Rewrite docker to be sensible and smaller

I have checked the image can build things and inspected `diff -ru`
compared to the old image. As far as I can tell it is more or less
the same besides the later git change.

Layers are now 65MB or less, and we aren't against the maxLayers limit
for the broken automatic layering to do anything but shove one store
path in a layer (which is good behaviour, actually).

This uses nix2container which streams images, so the build time is much
shorter.

I have also taken the opportunity to, in addition to fixing the 400MB
single layer (terrible, and what motivated this in the first place),
delete about 200MB of closure size inflicted by git vs gitMinimal
causing both perl and python to get into closure.

People mostly use this thing for CI, so I don't really think you need
advanced git operations, and large git can be added at the user side if
really motivated.

With love for whichever container developer somewhat ironically assumed
that one would not run skopeo in a minimal container that doesn't have a
/var/tmp.

Fixes: lix-project/lix#378

Change-Id: Icc3aa20e64446276716fbbb87535fd5b50628010

docker.nix

@@ -1,5 +1,6 @@
 {
   pkgs ? import <nixpkgs> { },
+  nix2container,
   lib ? pkgs.lib,
   name ? "lix",
   tag ? "latest",
@@ -12,26 +13,51 @@
   flake-registry ? null,
 }:
 let
-  defaultPkgs =
-    with pkgs;
-    [
-      nix
+  layerContents = with pkgs; [
+    # pulls in glibc and openssl, about 60MB
+    { contents = [ coreutils-full ]; }
+    # some stuff that is low in the closure graph and small ish, mostly to make
+    # incremental lix updates cheaper
+    {
+      contents = [
+        curl
+        libxml2
+        sqlite
+      ];
+    }
+    # 50MB of git
+    { contents = [ gitMinimal ]; }
+    # 144MB of nixpkgs
+    {
+      contents = [ channel ];
+      inProfile = false;
+    }
+  ];
+
+  # These packages are left to be auto layered by nix2container, since it is
+  # less critical that they get layered sensibly and they tend to not be deps
+  # of anything in particular
+  autoLayered = with pkgs; [
     bashInteractive
-      coreutils-full
     gnutar
     gzip
     gnugrep
     which
-      curl
     less
     wget
     man
     cacert.out
     findutils
     iana-etc
-      git
     openssh
-    ]
+    nix
+  ];
+
+  defaultPkgs =
+    lib.lists.flatten (
+      map (x: if !(x ? inProfile) || x.inProfile then x.contents else [ ]) layerContents
+    )
+    ++ autoLayered
     ++ extraPkgs;
 
   users =
@@ -139,16 +165,17 @@ let
     ))
     + "\n";
 
-  baseSystem =
-    let
   nixpkgs = pkgs.path;
-      channel = pkgs.runCommand "channel-nixos" { inherit bundleNixpkgs; } ''
+  channel = pkgs.runCommand "channel-nixpkgs" { } ''
     mkdir $out
-        if [ "$bundleNixpkgs" ]; then
+    ${lib.optionalString bundleNixpkgs ''
       ln -s ${nixpkgs} $out/nixpkgs
       echo "[]" > $out/manifest.nix
-        fi
+    ''}
   '';
+
+  baseSystem =
+    let
       rootEnv = pkgs.buildPackages.buildEnv {
         name = "root-profile-env";
         paths = defaultPkgs;
@@ -187,7 +214,7 @@ let
       profile = pkgs.buildPackages.runCommand "user-environment" { } ''
         mkdir $out
         cp -a ${rootEnv}/* $out/
-        ln -s ${manifest} $out/manifest.nix
+        ln -sf ${manifest} $out/manifest.nix
       '';
       flake-registry-path =
         if (flake-registry == null) then
@@ -236,6 +263,7 @@ let
           ln -s /nix/var/nix/profiles/share $out/usr/
 
           mkdir -p $out/nix/var/nix/gcroots
+          ln -s /nix/var/nix/profiles $out/nix/var/nix/gcroots/profiles
 
           mkdir $out/tmp
 
@@ -248,14 +276,14 @@ let
           mkdir -p $out/nix/var/nix/profiles/per-user/root
 
           ln -s ${profile} $out/nix/var/nix/profiles/default-1-link
-          ln -s $out/nix/var/nix/profiles/default-1-link $out/nix/var/nix/profiles/default
+          ln -s /nix/var/nix/profiles/default-1-link $out/nix/var/nix/profiles/default
           ln -s /nix/var/nix/profiles/default $out/root/.nix-profile
 
           ln -s ${channel} $out/nix/var/nix/profiles/per-user/root/channels-1-link
-          ln -s $out/nix/var/nix/profiles/per-user/root/channels-1-link $out/nix/var/nix/profiles/per-user/root/channels
+          ln -s /nix/var/nix/profiles/per-user/root/channels-1-link $out/nix/var/nix/profiles/per-user/root/channels
 
           mkdir -p $out/root/.nix-defexpr
-          ln -s $out/nix/var/nix/profiles/per-user/root/channels $out/root/.nix-defexpr/channels
+          ln -s /nix/var/nix/profiles/per-user/root/channels $out/root/.nix-defexpr/channels
           echo "${channelURL} ${channelName}" > $out/root/.nix-channels
 
           mkdir -p $out/bin $out/usr/bin
@@ -273,21 +301,35 @@ let
           ln -s $globalFlakeRegistryPath $out/nix/var/nix/gcroots/auto/$rootName
         '')
       );
-in
-pkgs.dockerTools.buildLayeredImageWithNixDb {
+
+  layers = builtins.foldl' (
+    layersList: el:
+    let
+      layer = nix2container.buildLayer {
+        deps = el.contents;
+        layers = layersList;
+      };
+    in
+    layersList ++ [ layer ]
+  ) [ ] layerContents;
+
+  image = nix2container.buildImage {
 
     inherit name tag maxLayers;
 
