nixos/tailscale: use systemctl restart during activation.

This avoids the scenario where you activate a new config over Tailscale,
and a long delay between the "stop services" and "start services" phases
of the activation script lead to your terminal freezing for tens of
seconds, until tailscaled finally gets started again and the session
recovers.

Per the documentation of stopIfChanged, this is only safe to do if the
service definition is robust to stopping the old process using the new
service definition. As the maintainer of the upstream systemd unit, I
can confirm that Tailscale is robust to this scenario: it has to be
in order to work right on several other distros that just do
unpack-then-restart, rather than the more complex stop-unpack-start
dance.

Signed-off-by: David Anderson <dave@natulte.net>

nixos/modules/services/networking/tailscale.nix

@@ -47,6 +47,18 @@ in {
       ] ++ (lib.optionals (cfg.permitCertUid != null) [
         "TS_PERMIT_CERT_UID=${cfg.permitCertUid}"
       ]);
+      # Restart tailscaled with a single `systemctl restart` at the
+      # end of activation, rather than a `stop` followed by a later
+      # `start`. Activation over Tailscale can hang for tens of
+      # seconds in the stop+start setup, if the activation script has
+      # a significant delay between the stop and start phases
+      # (e.g. script blocked on another unit with a slow shutdown).
+      #
+      # Tailscale is aware of the correctness tradeoff involved, and
+      # already makes its upstream systemd unit robust against unit
+      # version mismatches on restart for compatibility with other
+      # linux distros.
+      stopIfChanged = false;
     };
   };
 }