Set system to x86_64-linux on non-linux platforms

Taken from NixOS#412 to generate a minimal change
as an improvement of the current situation regarding remote building.

Adjusted to use "mkOverride 900" to make it consistent with all other backends.

If building a container on a non-linux system and "nixpkgs.system" is not set,
then we default now to "x86_64-linux".

nix/container.nix

@@ -33,6 +33,7 @@ in
 
     boot.isContainer = true;
 
+    nixpkgs.system = mkIf (! any (platform: platform == builtins.currentSystem) platforms.linux) (mkOverride 900 "x86_64-linux");
   };
 
 }

nixops/backends/container.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 from nixops.backends import MachineDefinition, MachineState
+from nixops.nix_expr import py2nix
 import nixops.util
 import nixops.ssh_util
 import subprocess
@@ -126,10 +127,14 @@ def create(self, defn, check, allow_reboot, allow_recreate):
         if self.vm_id is None:
             self.log("building initial configuration...")
 
+            eval_args = self.depl._eval_args(self.depl.nix_exprs)
+            eval_args['checkConfigurationOptions'] = False
             expr = " ".join([
                 '{ imports = [ <nixops/container-base.nix> ];',
                 '  boot.isContainer = true;',
                 '  networking.hostName = "{0}";'.format(self.name),
+                '  nixpkgs.system = let final = import <nixops/eval-machine-info.nix> {0};'.format(py2nix(eval_args, inline=True)),
+                '  in final.resources.machines.{0}.nixpkgs.system;'.format(self.name),
                 '  users.extraUsers.root.openssh.authorizedKeys.keys = [ "{0}" ];'.format(self.client_public_key),
                 '}'])
 

nixops/deployment.py

@@ -256,16 +256,26 @@ def _nix_path_flags(self):
         flags.extend(["-I", "nixops=" + self.expr_path])
         return flags
 
+    def _eval_args(self, exprs):
+        args = {key: RawValue(val) for key, val in self.args.iteritems()}
+        exprs_ = [RawValue(x) if x[0] == '<' else x for x in exprs]
+        eval_args = {
+            'networkExprs' : exprs_,
+            'args' : args,
+            'uuid' : self.uuid,
+            'deploymentName' : self.name if self.name else ""
+        }
+        return eval_args
+
 
     def _eval_flags(self, exprs):
         flags = self._nix_path_flags()
-        args = {key: RawValue(val) for key, val in self.args.iteritems()}
-        exprs_ = [RawValue(x) if x[0] == '<' else x for x in exprs]
+        args = self._eval_args(exprs)
         flags.extend(
-            ["--arg", "networkExprs", py2nix(exprs_, inline=True),
-             "--arg", "args", py2nix(args, inline=True),
-             "--argstr", "uuid", self.uuid,
-             "--argstr", "deploymentName", self.name if self.name else "",
+            ["--arg", "networkExprs", py2nix(args['networkExprs'], inline=True),
+             "--arg", "args", py2nix(args['args'], inline=True),
+             "--argstr", "uuid", args['uuid'],
+             "--argstr", "deploymentName", args['deploymentName'],
              "<nixops/eval-machine-info.nix>"])
         return flags