cluster/services/consul: implement consul-ready

cluster/services/consul/default.nix

@@ -15,6 +15,7 @@ in
     nixos.agent = [
       ./agent.nix
       ./remote-api.nix
+      ./ready.nix
     ];
   };
 

cluster/services/consul/ready.nix

@@ -0,0 +1,54 @@
+{ config, lib, pkgs, ... }:
+
+let
+  consulReady = pkgs.writers.writeHaskellBin "consul-ready" {
+    libraries = with pkgs.haskellPackages; [ aeson http-conduit watchdog ];
+  } ''
+    {-# LANGUAGE OverloadedStrings #-}
+    import Control.Watchdog
+    import Control.Exception
+    import System.IO
+    import Network.HTTP.Simple
+    import Data.Aeson
+
+    flushLogger :: WatchdogLogger String
+    flushLogger taskErr delay = do
+      defaultLogger taskErr delay
+      hFlush stdout
+
+    data ConsulHealth = ConsulHealth {
+      healthy :: Bool
+    }
+
+    instance FromJSON ConsulHealth where
+      parseJSON (Object v) = ConsulHealth <$> (v .: "Healthy")
+
+    handleException ex = case ex of
+      (SomeException _) -> return $ Left "Consul is not active"
+
+    main :: IO ()
+    main = watchdog $ do
+      setInitialDelay 300_000
+      setMaximumDelay 30_000_000
+      setLoggingAction flushLogger
+      watch $ handle handleException $ do
+          res <- httpJSON "${config.links.consulAgent.url}/v1/operator/autopilot/health"
+          case getResponseBody res of
+            ConsulHealth True -> return $ Right ()
+            ConsulHealth False -> return $ Left "Consul is unhealthy"
+  '';
+in
+
+{
+  systemd.services.consul-ready = {
+    description = "Wait for Consul";
+    requires = [ "consul.service" ];
+    after = [ "consul.service" ];
+    serviceConfig = {
+      ExecStart = lib.getExe consulReady;
+      DynamicUser = true;
+      TimeoutStartSec = "5m";
+      Type = "oneshot";
+    };
+  };
+}