fix(node): ignore benign port exits in trap_exit device servers

ThinPool, Img.Server, Layer.Server and Img.Mutable each run privileged
commands through `System.cmd`, which links a transient port to the caller.
Because these servers trap exits (for `terminate/2` teardown), the port's
normal close arrives as `{:EXIT, port, :normal}`. With no matching
handle_info clause the server crash-looped on it -- Layer.Server crashed
while mounting a layer, which surfaced to create_vm as :no_capacity.

Add the same `{:EXIT, _port, :normal} -> :noreply` ignore clause to all
four servers. An abnormal exit carries a non-`:normal` reason and still
falls through to the default handler.

Author: markovejnovic

lib/hyper/node/img/mutable.ex

@@ -119,6 +119,12 @@ defmodule Hyper.Node.Img.Mutable do
   @impl true
   def handle_info(:idle_timeout, state), do: {:noreply, state}
 
+  @impl true
+  # Each privileged command runs through `System.cmd`, which links a transient
+  # port to this process; because we trap exits (for `terminate/2` teardown),
+  # the port's normal close is delivered here. Ignore it.
+  def handle_info({:EXIT, _port, :normal}, state), do: {:noreply, state}
+
   @impl true
   def terminate(_reason, state) do
     # Destroy the thin volume, then release the image (its monitor on us also

lib/hyper/node/img/server.ex

@@ -128,6 +128,12 @@ defmodule Hyper.Node.Img.Server do
     {:noreply, state}
   end
 
+  @impl true
+  # Each privileged command runs through `System.cmd`, which links a transient
+  # port to this process; because we trap exits (for `terminate/2` teardown),
+  # the port's normal close is delivered here. Ignore it.
+  def handle_info({:EXIT, _port, :normal}, state), do: {:noreply, state}
+
   @impl true
   def terminate(_reason, %State{dm_names: dm_names}) do
     # Remove top-down (a snapshot's origin is the device below it). Layers are

lib/hyper/node/img/thin_pool.ex

@@ -94,6 +94,12 @@ defmodule Hyper.Node.Img.ThinPool do
     {:reply, :ok, id_free(state, id)}
   end
 
+  @impl true
+  # Each privileged command runs through `System.cmd`, which links a transient
+  # port to this process; because we trap exits (for `terminate/2` teardown),
+  # the port's normal close is delivered here. Ignore it.
+  def handle_info({:EXIT, _port, :normal}, state), do: {:noreply, state}
+
   @impl true
   def terminate(_reason, state) do
     _ = SuidHelper.Dmsetup.remove(@pool_name)

lib/hyper/node/layer/server.ex

@@ -119,6 +119,12 @@ defmodule Hyper.Node.Layer.Server do
     {:noreply, state}
   end
 
+  @impl true
+  # Each privileged command runs through `System.cmd`, which links a transient
+  # port to this process; because we trap exits (for `terminate/2` teardown),
+  # the port's normal close is delivered here. Ignore it.
+  def handle_info({:EXIT, _port, :normal}, state), do: {:noreply, state}
+
   @impl true
   def terminate(_reason, %State{blk_path: blk_path}) do
     case SuidHelper.Losetup.detach(blk_path) do