An error returned by a Linx.User operation.

Built from a failed File.read/1 / File.write/2 against one of the user-namespace procfs files (/proc/<pid>/uid_map, /proc/<pid>/gid_map, /proc/<pid>/setgroups). The shape:

• :path — the procfs path the operation targeted.
• :operation — what we were trying to do, as an atom (:set_uid_map, :set_gid_map, :deny_setgroups, :read_uid_map, :read_gid_map).
• :errno — the POSIX errno as an atom (:enoent, :eperm, :einval, …).
• :code — the matching positive errno integer, or nil if we don't have a mapping for this atom. Included for symmetry with Linx.Mount.Error / Linx.Cgroup.Error.

Pattern-match on :errno and :operation to handle specific failures:

case Linx.User.set_uid_map(pid, [{0, 1000, 1}]) do
  :ok ->
    :mapped

  {:error, %Linx.User.Error{errno: :eperm}} ->
    # No CAP_SETUID in the parent user ns; the map is too
    # broad for an unprivileged caller, or the map was
    # already written (write-once).
    :no_perm

  {:error, %Linx.User.Error{errno: :enoent}} ->
    # The target pid is gone.
    :pid_dead
end

Implements Exception, so an error can be raised or rendered with Exception.message/1.

Caller-side input mistakes (e.g. a non-list mappings, negative ids, zero-length entries) surface as {:error, {:bad_map, reason}} instead — distinct from kernel rejections, mirroring Linx.Mount's :bad_flag / %Error{} split.