image-builder: Adds alignment tests

lib/image-builder/tests/alignment/aligned.nix

@@ -0,0 +1,50 @@
+# Verifies that filesystems sized to be aligned works.
+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  inherit (pkgs) imageBuilder;
+  makeNull = size: pkgs.runCommandNoCC "filesystems-test" {
+    filename = "null.img";
+    filesystemType = "FAT32"; # meh, good enough
+  } ''
+    mkdir -p $out
+    dd if=/dev/zero of=$out/$filename bs=${toString size} count=1
+  '';
+in
+
+with imageBuilder;
+
+{
+  one = diskImage.makeMBR {
+    name = "diskimage";
+    diskID = "012345678";
+    partitions = [
+      (makeNull (size.MiB 1))
+      (makeNull (size.MiB 1))
+    ];
+  };
+  nine = diskImage.makeMBR {
+    name = "diskimage";
+    diskID = "012345678";
+    partitions = [
+      (makeNull (size.MiB 9))
+      (makeNull (size.MiB 9))
+    ];
+  };
+  ten = diskImage.makeMBR {
+    name = "diskimage";
+    diskID = "012345678";
+    partitions = [
+      (makeNull (size.MiB 10))
+      (makeNull (size.MiB 10))
+    ];
+  };
+  eleven = diskImage.makeMBR {
+    name = "diskimage";
+    diskID = "012345678";
+    partitions = [
+      (makeNull (size.MiB 11))
+      (makeNull (size.MiB 11))
+    ];
+  };
+}

lib/image-builder/tests/alignment/unaligned.nix

@@ -0,0 +1,42 @@
+# Verifies that filesystems sized to be unaligned will work.
+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  inherit (pkgs) imageBuilder;
+  makeNull = size: pkgs.runCommandNoCC "filesystems-test" {
+    filename = "null.img";
+    filesystemType = "FAT32"; # meh, good enough
+  } ''
+    mkdir -p $out
+    dd if=/dev/zero of=$out/$filename bs=${toString size} count=1
+  '';
+in
+
+with imageBuilder;
+
+# Through empirical testing, it was found out that the defaults from `sfdisk`
+# are not as documented.
+# First of all, on small disks no alignment will be made.
+# Starting with 3MiB (empirically derived) alignment will be made.
+# The alignment is documented as being based on the I/O limits. It seems like
+# for files it ends up causing alignments at the 2MiB boundaries.
+# Such, `grain: 1024` has to be set to configure sfdisk for the sane default
+# documented in its manpage
+#
+# > grain  Specify minimal size in bytes used to calculate partitions alignment.
+# >        The default is 1MiB and it's strongly recommended to use the default.
+# >        Do not modify this variable if you're not sure.
+#
+# The default is *not* 1MiB and will break the generation of images if it tries
+# to align a small partition at the very end of the disk, when the disk is sized
+# just right to fit.
+#
+# This is what this test validates.
+diskImage.makeMBR {
+  name = "diskimage";
+  diskID = "012345678";
+  partitions = [
+    (makeNull (size.MiB 3))
+    (makeNull (size.MiB 1))
+  ];
+}