Reduce closure size of nix-tools (#178)

* Buildkite: Add a test for nix-tools closure size

The limit is a rather generous 500MB.

* nix-tools: Reduce closure size

1. Run justStackExecutables on hpack to remove dependency on ghc and
   libraries.

   Ideally we would be using the component builder on hpack.

2. Replace nix-prefetch-scripts with nix-prefetch-git. stack.yaml
   supports mercurial repositories but I've never seen it used.

.buildkite/pipeline.yml

@@ -11,6 +11,14 @@ steps:
     agents:
       system: x86_64-linux
 
+  - label: 'Check closure size'
+    command:
+      - nix-build build.nix -A maintainer-scripts.check-closure-size -o check-closure-size.sh
+      - echo "+++ Closure size (MB)"
+      - ./check-closure-size.sh
+    agents:
+      system: x86_64-linux
+
   - label: 'Update docs'
     command:
       - nix-build build.nix -A maintainer-scripts.update-docs -o update-docs.sh

build.nix

@@ -38,5 +38,6 @@ in {
       };
     };
     check-hydra = haskell.callPackage ./scripts/check-hydra.nix {};
+    check-closure-size = haskell.callPackage ./scripts/check-closure-size.nix {};
   };
 }

default.nix

@@ -125,7 +125,8 @@ let
     # we never want to actually cross compile nix-tools on it's own.
     nix-tools = pkgs.buildPackages.callPackage ./nix-tools {
       inherit fetchExternal cleanSourceHaskell;
-      inherit (pkgs.buildPackages.haskellPackages) hpack;
+      hpack = pkgs.buildPackages.haskell.lib.justStaticExecutables
+        (pkgs.buildPackages.haskellPackages.hpack);
       inherit (self) mkCabalProjectPkgSet;
     };
 

nix-tools/default.nix

@@ -1,5 +1,5 @@
 { lib, symlinkJoin, makeWrapper
-, hpack, git, nix, nix-prefetch-scripts
+, hpack, git, nix, nix-prefetch-git
 , fetchExternal, cleanSourceHaskell, mkCabalProjectPkgSet }:
 
 let
@@ -27,7 +27,7 @@ let
 
   hsPkgs = pkgSet.config.hsPkgs;
 
-  tools = [ hpack git nix nix-prefetch-scripts ];
+  tools = [ hpack git nix nix-prefetch-git ];
 in
   symlinkJoin {
     name = "nix-tools";

scripts/check-closure-size.nix

@@ -0,0 +1,29 @@
+{ stdenv, writeScript, coreutils, gawk, nix
+, nix-tools
+, limitMB ? 500
+}:
+
+with stdenv.lib;
+
+writeScript "check-closure-size.sh" ''
+  #!${stdenv.shell}
+
+  set -euo pipefail
+
+  export PATH="${makeBinPath [ coreutils gawk nix ]}"
+
+  get_closure_size() {
+    du -scm $(nix-store -qR $1) | sort -n | tail -n25
+  }
+
+  nt="$(get_closure_size ${nix-tools})"
+  echo '	${nix-tools}'
+  echo "$nt"
+
+  total=$(awk '$2 == "total" { print $1; }' <<< "$nt")
+  
+  if [ $total -gt ${toString limitMB} ]; then
+    echo "Closure size exceeds limit of ${toString limitMB}MB!"
+    exit 1
+  fi
+''