cmd/tailscale/cli: show last seen time on status command (#16588)

Add a last seen time on the cli's status command, similar to the web
portal.

Before:
```
100.xxx.xxx.xxx    tailscale-operator   tagged-devices linux   offline
```

After:
```
100.xxx.xxx.xxx    tailscale-operator   tagged-devices linux   offline, last seen 20d ago
```

Fixes #16584

Signed-off-by: Mahyar Mirrashed <mah.mirr@gmail.com>

cmd/tailscale/cli/cli.go

@@ -18,6 +18,7 @@ import (
 	"strings"
 	"sync"
 	"text/tabwriter"
+	"time"
 
 	"github.com/mattn/go-colorable"
 	"github.com/mattn/go-isatty"
@@ -538,3 +539,19 @@ func jsonDocsWalk(cmd *ffcli.Command) *commandDoc {
 	}
 	return res
 }
+
+func lastSeenFmt(t time.Time) string {
+	if t.IsZero() {
+		return ""
+	}
+	d := max(time.Since(t), time.Minute) // at least 1 minute
+
+	switch {
+	case d < time.Hour:
+		return fmt.Sprintf(", last seen %dm ago", int(d.Minutes()))
+	case d < 24*time.Hour:
+		return fmt.Sprintf(", last seen %dh ago", int(d.Hours()))
+	default:
+		return fmt.Sprintf(", last seen %dd ago", int(d.Hours()/24))
+	}
+}

cmd/tailscale/cli/exitnode.go

@@ -173,11 +173,13 @@ func hasAnyExitNodeSuggestions(peers []*ipnstate.PeerStatus) bool {
 // a peer. If there is no notable state, a - is returned.
 func peerStatus(peer *ipnstate.PeerStatus) string {
 	if !peer.Active {
+		lastseen := lastSeenFmt(peer.LastSeen)
+
 		if peer.ExitNode {
-			return "selected but offline"
+			return "selected but offline" + lastseen
 		}
 		if !peer.Online {
-			return "offline"
+			return "offline" + lastseen
 		}
 	}
 

cmd/tailscale/cli/status.go

@@ -164,7 +164,7 @@ func runStatus(ctx context.Context, args []string) error {
 		anyTraffic := ps.TxBytes != 0 || ps.RxBytes != 0
 		var offline string
 		if !ps.Online {
-			offline = "; offline"
+			offline = "; offline" + lastSeenFmt(ps.LastSeen)
 		}
 		if !ps.Active {
 			if ps.ExitNode {
@@ -174,7 +174,7 @@ func runStatus(ctx context.Context, args []string) error {
 			} else if anyTraffic {
 				f("idle" + offline)
 			} else if !ps.Online {
-				f("offline")
+				f("offline" + lastSeenFmt(ps.LastSeen))
 			} else {
 				f("-")
 			}
@@ -193,7 +193,7 @@ func runStatus(ctx context.Context, args []string) error {
 				f("peer-relay %s", ps.PeerRelay)
 			}
 			if !ps.Online {
-				f("; offline")
+				f(offline)
 			}
 		}
 		if anyTraffic {