nixos/tests/docker-tools: check layer sharing with buildLayeredImage

Adapted from grahamc's blog post on layered Docker images in Nix: https://grahamc.com/blog/nix-and-layered-docker-images
NixOS · Jun 6, 2019 · 3b0d5b5 · 3b0d5b5
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diff --git a/nixos/tests/docker-tools.nix b/nixos/tests/docker-tools.nix
@@ -68,6 +68,12 @@ import ./make-test.nix ({ pkgs, ... }: {
       $docker->succeed("docker load --input='${pkgs.dockerTools.examples.layered-on-top}'");
       $docker->succeed("docker run --rm ${pkgs.dockerTools.examples.layered-on-top.imageName}");
 
+      # Ensure layers are shared between images
+      $docker->succeed("docker load --input='${pkgs.dockerTools.examples.another-layered-image}'");
+      $docker->succeed("docker inspect ${pkgs.dockerTools.examples.layered-image.imageName} | ${pkgs.jq}/bin/jq -r '.[] | .RootFS.Layers | .[]' | sort > layers1.sha256");
+      $docker->succeed("docker inspect ${pkgs.dockerTools.examples.another-layered-image.imageName} | ${pkgs.jq}/bin/jq -r '.[] | .RootFS.Layers | .[]' | sort > layers2.sha256");
+      $docker->succeed('[ $(comm -1 -2 layers1.sha256 layers2.sha256 | wc -l) -ne 0 ]');
+
       # Ensure order of layers is correct
       $docker->succeed("docker load --input='${pkgs.dockerTools.examples.layersOrder}'");
       $docker->succeed("docker run --rm  ${pkgs.dockerTools.examples.layersOrder.imageName} cat /tmp/layer1 | grep -q layer1");

diff --git a/pkgs/build-support/docker/examples.nix b/pkgs/build-support/docker/examples.nix
@@ -226,4 +226,12 @@ rec {
     '';
   };
 
+  # 14. Create another layered image, for comparing layers with image 10.
+  another-layered-image = pkgs.dockerTools.buildLayeredImage {
+    name = "another-layered-image";
+    tag = "latest";
+    config.Cmd = [ "${pkgs.hello}/bin/hello" ];
+    contents = [ pkgs.hello ];
+  };
+
 }