while building some rpi disk images, i discovered ext4 images generated by nixos/lib/make-ext4-fs.nix not being reproducible

using debugfs on the images, and running show_super_stats, i found the following difference:

Directory Hash Seed:      153a13f8-fd63-4dec-9295-e702caea9421
Directory Hash Seed:      7ab5f8a0-ae47-4beb-a549-b32270a9ee1d

2021-08-30 11:15:54 < slyfox> e2fsprogs' tests force UUID to a constant to get stable output: tests/t_change_uuid_mcsum/script:$MKE2FS -O metadata_csum,^metadata_csum_seed -U 6b33f586-a183-4383-921d-30da3fef2e5c -F $TMPFILE > /dev/null 2>&1

but when applying that to nixpkgs:

diff --git a/nixos/lib/make-ext4-fs.nix b/nixos/lib/make-ext4-fs.nix
index 416beeb32f2..179ab5998e3 100644
--- a/nixos/lib/make-ext4-fs.nix
+++ b/nixos/lib/make-ext4-fs.nix
@@ -64,7 +64,7 @@ pkgs.stdenv.mkDerivation {

       truncate -s $bytes $img

-      faketime -f "1970-01-01 00:00:01" fakeroot mkfs.ext4 -L ${volumeLabel} -U ${uuid} -d ./rootImage $img
+      faketime -f "1970-01-01 00:00:01" fakeroot mkfs.ext4 -O metadata_csum,^metadata_csum_seed -L ${volumeLabel} -U ${uuid} -d ./rootImage $img

       export EXT2FS_NO_MTAB_OK=yes
       # I have ended up with corrupted images sometimes, I suspect that happens when the build machine's disk gets full during the build.

the build fails with:

ext4-fs.img> Creating journal (8192 blocks): done
ext4-fs.img> Copying files into the device: ext2fs_mkdir: Directory block checksum does not match directory block while creating directory "lib"
ext4-fs.img> __populate_fs: Directory block checksum does not match directory block while making dir "lib"
ext4-fs.img> mkfs.ext4: Directory block checksum does not match directory block while populating file system
ext4-fs.img> builder for '/nix/store/i6y239h646p67l6pa9qyzw2v50cyf432-ext4-fs.img.drv' failed with exit code 1

In reply to @baloo_:matrix.org
any chance you could strace -Ff -s 1500 -p $(pidof nix-daemon) 2> log and upload that somewhere?

This is /run/systemd/resolve/stub-resolv.conf managed by man:systemd-resolved(8).\n# Do not edit.\n#\n# This file might be symlinked as /etc/resolv.conf. If you're looking at\n# /etc/resolv.conf and seeing this text, you have followed the symlink.\n#\n# This is a dynamic resolv.conf file for connecting local clients to the\n# internal DNS stub resolver of systemd-resolved. This file lists all\n# configured search domains.\n#\n# Run "resolvectl status" to see details about the uplink DNS servers\n# currently in use.\n#\n# Third party programs should typically not access this file directly, but only\n# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a\n# different way, replace this symlink by a static file or a different symlink.\n#\n# See man:systemd-resolved.service(8) for details about the supported modes of\n# operation for /etc/resolv.conf.\n\nnameserver 127.0.0.53\noptions edns0 trust-ad\nsearch naive.network\n