Add a test to confirm the initrd's are the same, and that rebuilds are fast

size-test/.gitignore

@@ -0,0 +1,3 @@
+netboot-base
+netboot-incremental
+netboot-incremental.drv

size-test/base.nix

@@ -0,0 +1,3 @@
+{
+  imports = [ ../quickly.nix ];
+}

size-test/build.sh

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+#!/bin/sh
+
+set -eux
+
+cd "$(dirname "$0")"
+
+nix-build '<nixpkgs/nixos>' \
+          -I nixos-config=./base.nix \
+          -A config.system.build.ipxeBootDir \
+          --out-link ./netboot-base
+
+nix-instantiate '<nixpkgs/nixos>' \
+          -I nixos-config=./incremental.nix \
+          -A config.system.build.ipxeBootDir \
+          --add-root ./netboot-incremental.drv --indirect
+
+time nix-build ./netboot-incremental.drv \
+     --out-link ./netboot-incremental
+
+ensureSame() (
+    test "$(realpath "./netboot-base/$1")" =  "$(realpath "./netboot-incremental/$1")"
+)
+
+ensureSame bzImage
+ensureSame initrd
+
+echo "ok!"

size-test/incremental.nix

@@ -0,0 +1,4 @@
+{
+  imports = [ ./base.nix ];
+  services.nginx.enable = true;
+}