-  contents = [ baseSystem ];
+    inherit layers;
 
-  extraCommands = ''
-    rm -rf nix-support
-    ln -s /nix/var/nix/profiles nix/var/nix/gcroots/profiles
-  '';
-  fakeRootCommands = ''
-    chmod 1777 tmp
-    chmod 1777 var/tmp
-  '';
+    copyToRoot = [ baseSystem ];
+
+    initializeNixDatabase = true;
+
+    perms = [
+      {
+        path = baseSystem;
+        regex = "(/var)?/tmp";
+        mode = "1777";
+      }
+    ];
 
     config = {
       Cmd = [ "/root/.nix-profile/bin/bash" ];
@@ -312,4 +354,37 @@ pkgs.dockerTools.buildLayeredImageWithNixDb {
         "NIX_PATH=/nix/var/nix/profiles/per-user/root/channels:/root/.nix-defexpr/channels"
       ];
     };
+  };
+in
+image
+// {
+  # We don't ship the tarball as the default output because it is a strange thing to want imo
+  tarball =
+    pkgs.buildPackages.runCommand "docker-image-tarball-${pkgs.nix.version}"
+      {
+        nativeBuildInputs = [ pkgs.buildPackages.bubblewrap ];
+        meta.description = "Docker image tarball with Lix for ${pkgs.system}";
+      }
+      ''
+        mkdir -p $out/nix-support
+        image=$out/image.tar
+        # bwrap for foolish temp dir selection code that forces /var/tmp:
+        # https://github.com/containers/skopeo.git/blob/60ee543f7f7c242f46cc3a7541d9ac8ab1c89168/vendor/github.com/containers/image/v5/internal/tmpdir/tmpdir.go#L15-L18
+        mkdir -p $TMPDIR/fake-var/tmp
+        args=(--unshare-user --bind "$TMPDIR/fake-var" /var)
+        for dir in /*; do
+          args+=(--dev-bind "/$dir" "/$dir")
+        done
+        bwrap ''${args[@]} -- ${lib.getExe image.copyTo} docker-archive:$image
+        gzip $image
+        echo "file binary-dist $image" >> $out/nix-support/hydra-build-products
+      '';
+  meta = image.meta // {
+    description = "Docker image for Lix. This is built with nix2container; see that project's README for details";
+    longDescription = ''
+      Docker image for Lix, built with nix2container.
+      To copy it to your docker daemon, nix run .#dockerImage.copyToDockerDaemon
+      To copy it to podman, nix run .#dockerImage.copyTo containers-storage:lix
+    '';
+  };
 }

flake.lock

@@ -16,6 +16,22 @@
         "type": "github"
       }
     },
+    "nix2container": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1712990762,
+        "narHash": "sha256-hO9W3w7NcnYeX8u8cleHiSpK2YJo7ecarFTUlbybl7k=",
+        "owner": "nlewo",
+        "repo": "nix2container",
+        "rev": "20aad300c925639d5d6cbe30013c8357ce9f2a2e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nlewo",
+        "repo": "nix2container",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1715123187,
@@ -67,6 +83,7 @@
     "root": {
       "inputs": {
         "flake-compat": "flake-compat",
+        "nix2container": "nix2container",
         "nixpkgs": "nixpkgs",
         "nixpkgs-regression": "nixpkgs-regression",
         "pre-commit-hooks": "pre-commit-hooks"

flake.nix

@@ -8,6 +8,10 @@
       url = "github:cachix/git-hooks.nix";
       flake = false;
     };
+    nix2container = {
+      url = "github:nlewo/nix2container";
+      flake = false;
+    };
     flake-compat = {
       url = "github:edolstra/flake-compat";
       flake = false;
@@ -20,6 +24,7 @@
       nixpkgs,
       nixpkgs-regression,
       pre-commit-hooks,
+      nix2container,
       flake-compat,
     }:
 
@@ -330,19 +335,13 @@
             dockerImage =
               let
                 pkgs = nixpkgsFor.${system}.native;
-                image = import ./docker.nix {
+                nix2container' = import nix2container { inherit pkgs system; };
+              in
+              import ./docker.nix {
                 inherit pkgs;
+                nix2container = nix2container'.nix2container;
                 tag = pkgs.nix.version;
               };
-              in
-              pkgs.runCommand "docker-image-tarball-${pkgs.nix.version}"
-                { meta.description = "Docker image with Lix for ${system}"; }
-                ''
-                  mkdir -p $out/nix-support
-                  image=$out/image.tar.gz
-                  ln -s ${image} $image
-                  echo "file binary-dist $image" >> $out/nix-support/hydra-build-products
-                '';
           }
           // builtins.listToAttrs (
             map (crossSystem: {

releng/release-jobs.nix

@@ -33,7 +33,7 @@ let
       targetName = "*.tar.xz";
     }) systems
     ++ builtins.map (system: {
-      target = hydraJobs.dockerImage.${system};
+      target = hydraJobs.dockerImage.${system}.tarball;
       targetName = "image.tar.gz";
       rename = "lix-${lix.version}-docker-image-${system}.tar.gz";
     }) dockerSystems;