1 files changed, 135 insertions, 0 deletions
diff --git a/nixos/modules/virtualisation/docker-preloader.nix b/nixos/modules/virtualisation/docker-preloader.nix
new file mode 100644
index 000000000000..faa94f53d98f
--- /dev/null
+++ b/nixos/modules/virtualisation/docker-preloader.nix
@@ -0,0 +1,135 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+with builtins;
+
+let
+  cfg = config.virtualisation;
+
+  sanitizeImageName = image: replaceStrings ["/"] ["-"] image.imageName;
+  hash = drv: head (split "-" (baseNameOf drv.outPath));
+  # The label of an ext4 FS is limited to 16 bytes
+  labelFromImage = image: substring 0 16 (hash image);
+
+  # The Docker image is loaded and some files from /var/lib/docker/
+  # are written into a qcow image.
+  preload = image: pkgs.vmTools.runInLinuxVM (
+    pkgs.runCommand "docker-preload-image-${sanitizeImageName image}" {
+      buildInputs = with pkgs; [ docker e2fsprogs utillinux curl kmod ];
+      preVM = pkgs.vmTools.createEmptyImage {
+        size = cfg.dockerPreloader.qcowSize;
+        fullName = "docker-deamon-image.qcow2";
+      };
+    }
+    ''
+      mkfs.ext4 /dev/vda
+      e2label /dev/vda ${labelFromImage image}
+      mkdir -p /var/lib/docker
+      mount -t ext4 /dev/vda /var/lib/docker
+
+      modprobe overlay
+
+      # from https://github.com/tianon/cgroupfs-mount/blob/master/cgroupfs-mount
+      mount -t tmpfs -o uid=0,gid=0,mode=0755 cgroup /sys/fs/cgroup
+      cd /sys/fs/cgroup
+      for sys in $(awk '!/^#/ { if ($4 == 1) print $1 }' /proc/cgroups); do
+        mkdir -p $sys
+        if ! mountpoint -q $sys; then
+          if ! mount -n -t cgroup -o $sys cgroup $sys; then
+            rmdir $sys || true
+          fi
+        fi
+      done
+
+      dockerd -H tcp://127.0.0.1:5555 -H unix:///var/run/docker.sock &
+
+      until $(curl --output /dev/null --silent --connect-timeout 2 http://127.0.0.1:5555); do
+        printf '.'
+        sleep 1
+      done
+
+      docker load -i ${image}
+
+      kill %1
+      find /var/lib/docker/ -maxdepth 1 -mindepth 1 -not -name "image" -not -name "overlay2" | xargs rm -rf
+    '');
+
+  preloadedImages = map preload cfg.dockerPreloader.images;
+
+in
+
+{
+  options.virtualisation.dockerPreloader = {
+    images = mkOption {
+      default = [ ];
+      type = types.listOf types.package;
+      description =
+      ''
+        A list of Docker images to preload (in the /var/lib/docker directory).
+      '';
+    };
+    qcowSize = mkOption {
+      default = 1024;
+      type = types.int;
+      description =
+      ''
+        The size (MB) of qcow files.
+      '';
+    };
+  };
+
+  config = {
+    assertions = [{
+      # If docker.storageDriver is null, Docker choose the storage
+      # driver. So, in this case, we cannot be sure overlay2 is used.
+      assertion = cfg.dockerPreloader.images == []
+        || cfg.docker.storageDriver == "overlay2"
+        || cfg.docker.storageDriver == "overlay"
+        || cfg.docker.storageDriver == null;
+      message = "The Docker image Preloader only works with overlay2 storage driver!";
+    }];
+
+    virtualisation.qemu.options =
+      map (path: "-drive if=virtio,file=${path}/disk-image.qcow2,readonly,media=cdrom,format=qcow2")
+      preloadedImages;
+
+
+    # All attached QCOW files are mounted and their contents are linked
+    # to /var/lib/docker/ in order to make image available.
+    systemd.services.docker-preloader = {
+      description = "Preloaded Docker images";
+      wantedBy = ["docker.service"];
+      after = ["network.target"];
+      path = with pkgs; [ mount rsync jq ];
+      script = ''
+        mkdir -p /var/lib/docker/overlay2/l /var/lib/docker/image/overlay2
+        echo '{}' > /tmp/repositories.json
+
+        for i in ${concatStringsSep " " (map labelFromImage cfg.dockerPreloader.images)}; do
+          mkdir -p /mnt/docker-images/$i
+
+          # The ext4 label is limited to 16 bytes
+          mount /dev/disk/by-label/$(echo $i | cut -c1-16) -o ro,noload /mnt/docker-images/$i
+
+          find /mnt/docker-images/$i/overlay2/ -maxdepth 1 -mindepth 1 -not -name l\
+             -exec ln -s '{}' /var/lib/docker/overlay2/ \;
+          cp -P /mnt/docker-images/$i/overlay2/l/* /var/lib/docker/overlay2/l/
+
+          rsync -a /mnt/docker-images/$i/image/ /var/lib/docker/image/
+
+          # Accumulate image definitions
+          cp /tmp/repositories.json /tmp/repositories.json.tmp
+          jq -s '.[0] * .[1]' \
+            /tmp/repositories.json.tmp \
+            /mnt/docker-images/$i/image/overlay2/repositories.json \
+            > /tmp/repositories.json
+        done
+
+        mv /tmp/repositories.json /var/lib/docker/image/overlay2/repositories.json
+      '';
+      serviceConfig = {
+        Type = "oneshot";
+      };
+    };
+  };
+}