An error returned by a Linx.Mount operation.

Built from a failed mount(2) / umount2(2) / pivot_root(2) call against the kernel. The shape:

• :path — the absolute filesystem path the operation targeted (the target argument of mount/4, the target of umount/2, etc.).
• :operation — what we were trying to do, as an atom:
  • :mount, :umount, :pivot_root — the target syscall itself failed.
  • :open_ns — couldn't open the target namespace file (target process is gone, BEAM lacks read access).
  • :unshare — couldn't detach the worker thread's fs_struct from the BEAM's. Prerequisite for setns on a mount namespace; extremely unlikely to fail.
  • :setns — couldn't enter the target namespace (lacks CAP_SYS_ADMIN in the right user namespace).
  • :chdir — couldn't chdir into new_root before pivot_root. Typically the path doesn't exist or isn't a directory.
  • :thread — couldn't create the worker thread that does the cross-namespace dance. Rare; typically EAGAIN from thread-creation pressure. The last three only appear when a cross-namespace :in option was used.
• :errno — the POSIX errno as an atom (:enoent, :eacces, :ebusy, :einval, :eperm, …).
• :code — the matching positive errno integer, or nil if we don't have a mapping for this atom. Included for symmetry with Linx.Cgroup.Error / Linx.Netlink.Error and for callers unfamiliar with POSIX mnemonics.

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

case Linx.Mount.mount("tmpfs", "/mnt/x", "tmpfs") do
  :ok -> :ok
  {:error, %Linx.Mount.Error{errno: :eacces}} ->
    # not enough privilege
    :no_perm
end

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