The file '/etc' already exists in the tar with perms when maxLayers is set to 1

[gytis-ivaskevicius]

When multiple derivations create the same directory and maxLayers=1 this error is thrown:

layers.json> The file '/etc' already exists in the tar with perms []types.Perm{types.Perm{Regex:"/etc/nix/nix.conf", Mode:"", Uid:0, Gid:0, Uname:"", Gname:""}, types.Perm{Regex:"/tmp", Mode:"1777", Uid:0, Gid:0, Uname:"", Gname:""}} but is overridden with perms []types.Perm(nil)
error: builder for '/nix/store/bryxlb2592i7zqavkb6j6m7rv59xvx35-layers.json.drv' failed with exit code 1;
       last 1 log lines:
       > The file '/etc' already exists in the tar with perms []types.Perm{types.Perm{Regex:"/etc/nix/nix.conf", Mode:"", Uid:0, Gid:0, Uname:"", Gname:""}, types.Perm{Regex:"/tmp", Mode:"1777", Uid:0, Gid:0, Uname:"", Gname:""}} but is overridden with perms []types.Perm(nil)
       For full logs, run 'nix log /nix/store/bryxlb2592i7zqavkb6j6m7rv59xvx35-layers.json.drv'.

If I set maxLayers=128 - its all good. It would be really nice to improve layers merging logic, if permissions are in fact different (I am not sure if that is the case) - set higher perms. And if file/dir and perms are identical - just keep one of them, ignore the error

If you'd like I could create a minimal example of this issue

[dr-housemd]

I've run into a similar issue where I've coreutils as one layer and tzdata as another.
This isn't an issue if I add both packages to a single layer.

 > The file '/bin' already exists in the graph with mode 'Lrwxrwxrwx' from '/nix/store/8kznh5nc5lmsqmkqlfkwsvd6ianwap2a-runtimeenv/bin' while it is added again with mode 'dr-xr-xr-x' by '/nix/store/7g55zq3h09rg244d343ljxijpi6hz6xb-devenv/bin'

Looks like this is where we're running into it.
https://github.com/nlewo/nix2container/blob/master/nix/graph.go#L85

Happy to help fix this issue if we can decide on what the fix should be..

[dr-housemd]

One way I can think of is to compute the nar hash of the directories/files. If they're the same, pick one with higher permissions.
If they're different and it is a directory,

• we need to walk the directory
• check if they're common files
• if files with same name in both layers exist, compute nar hash. Only fail if there's a hash mismatch.
• add non-overlapping files

[nlewo]

If you'd like I could create a minimal example of this issue

Yes, that would be great!

(I don't exactly understand which situations lead to this issue and an example could help me.)