Keep expected overlay behaviour - make API overlays the lesser authority

1 files changed, 8 insertions, 4 deletions

diff --git a/src/rlx_prv_overlay.erl b/src/rlx_prv_overlay.erl
index 5ba3f75..ec8a97f 100644
--- a/src/rlx_prv_overlay.erl
+++ b/src/rlx_prv_overlay.erl
@@ -152,10 +152,15 @@ get_overlay_vars_from_file(State, OverlayVars) ->
 -spec read_overlay_vars(rlx_state:t(), proplists:proplist(), [file:name()]) ->
                                proplists:proplist() | relx:error().
 read_overlay_vars(State, OverlayVars, FileNames) ->
+    ApiCallerVars = rlx_state:api_caller_overlays(State),
     Terms = merge_overlay_vars(State, FileNames),
-    case render_overlay_vars(OverlayVars, Terms, []) of
+    case render_overlay_vars(OverlayVars ++ ApiCallerVars, Terms, []) of
         {ok, NewTerms} ->
-            OverlayVars ++ NewTerms;
+            % We place `ApiCallerVars' at the end on purpose; their
+            % definitions should be overwrittenable by both internal
+            % and rendered vars, as not to change behaviour in
+            % setups preceding the support for API caller overlays.
+            OverlayVars ++ NewTerms ++ ApiCallerVars;
         Error ->
             Error
     end.
@@ -273,8 +278,7 @@ generate_state_vars(State) ->
                                erlang:atom_to_list(Name1);
                            {Name1, Vsn1} ->
                                erlang:atom_to_list(Name1) ++ "-" ++ Vsn1
-                       end}
-     | rlx_state:api_caller_overlays(State)].
+                       end}].
 
 -spec do_overlay(rlx_state:t(), list(), proplists:proplist()) ->
                                    {ok, rlx_state:t()} | relx:error().