Fix toolchain tests tmp_dir tag; propagate .tool-versions read errors

ExUnit only injects context.tmp_dir when :tmp_dir is tagged; add @moduletag
so Desktop.ToolchainTest setup runs.

Stop mapping all File.read failures to :enoent; surface real I/O errors via
Desktop.Toolchain.verify with a clear message for the Mix task.

Co-authored-by: Dominic Letz <dominicletz@users.noreply.github.com>

Author: cursoragent

lib/desktop/tool_versions.ex

@@ -34,16 +34,15 @@ defmodule Desktop.ToolVersions do
 
   @doc """
   Reads `.tool-versions` from `directory` and parses it. Returns `{:ok, map}` or
-  `{:error, :enoent}` when the file is missing.
+  `{:error, reason}` from `File.read/1` (typically `:enoent` when the file is missing).
   """
-  @spec read_from_dir(String.t()) :: {:ok, map()} | {:error, :enoent}
+  @spec read_from_dir(String.t()) :: {:ok, map()} | {:error, :enoent | File.posix() | :badarg}
   def read_from_dir(directory) do
     path = Path.join(directory, ".tool-versions")
 
     case File.read(path) do
       {:ok, body} -> {:ok, parse(body)}
-      {:error, :enoent} -> {:error, :enoent}
-      {:error, _} -> {:error, :enoent}
+      {:error, reason} -> {:error, reason}
     end
   end
 end

lib/desktop/toolchain.ex

@@ -22,6 +22,9 @@ defmodule Desktop.Toolchain do
       {:error, :enoent} ->
         {:ok, :no_tool_versions}
 
+      {:error, reason} ->
+        {:error, [read_tool_versions_failed_message(reason)]}
+
       {:ok, tools} ->
         errors =
           []
@@ -75,6 +78,14 @@ defmodule Desktop.Toolchain do
     end
   end
 
+  defp read_tool_versions_failed_message(reason) when is_atom(reason) do
+    "Could not read .tool-versions: #{:file.format_error(reason)}"
+  end
+
+  defp read_tool_versions_failed_message(reason) do
+    "Could not read .tool-versions: #{inspect(reason)}"
+  end
+
   defp otp_major_from_declared(declared) do
     declared
     |> String.split(".", parts: 2)

test/desktop/toolchain_test.exs

@@ -1,6 +1,8 @@
 defmodule Desktop.ToolchainTest do
   use ExUnit.Case, async: true
 
+  @moduletag :tmp_dir
+
   setup context do
     root = Path.join(context.tmp_dir, "proj")
     File.mkdir_p!(root)
@@ -23,14 +25,18 @@ defmodule Desktop.ToolchainTest do
   test "error when OTP major mismatches", %{root: root} do
     File.write!(Path.join(root, ".tool-versions"), "erlang 26.0.1\n")
 
-    assert {:error, [msg]} = Desktop.Toolchain.verify(root, otp_release: "25", elixir_version: "1.19.1")
+    assert {:error, [msg]} =
+             Desktop.Toolchain.verify(root, otp_release: "25", elixir_version: "1.19.1")
+
     assert msg =~ "OTP"
   end
 
   test "error when Elixir semver mismatches", %{root: root} do
     File.write!(Path.join(root, ".tool-versions"), "elixir 1.19.1-otp-26\n")
 
-    assert {:error, [msg]} = Desktop.Toolchain.verify(root, otp_release: "26", elixir_version: "1.18.0")
+    assert {:error, [msg]} =
+             Desktop.Toolchain.verify(root, otp_release: "26", elixir_version: "1.18.0")
+
     assert msg =~ "Elixir mismatch"
   end
 end