fix(yocto): force NFSv4.1 for the shared Yocto cache

NFSv4.2's READ_PLUS corrupts large-file reads over the shared download/
sstate cache: multi-GB BitBake git pack mirrors (linux, u-boot) come back
with mangled bytes, so `do_unpack` / `devtool modify` fail with zlib
"inflate: data stream error" / "pack checksum mismatch" at varying
offsets. Cloning the same repo to local disk is bit-perfect, isolating
the NFSv4.2 read path as the cause.

Cap nfsd at 4.1 on the hyper server (disables READ_PLUS for all clients)
and pin nfsvers=4.1 on the client mounts as belt-and-suspenders.

Author: mdrodrigo

nix/hosts/features/shared-state-yocto/default.nix

@@ -16,13 +16,18 @@
     mkdir -p /srv/yocto/sstate-cache
   '';
 
+  # Pin NFSv4.1 on both shares. NFSv4.2's READ_PLUS corrupts large-file reads,
+  # so multi-GB BitBake git pack mirrors come back mangled and do_unpack /
+  # devtool modify fail with zlib "inflate: data stream error". The hyper server
+  # also disables 4.2, but pinning here keeps clients on 4.1 regardless.
   fileSystems."/srv/yocto/sstate-cache" = {
     device = "10.5.3.187:/srv/nfs/yocto/sstate-cache";
     fsType = "nfs";
     options = [
       "auto"
       "rw"
       "defaults"
+      "nfsvers=4.1"
       "_netdev"
       "x-systemd.automount"
     ];
@@ -35,6 +40,7 @@
       "auto"
       "rw"
       "defaults"
+      "nfsvers=4.1"
       "_netdev"
       "x-systemd.automount"
     ];

nix/hosts/hyper/configuration.nix

@@ -68,6 +68,15 @@ in
     '';
   };
 
+  # Cap the server at NFSv4.1. NFSv4.2's READ_PLUS (sparse-aware reads) corrupts
+  # large multi-GB files read back over this share: BitBake's git pack mirrors
+  # (linux/u-boot, >4GB) come back with mangled bytes, so do_unpack / devtool
+  # modify fail with zlib "inflate: data stream error / pack checksum mismatch"
+  # at varying offsets, while a clone of the same repo to local disk is
+  # bit-perfect. Disabling 4.2 makes clients negotiate down to 4.1 (no
+  # READ_PLUS), which the shared Yocto cache doesn't otherwise need.
+  services.nfs.settings.nfsd."vers4.2" = false;
+
   services.bitbake = {
     enable = true;
     versions = {