Resolve [terrain_mask]mask_file= relative to the [binary_path] like map_file= is

data/lua/wml-tags.lua

@@ -967,7 +967,9 @@ function wml_actions.terrain_mask(cfg)
 		rules[#rules + 1] = rule
 	end
 	if cfg.mask_file then
-		mask = filesystem.read_file(cfg.mask_file)
+		local resolved = filesystem.resolve_asset(filesystem.asset_type.MAP, cfg.mask_file)
+		resolved = resolved:sub(6) -- strip off 'data/' prefix
+		mask = filesystem.read_file(resolved)
 	end
 	wesnoth.current.map:terrain_mask({x, y}, mask, {
 		is_odd = is_odd,

data/test/_main.cfg

@@ -22,6 +22,10 @@
 
 #ifdef TEST
 
+[binary_path]
+    path="data/test"
+[/binary_path]
+
 #Load test macros
 {test/macros}
 

data/test/maps/test_hills_square.mask

@@ -0,0 +1,2 @@
+Hh, Hh
+Hh, Hh

data/test/scenarios/test_terrain_mask.cfg

@@ -11,6 +11,8 @@ raw#endarg
 #endarg
 #arg RULES
 #endarg
+#arg MASK_TYPE
+mask#endarg
 {GENERIC_UNIT_TEST test_terrain_mask_{SUFFIX} (
     [event]
         name=start
@@ -18,7 +20,7 @@ raw#endarg
         [terrain_mask]
             x,y={POS}
             alignment={ALIGNMENT}
-            mask={MASK}
+            {MASK_TYPE}={MASK}
             {RULES}
         [/terrain_mask]
         {POST_MASK}
@@ -138,6 +140,14 @@ Hh, Hh"#enddef
     {ASSERT_LOCATION_TERRAIN 8 4 Hh}
 )}
 
+# mask_file= works (this is the same as the above test in all other respects)
+{TERRAIN_MASK_TEST file "test_hills_square.mask" (POS=7,3) (RULES={TERRAIN_RULE_SIMPLE Kha^Kov Hh Ko}{TERRAIN_RULE_SIMPLE Cha^Kov Hh Co}) (POST_MASK=
+    {ASSERT_LOCATION_TERRAIN 7 3 Ko}
+    {ASSERT_LOCATION_TERRAIN 7 4 Co}
+    {ASSERT_LOCATION_TERRAIN 8 3 Hh}
+    {ASSERT_LOCATION_TERRAIN 8 4 Hh}
+) (MASK_TYPE=mask_file)}
+
 # TODO: Add more terrain rule tests?
 
 #undef TERRAIN_MASK_TEST