From 8d6c8d77802af4d6688842b6d557e9ff781fcc24 Mon Sep 17 00:00:00 2001
From: Vincent Ambo <tazjin@google.com>
Date: Thu, 7 Nov 2019 17:19:06 +0000
Subject: [PATCH] fix(build-image): Allow "cross-builds" of images for
 different arch

Imports the package set twice in the builder expression: Once
configured for the target system, once configured for the native
system.

This makes it possible to fetch the actual image contents for the
required architecture, but use local tools to assemble the symlink
layer and metadata.
---
 build-image/build-image.nix | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/build-image/build-image.nix b/build-image/build-image.nix
index eb14d52..4393f2b 100644
--- a/build-image/build-image.nix
+++ b/build-image/build-image.nix
@@ -45,8 +45,13 @@ let
     toFile
     toJSON;
 
-  inherit (pkgs) lib runCommand writeText;
+  # Package set to use for sourcing utilities
+  nativePkgs = import loadPkgs { inherit srcType srcArgs importArgs; };
+  inherit (nativePkgs) coreutils jq openssl lib runCommand writeText symlinkJoin;
 
+  # Package set to use for packages to be included in the image. This
+  # package set is imported with the system set to the target
+  # architecture.
   pkgs = import loadPkgs {
     inherit srcType srcArgs;
     importArgs = importArgs // {
@@ -115,7 +120,7 @@ let
   runtimeGraph = runCommand "runtime-graph.json" {
     __structuredAttrs = true;
     exportReferencesGraph.graph = allContents.contents;
-    PATH = "${pkgs.coreutils}/bin";
+    PATH = "${coreutils}/bin";
     builder = toFile "builder" ''
       . .attrs.sh
       cp .attrs.json ''${outputs[out]}
@@ -124,7 +129,7 @@ let
 
   # Create a symlink forest into all top-level store paths of the
   # image contents.
-  contentsEnv = pkgs.symlinkJoin {
+  contentsEnv = symlinkJoin {
     name = "bulk-layers";
     paths = allContents.contents;
   };
@@ -141,7 +146,7 @@ let
   # Two different hashes are computed for different usages (inclusion
   # in manifest vs. content-checking in the layer cache).
   symlinkLayerMeta = fromJSON (readFile (runCommand "symlink-layer-meta.json" {
-    buildInputs = with pkgs; [ coreutils jq openssl ];
+    buildInputs = [ coreutils jq openssl ];
   }''
     tarHash=$(sha256sum ${symlinkLayer} | cut -d ' ' -f1)
     layerSize=$(stat --printf '%s' ${symlinkLayer})