wgengine/magicsock: track per-endpoint changes in ringbuffer

This change adds a ringbuffer to each magicsock endpoint that keeps a
fixed set of "changes"–debug information about what updates have been
made to that endpoint.

Additionally, this adds a LocalAPI endpoint and associated
"debug peer-status" CLI subcommand to fetch the set of changes for a given
IP or hostname.

Updates tailscale/corp#9364

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I34f726a71bddd0dfa36ec05ebafffb24f6e0516a

cmd/tailscale/cli/debug.go

@@ -213,6 +213,11 @@ var debugCmd = &ffcli.Command{
 				return fs
 			})(),
 		},
+		{
+			Name:      "peer-endpoint-changes",
+			Exec:      runPeerEndpointChanges,
+			ShortHelp: "prints debug information about a peer's endpoint changes",
+		},
 	},
 }
 
@@ -822,3 +827,61 @@ func debugPortmap(ctx context.Context, args []string) error {
 	_, err = io.Copy(os.Stdout, rc)
 	return err
 }
+
+func runPeerEndpointChanges(ctx context.Context, args []string) error {
+	st, err := localClient.Status(ctx)
+	if err != nil {
+		return fixTailscaledConnectError(err)
+	}
+	description, ok := isRunningOrStarting(st)
+	if !ok {
+		printf("%s\n", description)
+		os.Exit(1)
+	}
+
+	if len(args) != 1 || args[0] == "" {
+		return errors.New("usage: peer-status <hostname-or-IP>")
+	}
+	var ip string
+
+	hostOrIP := args[0]
+	ip, self, err := tailscaleIPFromArg(ctx, hostOrIP)
+	if err != nil {
+		return err
+	}
+	if self {
+		printf("%v is local Tailscale IP\n", ip)
+		return nil
+	}
+
+	if ip != hostOrIP {
+		log.Printf("lookup %q => %q", hostOrIP, ip)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "GET", "http://local-tailscaled.sock/localapi/v0/debug-peer-endpoint-changes?ip="+ip, nil)
+	if err != nil {
+		return err
+	}
+
+	resp, err := localClient.DoLocalRequest(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return err
+	}
+
+	var dst bytes.Buffer
+	if err := json.Indent(&dst, body, "", "  "); err != nil {
+		return fmt.Errorf("indenting returned JSON: %w", err)
+	}
+
+	if ss := dst.String(); !strings.HasSuffix(ss, "\n") {
+		dst.WriteByte('\n')
+	}
+	fmt.Printf("%s", dst.String())
+	return nil
+}

cmd/tailscaled/depaware.txt

@@ -303,6 +303,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/util/osshare                                   from tailscale.com/ipn/ipnlocal+
    W    tailscale.com/util/pidowner                                  from tailscale.com/ipn/ipnauth
         tailscale.com/util/racebuild                                 from tailscale.com/logpolicy
+        tailscale.com/util/ringbuffer                                from tailscale.com/wgengine/magicsock
         tailscale.com/util/set                                       from tailscale.com/health+
         tailscale.com/util/singleflight                              from tailscale.com/control/controlclient+
         tailscale.com/util/slicesx                                   from tailscale.com/net/dnscache+