cmd/tailscale,net/netcheck: add debug feature to force preferred DERP

This provides an interface for a user to force a preferred DERP outcome
for all future netchecks that will take precedence unless the forced
region is unreachable.

The option does not persist and will be lost when the daemon restarts.

Updates tailscale/corp#18997
Updates tailscale/corp#24755

Signed-off-by: James Tucker <james@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -175,6 +175,12 @@ var debugCmd = &ffcli.Command{
 			Exec:       localAPIAction("pick-new-derp"),
 			ShortHelp:  "Switch to some other random DERP home region for a short time",
 		},
+		{
+			Name:       "force-prefer-derp",
+			ShortUsage: "tailscale debug force-prefer-derp",
+			Exec:       forcePreferDERP,
+			ShortHelp:  "Prefer the given region ID if reachable (until restart, or 0 to clear)",
+		},
 		{
 			Name:       "force-netmap-update",
 			ShortUsage: "tailscale debug force-netmap-update",
@@ -577,6 +583,25 @@ func runDERPMap(ctx context.Context, args []string) error {
 	return nil
 }
 
+func forcePreferDERP(ctx context.Context, args []string) error {
+	var n int
+	if len(args) != 1 {
+		return errors.New("expected exactly one integer argument")
+	}
+	n, err := strconv.Atoi(args[0])
+	if err != nil {
+		return fmt.Errorf("expected exactly one integer argument: %w", err)
+	}
+	b, err := json.Marshal(n)
+	if err != nil {
+		return fmt.Errorf("failed to marshal DERP region: %w", err)
+	}
+	if err := localClient.DebugActionBody(ctx, "force-prefer-derp", bytes.NewReader(b)); err != nil {
+		return fmt.Errorf("failed to force preferred DERP: %w", err)
+	}
+	return nil
+}
+
 func localAPIAction(action string) func(context.Context, []string) error {
 	return func(ctx context.Context, args []string) error {
 		if len(args) > 0 {