image-builder: makeFilesystem: reviews minimum size with blockSize

First of all, `du -s` will always combine the sizes into the multiple of
its given --block-size, so 4×1 byte files will show up as 1 block size
(assuming a block size > 4). The expected size would have been at least
one block size per file, so for 512, 512×4 thus 2048.

With this change, we sum the sizes ourselves.

This adds a new requirement to the filesystems, that they pass the block
size to the generic builder, though this is good, as it will increases
reproducibility. In addition, filesystems can present the option to
users, allowing the user to set their block sizes as expected.

lib/image-builder/makeFilesystem.nix

@@ -13,6 +13,7 @@ in
   , populateCommands ? null
   # When size is not given, it is assumed that `populateCommands` will populate
   # the filesystem.
+  , blockSize
   , size ? null
   , ...
 } @ args:
@@ -27,7 +28,7 @@ in
 stdenvNoCC.mkDerivation (args // rec {
   # Do not inherit `size`; we don't want to accidentally use it. The `size` can
   # be dynamic depending on the contents.
-  inherit partName;
+  inherit partName blockSize;
 
   name = "partition-${partName}";
   filename = "${partName}.img";
@@ -37,6 +38,16 @@ stdenvNoCC.mkDerivation (args // rec {
   ] ++ optionals (args ? nativeBuildInputs) args.nativeBuildInputs;
 
   buildCommand = ''
+    sum-lines() {
+      local acc=0
+      while read -r number; do
+          acc=$((acc+number))
+      done
+
+      echo "$acc"
+    }
+
+
     # The default stdenv/generic clashes with `runHook`.
     # It doesn't override as expected.
     unset -f checkPhase
@@ -54,7 +65,11 @@ stdenvNoCC.mkDerivation (args // rec {
     )
     ''}
     ${optionalString (size == null) ''
-    size=$(cd files; du -sb --apparent-size . | tr -cd '[:digit:]')
+      # Size rounded in blocks. This assumes all files are to be rounded to a
+      # multiple of blockSize.
+      size=$(du -akh --block-size "$blockSize" . | cut -f1 | sum-lines)
+      # Size in bytes
+      size=$((size * blockSize))
     ''}
 
     if (( size < minimumSize )); then