create a module for round-robin

examples/l2tp.nix

@@ -47,6 +47,7 @@ in rec {
     ../modules/watchdog
     ../modules/mount
     ../modules/ppp
+    ../modules/round-robin
   ];
   hostname = "thing";
 
@@ -59,17 +60,7 @@ in rec {
 
   services.wan =
     let
-      controller = longrun rec {
+      pppoe = svc.pppoe.build {
-        name = "wan-switcher";
-        run = ''
-            in_outputs ${name}
-            exec ${pkgs.s6-rc-round-robin}/bin/s6-rc-round-robin \
-               -p ${proxy.name} \
-               ${lib.concatStringsSep " "
-                 (builtins.map (f: f.name) [pppoe l2tp])}
-          '';
-      };
-      pppoe = (svc.pppoe.build {
         interface = config.hardware.networkInterfaces.wan;
 
         ppp-options = [
@@ -77,7 +68,7 @@ in rec {
           "name" rsecrets.l2tp.name
           "password" rsecrets.l2tp.password
         ];
-      }).overrideAttrs(o: { inherit controller; });
+      };
 
       l2tp =
         let
@@ -91,7 +82,7 @@ in rec {
             target = lns.address;
             dependencies = [services.dhcpc check-address];
           };
-        in (svc.l2tp.build {
+        in svc.l2tp.build {
           lns = lns.address;
           ppp-options = [
             "debug" "+ipv6" "noauth"
@@ -99,20 +90,11 @@ in rec {
             "password" rsecrets.l2tp.password
           ];
           dependencies = [config.services.lns-address route check-address];
-        }).overrideAttrs(o: { inherit controller; });
+        };
-      proxy = oneshot rec {
+    in svc.round-robin.build {
-        name = "wan-proxy";
+      name = "wan";
-        inherit controller;
+      services = [ l2tp pppoe ];
-        buildInputs = [ pppoe l2tp];
+    };
-        up = ''
-            echo start proxy ${name}
-            set -x
-            (in_outputs ${name}
-             cp -rv $(output_path ${controller} active)/* .
-            )
-          '';
-      };
-    in proxy;
 
   services.sshd = svc.ssh.build { };
 

modules/round-robin/default.nix

@@ -0,0 +1,27 @@
+## Round Robin
+##
+## Given a list of services, run each in turn until it exits, then
+## runs the next.
+
+
+{ lib, pkgs, config, ...}:
+let
+  inherit (lib) mkOption types;
+  inherit (pkgs) liminix;
+  inherit (pkgs.liminix.services) longrun;
+in {
+  options = {
+    system.service.round-robin = mkOption {
+      description = "run services one at a time and failover to next";
+      type = liminix.lib.types.serviceDefn;
+    };
+  };
+  config.system.service.round-robin = config.system.callService ./service.nix {
+    services = mkOption {
+      type = types.listOf  liminix.lib.types.service;
+    };
+    name =  mkOption {
+      type = types.str;
+    };
+  };
+}

modules/round-robin/service.nix

@@ -0,0 +1,32 @@
+{
+  liminix, lib, s6-rc-round-robin
+}:
+{ services, name} :
+let
+   inherit (liminix.services) oneshot longrun;
+   controlled-services = builtins.map
+    (s: s.overrideAttrs(o: { inherit controller; }))
+    services;
+  controller = let name' = "control-${name}"; in longrun {
+    name = name';
+    run = ''
+      in_outputs ${name'}
+      exec ${s6-rc-round-robin}/bin/s6-rc-round-robin \
+         -p ${proxy.name} \
+         ${lib.concatStringsSep " "
+           (builtins.map (f: f.name) controlled-services)}
+    '';
+  };
+  proxy = oneshot rec {
+    inherit name;
+    inherit controller;
+    buildInputs = controlled-services;
+    up = ''
+      echo start proxy ${name}
+      set -x
+      (in_outputs ${name}
+       cp -rv $(output_path ${controller} active)/* .
+      )
+    '';
+  };
+in proxy

pkgs/s6-rc-round-robin/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation {
   propagatedBuildInputs = [ s6-rc-up-tree ];
   installPhase = ''
     mkdir -p $out/bin
-    cp -p ${writeFennel "uevent-watch" {
+    cp -p ${writeFennel "s6-rc-round-robin" {
       packages = [fennel anoia linotify lualinux s6-rc-up-tree] ;
       mainFunction = "run";
     } ./robin.fnl} $out/bin/s6-rc-round-robin