofjm@gmail.com>

Author: amarkdotdev

pkg/infoutil/infoutil.go

@@ -35,6 +35,7 @@ import (
 	"github.com/containerd/nerdctl/v2/pkg/buildkitutil"
 	"github.com/containerd/nerdctl/v2/pkg/inspecttypes/dockercompat"
 	"github.com/containerd/nerdctl/v2/pkg/inspecttypes/native"
+	"github.com/containerd/nerdctl/v2/pkg/rootlessutil"
 	"github.com/containerd/nerdctl/v2/pkg/version"
 )
 
@@ -142,6 +143,9 @@ func ServerVersion(ctx context.Context, client *containerd.Client) (*dockercompa
 			runcVersion(),
 		},
 	}
+	if rootlessutil.IsRootless() {
+		v.Components = append(v.Components, rootlessKitVersion())
+	}
 	return v, nil
 }
 
@@ -233,6 +237,32 @@ func parseRuncVersion(runcVersionStdout []byte) (*dockercompat.ComponentVersion,
 }
 
 // getMobySysInfo returns the moby system info for the given cgroup manager
+
+func rootlessKitVersion() dockercompat.ComponentVersion {
+	stdout, err := exec.Command("rootlesskit", "--version").Output()
+	if err != nil {
+		log.L.WithError(err).Warnf("unable to determine RootlessKit version")
+		return dockercompat.ComponentVersion{Name: "rootlesskit"}
+	}
+	v, err := parseRootlessKitVersion(stdout)
+	if err != nil {
+		log.L.Warn(err)
+		return dockercompat.ComponentVersion{Name: "rootlesskit"}
+	}
+	return *v
+}
+
+func parseRootlessKitVersion(stdout []byte) (*dockercompat.ComponentVersion, error) {
+	// Expected format: "rootlesskit version 2.0.0" or "rootlesskit version 2.3.1+dev"
+	fields := strings.Fields(strings.TrimSpace(string(stdout)))
+	if len(fields) < 3 || fields[0] != "rootlesskit" {
+		return nil, fmt.Errorf("unable to parse RootlessKit version, got: %s", string(stdout))
+	}
+	return &dockercompat.ComponentVersion{
+		Name:    "rootlesskit",
+		Version: fields[2],
+	}, nil
+}
 func getMobySysInfo(cgroupManager string) *sysinfo.SysInfo {
 	var info dockercompat.Info
 	info.CgroupVersion = CgroupsVersion()

pkg/infoutil/infoutil_test.go

@@ -83,3 +83,26 @@ libseccomp: 2.5.1`: {
 		}
 	}
 }
+
+func TestParseRootlessKitVersion(t *testing.T) {
+	testCases := map[string]*dockercompat.ComponentVersion{
+		"rootlesskit version 2.3.1\n": {
+			Name:    "rootlesskit",
+			Version: "2.3.1",
+		},
+		"rootlesskit version 2.0.0+dev\n": {
+			Name:    "rootlesskit",
+			Version: "2.0.0+dev",
+		},
+		"unexpected output": nil,
+	}
+	for s, expected := range testCases {
+		got, err := parseRootlessKitVersion([]byte(s))
+		if expected != nil {
+			assert.NilError(t, err)
+			assert.DeepEqual(t, expected, got)
+		} else {
+			assert.Assert(t, err != nil)
+		}
+	}
+}