An error returned by a Linx.Sysctl operation.

Built from a failed File.read/1 / File.write/2 against a /proc/sys/... path, or — for the cross-namespace verbs — from a failure during the setns dance the NIF performs on a throwaway pthread.

The shape:

• :key — the dot-form sysctl key the caller passed in, e.g. "net.ipv4.ip_forward".
• :path — the resolved procfs path the operation targeted, e.g. "/proc/sys/net/ipv4/ip_forward".
• :operation — what we were trying to do, as an atom:
  • :read, :write, :list — the procfs I/O itself failed.
  • :open_ns, :unshare, :setns, :chdir, :thread — namespace-acquisition failures from the cross-namespace NIF (only seen when a cross-namespace :in option was used).
• :errno — the POSIX errno as an atom (:enoent, :eperm, :eacces, :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.User.Error / Linx.Cgroup.Error.

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

case Linx.Sysctl.write("net.ipv4.ip_forward", 1) do
  :ok ->
    :ok

  {:error, %Linx.Sysctl.Error{errno: :eacces}} ->
    # Needs root.
    :no_perm

  {:error, %Linx.Sysctl.Error{errno: :enoent}} ->
    # No such sysctl on this kernel.
    :unknown_knob
end

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

Caller-side input mistakes (a malformed key, a multi-line value the kernel would silently truncate) surface as {:error, {:bad_key, reason}} / {:error, {:bad_value, reason}} instead — distinct from kernel rejections, mirroring Linx.User's :bad_map / %Error{} split.