image-builder: WIP adds makeGPT

TODO : review makeMBR
TODO : share common code into makeFromSfdisk
TODO : add tests

lib/image-builder/default.nix

@@ -30,6 +30,7 @@ makeScope newScope (self:
     # All supported disk formats for image generation.
     diskImage = {
       makeMBR = callPackage ./makeMBR.nix {};
+      makeGPT = callPackage ./makeGPT.nix {};
     };
 
     # Don't do maths yourselves, just use the helpers.

lib/image-builder/makeGPT.nix

@@ -0,0 +1,159 @@
+{ stdenvNoCC, lib
+, imageBuilder
+, utillinux
+}:
+
+/*  */ let scope = { "diskImage.makeGPT" =
+
+let
+  inherit (lib) concatMapStringsSep optionalString;
+
+  # List of known mappings of GPT partition types to filesystems.
+  # This is not exhaustive, only used as a default.
+  types = {
+    "FAT32" = "EBD0A0A2-B9E5-4433-87C0-68B6B72699C7";
+    "ESP"   = "C12A7328-F81F-11D2-BA4B-00A0C93EC93B";
+    "ext2"  = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
+    "ext3"  = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
+    "ext4"  = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
+  };
+in
+{
+  name
+  , partitions
+  , diskID
+}:
+
+let
+  _name = name;
+
+  eachPart = partitions: fn: (
+    concatMapStringsSep "\n" (partition:
+      fn partition
+  ) partitions);
+
+  # Default alignment.
+  alignment = toString (imageBuilder.size.MiB 1);
+
+  image = partition: 
+    if lib.isDerivation partition then
+      "${partition}/${partition.filename}"
+    else
+      partition.filename
+  ;
+in
+stdenvNoCC.mkDerivation rec {
+  name = "disk-image-${_name}";
+  filename = "${_name}.img";
+  img = "${placeholder "out"}/${filename}";
+
+  nativeBuildInputs = [
+    utillinux
+  ];
+
+  buildCommand = let
+    # This fragment is used to compute the (aligned) size of the partition.
+    # It is used *only* to track the tally of the space used, thus the starting
+    # offset of the next partition. The filesystem sizes are untouched.
+    sizeFragment = partition: ''
+      start=$totalSize
+      ${
+        if partition ? length then
+        ''size=$((${toString partition.length}))''
+        else
+        ''size=$(($(du --apparent-size -B 512 "$input_img" | awk '{ print $1 }') * 512))''
+      }
+      size=$(( $(if (($size % ${alignment})); then echo 1; else echo 0; fi ) + size / ${alignment} ))
+      size=$(( size * ${alignment} ))
+      totalSize=$(( totalSize + size ))
+      echo "Partition: start $start | size $size | totalSize $totalSize"
+    '';
+
+    # This fragment is used to add the desired gap to `totalSize`.
+    # We're setting `start` and `size` only to mirror the information shown
+    # for partitions.
+    # Do note that gaps are always aligned, so two gaps sized half the alignment
+    # would create 2× the space expected.
+    # What may *instead* be done at one point is always align `start` for partitions.
+    gapFragment = partition: ''
+      start=$totalSize
+      size=${toString partition.length}
+      size=$(( $(if (($size % ${alignment})); then echo 1; else echo 0; fi ) + size / ${alignment} ))
+      totalSize=$(( totalSize + size ))
+      echo "Gap: start $start | size $size | totalSize $totalSize"
+    '';
+  in ''
+    mkdir -p $out
+
+    cat <<EOF > script.sfdisk
+    label: gpt
+    grain: 1024
+    label-id: ${diskID}
+    EOF
+
+    totalSize=${alignment}
+    echo
+    echo "Gathering information about partitions."
+    ${eachPart partitions (partition:
+      if partition ? isGap && partition.isGap then
+        (gapFragment partition)
+      else
+        ''
+          input_img="${image partition}"
+          ${sizeFragment partition}
+          echo " -> ${partition.name}: $size / ${if partition ? filesystemType then partition.filesystemType else ""}"
+
+          (
+          # The size is /1024; otherwise it's in sectors.
+          echo -n 'start='"$((start/1024))"'KiB'
+          echo -n ', size='"$((size/1024))"'KiB'
+          echo -n ', type=${
+            if partition ? partitionType then
+              partition.partitionType
+            else
+              types.${partition.filesystemType}
+          }'
+          ${optionalString (partition ? bootable && partition.bootable)
+              "echo -n ', bootable'"}
+          echo "" # Finishes the command
+          ) >> script.sfdisk
+        ''
+    )}
+
+    # Allow space for alignment + secondary partition table / header.
+    totalSize=$(( totalSize + ${alignment} ))
+
+    echo "--- script ----"
+    cat script.sfdisk
+    echo "--- script ----"
+
+    echo
+    echo "Making image, $totalSize bytes..."
+    truncate -s $((totalSize)) $img
+    sfdisk $img < script.sfdisk
+
+    totalSize=${alignment}
+    echo
+    echo "Writing partitions into image"
+    ${eachPart partitions (partition: 
+      if partition ? isGap && partition.isGap then
+        (gapFragment partition)
+      else
+        ''
+          input_img="${image partition}"
+          ${sizeFragment partition}
+          echo " -> ${partition.name}: $size / ${if partition ? filesystemType then partition.filesystemType else ""}"
+
+          echo "$start / $size"
+          dd conv=notrunc if=$input_img of=$img seek=$((start/512)) count=$((size/512)) bs=512
+        ''
+    )}
+
+    echo
+    echo "Information about the image:"
+    ls -lh $img
+    sfdisk -V --list $img
+  '';
+}
+
+/*  */ ;}; in scope."diskImage.makeGPT"