core/test.mk: Rebuild out-of-date test modules only

... instead of always rebuilding all of them each time a testsuite is
executed.

The only exception is when a Makefile was modified: like for main
source files, test modules are all recompiled in this case.

core/test.mk

@@ -31,9 +31,23 @@ test-deps: $(ALL_TEST_DEPS_DIRS)
 endif
 
 ifneq ($(wildcard $(TEST_DIR)),)
-test-dir:
-	$(gen_verbose) erlc -v $(TEST_ERLC_OPTS) -o $(TEST_DIR) \
-		-pa ebin/ -I include/ $(call core_find,$(TEST_DIR)/,*.erl)
+test-dir: $(ERLANG_MK_TMP)/$(PROJECT).last-testdir-build
+	@:
+
+test_erlc_verbose_0 = @echo " ERLC  " $(filter-out $(patsubst %,%.erl,$(ERLC_EXCLUDE)),\
+	$(filter %.erl %.core,$(notdir $(FILES_TO_COMPILE))));
+test_erlc_verbose_2 = set -x;
+test_erlc_verbose = $(test_erlc_verbose_$(V))
+
+define compile_test_erl
+	$(test_erlc_verbose) erlc -v $(TEST_ERLC_OPTS) -o $(TEST_DIR) \
+		-pa ebin/ -I include/ $(1)
+endef
+
+ERL_TEST_FILES = $(call core_find,$(TEST_DIR)/,*.erl)
+$(ERLANG_MK_TMP)/$(PROJECT).last-testdir-build: $(ERL_TEST_FILES) $(MAKEFILE_LIST)
+	$(eval FILES_TO_COMPILE := $(if $(filter $(MAKEFILE_LIST),$?),$(filter $(ERL_TEST_FILES),$^),$?))
+	$(if $(strip $(FILES_TO_COMPILE)),$(call compile_test_erl,$(FILES_TO_COMPILE)); touch $@)
 endif
 
 test-build:: IS_TEST=1
@@ -70,5 +84,5 @@ clean:: clean-test-dir
 
 clean-test-dir:
 ifneq ($(wildcard $(TEST_DIR)/*.beam),)
-	$(gen_verbose) rm -f $(TEST_DIR)/*.beam
+	$(gen_verbose) rm -f $(TEST_DIR)/*.beam $(ERLANG_MK_TMP)/$(PROJECT).last-testdir-build
 endif

test/core_app.mk

@@ -1682,7 +1682,7 @@ core-app-yrl-header: init
 	$t mkdir $(APP)/
 	$t cp ../erlang.mk $(APP)/
 	$t $(MAKE) -C $(APP) -f erlang.mk bootstrap-lib $v
-	
+
 	$i "Create a .yrl file"
 	$t echo "Nonterminals E T F." > $(APP)/src/y_parse.yrl
 	$t echo "Terminals '+' '*' '(' ')' number." >> $(APP)/src/y_parse.yrl
@@ -2743,3 +2743,46 @@ endif
 			= application:get_key(my_app, modules), \
 		[{module, M} = code:load_file(M) || M <- Mods], \
 		halt()"
+
+core-app-test-build-outofdate-files-only: init
+
+	$i "Bootstrap a new OTP library named $(APP)"
+	$t mkdir $(APP)/
+	$t cp ../erlang.mk $(APP)/
+	$t $(MAKE) -C $(APP) -f erlang.mk bootstrap-lib $v
+
+	$i "Generate .erl test files"
+	$t mkdir $(APP)/test/
+	$t printf "%s\n" "-module(use_blue)." > $(APP)/test/use_blue.erl
+	$t printf "%s\n" "-module(use_red)." > $(APP)/test/use_red.erl
+
+	$i "Build the application testsuite"
+	$t $(MAKE) -C $(APP) test-build $v
+
+	$i "Check that all compiled files exist"
+	$t test -f $(APP)/test/use_blue.beam
+	$t test -f $(APP)/test/use_red.beam
+
+# We will sleep for 1 second in several places, in case the underlying
+# filesystem does not support sub-second timestamps. This is to make
+# sure that we can compare file timestamps effectively.
+	$t sleep 1 && touch $(APP)/build-1
+
+	$i "Re-un the make command; check that nothing is rebuilt"
+	$t $(MAKE) -C $(APP) test-build $v
+	$t test $(APP)/test/use_blue.beam -ot $(APP)/build-1
+	$t test $(APP)/test/use_red.beam -ot $(APP)/build-1
+
+	$i "Touch one .erl file; check that only required files are rebuilt"
+	$t sleep 1 && touch $(APP)/test/use_blue.erl
+	$t $(MAKE) -C $(APP) test-build $v
+	$t test $(APP)/test/use_blue.beam -nt $(APP)/build-1
+	$t test $(APP)/test/use_red.beam -ot $(APP)/build-1
+
+	$t touch $(APP)/build-2
+
+	$i "Touch one Makefile; check that all files are rebuilt"
+	$t sleep 1 && touch $(APP)/Makefile
+	$t $(MAKE) -C $(APP) test-build $v
+	$t test $(APP)/test/use_blue.beam -nt $(APP)/build-2
+	$t test $(APP)/test/use_red.beam -nt $(APP)/build-2