From e2015bbfcbcf7594824755e39f838d7aab258b6e Mon Sep 17 00:00:00 2001
From: Graham Christensen <graham@grahamc.com>
Date: Sat, 13 May 2017 08:53:07 -0400
Subject: [PATCH] Support multiple versions of nixpkgs in one network

Having a machine named foo, described as:

    foo = { # ...snipped...
      deployment.nix_path.nixpkgs = (builtins.filterSource
        (path: type: type != "directory" || baseNameOf path != ".git")
        ./../nixpkgs);
    });

will have the custom nixpkgs set in the `NIX_PATH` as
`nixpkgs=path-to-custom-nixpkgs`.

Note this does not work with foo = { config, ... }: {... machines, but
having a second nix file in the network would work, and also:

    let
      canary = machine: {
        deployment.nix_path.nixpkgs = (builtins.filterSource
          (path: type: type != "directory" || baseNameOf path != ".git")
          ./../nixpkgs);
        imports = [machine];
      };

      machine = { ... }: {
        # your machine config
      };
    in {
      machineA = machine;
      machineB = canary machine;
    }

Note that because this uses scopedImport, the nixops network and
machines may use `import <nixpkgs>` and have a consistent view of
nixpkgs.
---
 nix/eval-machine-info.nix | 35 ++++++++++++++++++++++++++++++++---
 nix/options.nix           |  7 +++++++
 2 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/nix/eval-machine-info.nix b/nix/eval-machine-info.nix
index 503b4c25d..085452cd4 100644
--- a/nix/eval-machine-info.nix
+++ b/nix/eval-machine-info.nix
@@ -39,13 +39,42 @@ rec {
         # Get the configuration of this machine from each network
         # expression, attaching _file attributes so the NixOS module
         # system can give sensible error messages.
+
         modules =
           concatMap (n: optional (hasAttr machineName n)
             { imports = [(getAttr machineName n)]; inherit (n) _file; })
           networks;
-      in
-      { name = machineName;
-        value = import <nixpkgs/nixos/lib/eval-config.nix> {
+
+        machineConfs =
+          concatMap (n: optional (hasAttr machineName n)
+            (getAttr machineName n))
+          networks;
+
+        nameToPath = attrs: name: {
+          prefix = name;
+          path = attrs."${name}";
+        };
+
+        attrsetToPaths = attrset: map (nameToPath attrset)
+          (builtins.attrNames attrset);
+
+        importSources =
+          (concatMap (module:
+              if (!builtins.isFunction module
+                  && builtins.hasAttr "deployment" module)
+                  && (builtins.hasAttr "nix_path" module.deployment)
+              then attrsetToPaths module.deployment.nix_path
+              else [])
+            machineConfs) ++ builtins.nixPath;
+
+        __nixPath = importSources;
+
+        machineImport = builtins.scopedImport {
+          inherit __nixPath;
+        };
+      in {
+        name = machineName;
+        value = machineImport <nixpkgs/nixos/lib/eval-config.nix> {
           modules =
             modules ++
             defaults ++
diff --git a/nix/options.nix b/nix/options.nix
index 0866c3ab8..117b44a7b 100644
--- a/nix/options.nix
+++ b/nix/options.nix
@@ -103,6 +103,13 @@ in
       '';
     };
 
+    deployment.nix_path = mkOption {
+      default = {};
+      type = types.attrsOf types.str;
+      description = ''
+      '';
+    };
+
     deployment.hasFastConnection = mkOption {
       default = false;
       type = types.bool;