fix(avahi): workaround weird hostname conflicts

nixos/avahi.nix

@@ -1,9 +1,12 @@
 {
+  pkgs,
   config,
   lib,
   ...
 }: let
-  inherit (lib.modules) mkDefault;
+  inherit (lib.modules) mkDefault mkIf mkOptionDefault;
+  inherit (lib.strings) makeBinPath;
+  cfg = config.services.avahi;
 in {
   services.avahi = {
     enable = mkDefault true;
@@ -16,4 +19,41 @@ in {
     };
     wideArea = mkDefault false;
   };
+
+  systemd.services = let
+    # work around a weird bug or interaction in avahi-daemon
+    RestartSec = 2;
+    daemon = "avahi-daemon.service";
+    avahi-daemon-watchdog = pkgs.writeShellScript "avahi-daemon-watchdog" ''
+      set -eu
+      export PATH="$PATH:${makeBinPath [ config.systemd.package pkgs.coreutils pkgs.gnugrep ]}"
+      while read -r line; do
+        if [[ $line = *"Host name conflict"* ]]; then
+          if systemctl is-active ${daemon} > /dev/null; then
+            echo restarting avahi-daemon due to host name conflict... >&2
+            systemctl stop ${daemon}
+            sleep ${toString RestartSec}
+            systemctl start ${daemon}
+          fi
+        fi
+      done < <(journalctl -o cat -feu ${daemon} | grep -F 'Host name conflict, retrying with ')
+    '';
+  in mkIf (cfg.enable && cfg.publish.enable) {
+    avahi-daemon = {
+      serviceConfig = {
+        inherit RestartSec;
+      };
+    };
+    avahi-daemon-watchdog = {
+      wantedBy = [ daemon ];
+      serviceConfig = {
+        Type = mkOptionDefault "exec";
+        ExecStart = [
+          "${avahi-daemon-watchdog}"
+        ];
+        Restart = mkOptionDefault "on-failure";
+        RestartSec = mkOptionDefault RestartSec;
+      };
+    };
+  };
 }