Support returning the current and next boot slots

This exposes the ops.fw's capability of returning which slot is running
and which will be the one that gets run on next boot. This actually has
been around since the ops.fw was added, but it was never exposed.

lib/nerves_runtime/fwup_ops.ex

@@ -104,6 +104,29 @@ defmodule Nerves.Runtime.FwupOps do
     end
   end
 
+  @doc """
+  Return boot status
+
+  This invokes the "status" task in the `ops.fw` to report the current
+  firmware slot and what slot will be tried on the next reboot. The `ops.fw`
+  is expected to print the slot name or two slot names separated by "->".
+  """
+  @spec status(options()) ::
+          {:ok, %{current: String.t(), next: String.t()}} | {:error, reason :: any}
+  def status(opts \\ []) do
+    with {:ok, raw_result} <- run_fwup("status", opts),
+         {:ok, result} <- deframe(raw_result, []) do
+      Enum.find_value(result, {:error, "Invalid status"}, &find_status/1)
+    end
+  end
+
+  defp find_status({:warning, <<slot::1-bytes>>}), do: {:ok, %{current: slot, next: slot}}
+
+  defp find_status({:warning, <<current::1-bytes, "->", next::1-bytes>>}),
+    do: {:ok, %{current: current, next: next}}
+
+  defp find_status(_status), do: nil
+
   defp run_fwup(task, opts) do
     devpath = Keyword.get(opts, :devpath, "/dev/rootdisk0")
     cmd_opts = [env: Keyword.get(opts, :env, %{})]

test/nerves_runtime/fwup_ops_test.exs

@@ -50,6 +50,20 @@ defmodule NervesRuntime.FwupOpsTest do
     assert {:error, "prevent-revert error"} = FwupOps.prevent_revert(@fwup_fail_options)
   end
 
+  defp status_ops(status) do
+    Keyword.put(@fwup_options, :env, %{"STATUS" => status})
+  end
+
+  test "status" do
+    # ops.conf lets you set the status via $STATUS
+    assert {:ok, %{current: "a", next: "b"}} = FwupOps.status(status_ops("a->b"))
+    assert {:ok, %{current: "b", next: "a"}} = FwupOps.status(status_ops("b->a"))
+    assert {:ok, %{current: "c", next: "c"}} = FwupOps.status(status_ops("c"))
+    assert {:error, "Invalid status"} = FwupOps.status(status_ops("xyz"))
+
+    assert {:error, "status error"} = FwupOps.status(@fwup_fail_options)
+  end
+
   test "missing ops.fw" do
     Application.put_env(:nerves_runtime, :revert_fw_path, "/does/not/exist/missing_ops.fw")