users/services: s6-rc: implement readiness polling

modules/users/s6-rc.nix

@@ -55,7 +55,7 @@ let
 
   # infers the service type from the arguments and creates an attrset usable by `fsToDerivation`.
   # also configures the service for logging, if applicable.
-  serviceToFs = { name, run, finish, depends, contents }: let
+  serviceToFs = { name, check, contents, depends, finish, run }: let
     type = if run != null then "longrun" else "bundle";
     logger = serviceToFs' {
       name = "logger:${name}";
@@ -64,8 +64,7 @@ let
       type = "longrun";
     };
   in (serviceToFs' {
-    inherit name type run finish;
-    depends = depends;
+    inherit name check depends finish run type;
     contents = maybe (type == "bundle") contents;
     producerFor = maybe (type == "longrun") "logger:${name}";
   }) // (lib.optionalAttrs (type == "longrun") logger);
@@ -73,19 +72,28 @@ let
   serviceToFs'= {
     name,
     type,
+    check ? null,  #< command to poll to check for service readiness
     consumerFor ? null,
     contents ? null,  #< bundle contents
     depends ? [],
     finish ? null,
     producerFor ? null,
     run ? null,
-  }: {
+  }: let
+    maybe-notify = lib.optionalString (check != null) "s6-notifyoncheck -n 0 ";
+  in {
     "${name}".dir = {
       "type".text = type;
       "contents".text = maybe (contents != null) (
         lib.concatStringsSep "\n" contents
       );
       "consumer-for".text = maybe (consumerFor != null) consumerFor;
+      "data".dir = {
+        "check".executable = maybe (check != null) ''
+          #!/bin/sh
+          exec ${check}
+        '';
+      };
       "dependencies.d".dir = lib.genAttrs
         depends
         (dep: { text = ""; })
@@ -94,11 +102,12 @@ let
         #!/bin/sh
         exec ${finish}
       '';
+      "notification-fd".text = maybe (check != null) "3";
       "producer-for".text = maybe (producerFor != null) producerFor;
       "run".executable = maybe (run != null) ''
         #!/bin/sh
         echo "starting: s6-${name}"
-        exec ${run} 2>&1
+        exec ${maybe-notify}${run} 2>&1
       '';
     };
   };
@@ -147,14 +156,15 @@ let
   s6SvcsFromConfigServices = services: lib.mapAttrsToList
     (name: service: {
       inherit name;
-      run = service.command;
-      finish = service.cleanupCommand;
-      depends = service.depends ++ builtins.attrNames (
-        lib.filterAttrs (_: cfg: lib.elem name cfg.dependencyOf) services
-      );
+      check = service.pollReadyCommand;
       contents = builtins.attrNames (
         lib.filterAttrs (_: cfg: lib.elem name cfg.partOf) services
       );
+      depends = service.depends ++ builtins.attrNames (
+        lib.filterAttrs (_: cfg: lib.elem name cfg.dependencyOf) services
+      );
+      finish = service.cleanupCommand;
+      run = service.command;
     })
     services
   ;
@@ -165,24 +175,30 @@ let
   implicitServices = {
     "default" = {
       command = null;
+      check = null;
       cleanupCommand = null;
       depends = [];
       dependencyOf = [];
       partOf = [];
+      pollReadyCommand = null;
     };
     "graphical-session" = {
       command = null;
+      check = null;
       cleanupCommand = null;
       depends = [];
       dependencyOf = [];
       partOf = [];
+      pollReadyCommand = null;
     };
     "sound" = {
       command = null;
+      check = null;
       cleanupCommand = null;
       depends = [];
       dependencyOf = [];
       partOf = [ "default" ];
+      pollReadyCommand = null;
     };
   };
 in