Allow using elastic IP resources by direct reference

Example:

  resources.elasticIPs.my-ip =
    { region = "eu-west-1"; ...
    };

  machine =
    { config, pkgs, resources, ... }:
    { deployment.targetEnv = "ec2"; ...
      deployment.ec2.elasticIPv4 = resources.elasticIPs.my-ip;
    };

nix/ec2.nix

@@ -330,7 +330,8 @@ in
     deployment.ec2.elasticIPv4 = mkOption {
       default = "";
       example = "203.0.113.123";
-      type = types.str;
+      type = union types.str (resource "elastic-ip");
+      apply = x: if builtins.isString x then x else "res-" + x._name;
       description = ''
         Elastic IPv4 address to be associated with this machine.
       '';

nix/elastic-ip.nix

@@ -25,4 +25,6 @@ with pkgs.lib;
 
   };
 
+  config._type = "elastic-ip";
+
 }

nixops/backends/ec2.py

@@ -13,6 +13,7 @@
 from nixops.backends import MachineDefinition, MachineState
 from nixops.nix_expr import Function, RawValue
 from nixops.resources.ebs_volume import EBSVolumeState
+from nixops.resources.elastic_ip import ElasticIPState
 import nixops.util
 import nixops.ec2_utils
 import nixops.known_hosts
@@ -811,7 +812,17 @@ def create(self, defn, check, allow_reboot, allow_recreate):
             # TODO: remove obsolete tags?
             self.tags = tags
 
-        self.assign_elastic_ip(defn.elastic_ipv4, instance, check)
+        # Assign the elastic IP.  If necessary, dereference the resource.
+        elastic_ipv4 = defn.elastic_ipv4
+        if elastic_ipv4.startswith("res-"):
+            res_name = elastic_ipv4[4:]
+            res = self.depl.active_resources.get(res_name, None)
+            if not res:
+                raise Exception("resource ‘{0}’ does not exist".format(res_name))
+            if not isinstance(res, ElasticIPState):
+                raise Exception("resource ‘{0}’ is not an elastic IP address".format(res_name))
+            elastic_ipv4 = res.public_ipv4
+        self.assign_elastic_ip(elastic_ipv4, instance, check)
 
         # Wait for the IP address.
         if not self.public_ipv4 or check: