Fix nested use of Mix.Config.eval!/2 (#7925)

Previously, if a config file uses Mix.Config.eval!/2 then it would fail with the
following error:

 > ** (RuntimeError) could not set configuration via Mix.Config. This usually
 >  means you are trying to execute a configuration file directly instead of
 > using the proper command, such as mix loadconfig
 > code: assert Mix.Config.eval!(fixture_path("configs/nested_import.exs")) ==
 > stacktrace:
 >   (mix) lib/mix/config.ex:71: Mix.Config.raise_improper_use!/0
 >   (mix) lib/mix/config.ex:222: Mix.Config.eval!/2
 >   lib/mix/test/mix/config_test.exs:110: (test)

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: Gazler

lib/mix/lib/mix/config.ex

@@ -47,7 +47,6 @@ defmodule Mix.Config do
 
   defp put_config(value) do
     Process.put(@config_key, value)
-    :ok
   end
 
   defp delete_config() do
@@ -60,7 +59,6 @@ defmodule Mix.Config do
 
   defp put_files(value) do
     Process.put(@files_key, value)
-    :ok
   end
 
   defp delete_files() do
@@ -215,20 +213,23 @@ defmodule Mix.Config do
   It returns a tuple with the configuration and the imported paths.
   """
   def eval!(file, imported_paths \\ []) do
-    put_config([])
-    put_files(imported_paths)
-    {eval_config, _} = eval_config!(Path.expand(file))
+    previous_config = put_config([])
+    previous_files = put_files(imported_paths)
 
-    case get_config!() do
-      [] when is_list(eval_config) ->
-        {validate!(eval_config), get_files!()}
+    try do
+      {eval_config, _} = eval_config!(Path.expand(file))
 
-      pdict_config ->
-        {pdict_config, get_files!()}
+      case get_config!() do
+        [] when is_list(eval_config) ->
+          {validate!(eval_config), get_files!()}
+
+        pdict_config ->
+          {pdict_config, get_files!()}
+      end
+    after
+      if previous_config, do: put_config(previous_config), else: delete_config()
+      if previous_files, do: put_files(previous_files), else: delete_files()
     end
-  after
-    delete_config()
-    delete_files()
   end
 
   @doc false

lib/mix/test/fixtures/configs/nested_import.exs

@@ -0,0 +1,3 @@
+use Mix.Config
+Mix.Config.eval!(Path.join([__DIR__, "good_config.exs"]))
+:done

lib/mix/test/mix/config_test.exs

@@ -107,6 +107,9 @@ defmodule Mix.ConfigTest do
              {[my_app: [key: :value]],
               [fixture_path("configs/good_config.exs"), fixture_path("configs/good_import.exs")]}
 
+    assert Mix.Config.eval!(fixture_path("configs/nested_import.exs")) ==
+             {[], [fixture_path("configs/nested_import.exs")]}
+
     assert_raise ArgumentError,
                  ~r"expected runtime config for app :sample to return keyword list",
                  fn -> Mix.Config.eval!(fixture_path("configs/bad_app.exs")) end