Accept a list of deps to forcibly rebuild in `$(FORCE_REBUILD)`

... in addition to the `$(force_rebuild_dep)` function.

It's easier for the common use case than specify a function.

core/deps.mk

@@ -120,6 +120,25 @@ ifeq ($(IS_APP)$(IS_DEP),)
 	$(verbose) rm -f $(ERLANG_MK_TMP)/apps.log $(ERLANG_MK_TMP)/deps.log
 endif
 
+# Erlang.mk does not rebuild dependencies after they were compiled
+# once. If a developer is working on the top-level project and some
+# dependencies at the same time, he may want to change this behavior.
+# There are two solutions:
+#     1. Set `FULL=1` so that all dependencies are visited and
+#        recursively recompiled if necessary.
+#     2. Set `FORCE_REBUILD=` to the specific list of dependencies that
+#        should be recompiled (instead of the whole set).
+
+FORCE_REBUILD ?=
+
+ifeq ($(origin FULL),undefined)
+ifneq ($(strip $(force_rebuild_dep)$(FORCE_REBUILD)),)
+define force_rebuild_dep
+echo "$(FORCE_REBUILD)" | grep -qw "$$(basename "$1")"
+endef
+endif
+endif
+
 ifneq ($(SKIP_DEPS),)
 deps::
 else

test/core_deps.mk

@@ -210,6 +210,36 @@ core-deps-dep-built-force-full: init
 	$t find $(APP)/deps/cowlib -type f -newer $(APP)/EXPECT | grep -v ".git" | sort | diff $(APP)/EXPECT -
 	$t rm $(APP)/EXPECT
 
+	$i "Touch one cowlib file to mark it for recompilation"
+	$t $(SLEEP)
+	$t touch $(APP)/deps/cowlib/src/cow_http.erl
+
+	$i "Check that cowlib is not rebuilt if \`FORCE_REBUILD\` is empty"
+	$t touch $(APP)/EXPECT
+	$t $(SLEEP)
+	$t $(MAKE) -C $(APP) FORCE_REBUILD= $v
+	$t find $(APP)/deps/cowlib -type f -newer $(APP)/EXPECT | sort | diff $(APP)/EXPECT -
+	$t rm $(APP)/EXPECT
+
+	$i "Check that cowlib is not rebuilt if \`FORCE_REBUILD\` does not mention cowlib"
+	$t touch $(APP)/EXPECT
+	$t $(SLEEP)
+	$t $(MAKE) -C $(APP) FORCE_REBUILD='other_dep' $v
+	$t find $(APP)/deps/cowlib -type f -newer $(APP)/EXPECT | sort | diff $(APP)/EXPECT -
+	$t rm $(APP)/EXPECT
+
+	$i "Check that cowlib is rebuilt if \`FORCE_REBUILD\` contains cowlib"
+	$t printf "%s\n" \
+		$(APP)/deps/cowlib/cowlib.d \
+		$(APP)/deps/cowlib/ebin/cowlib.app \
+		$(APP)/deps/cowlib/ebin/cow_http.beam \
+		$(APP)/deps/cowlib/ebin/dep_built | sort > $(APP)/EXPECT
+	$t $(SLEEP)
+	$t $(MAKE) -C $(APP) FORCE_REBUILD='other_dep cowlib' $v
+# Files in .git might end up modified due to the id generation in the .app file.
+	$t find $(APP)/deps/cowlib -type f -newer $(APP)/EXPECT | grep -v ".git" | sort | diff $(APP)/EXPECT -
+	$t rm $(APP)/EXPECT
+
 core-deps-dep-built-ln: init
 
 	$i "Bootstrap a new OTP library named $(APP)"
@@ -1015,8 +1045,8 @@ core-deps-dep-makefile-change: init
 	$t $(MAKE) -C $(APP)/my_dep/ -f erlang.mk bootstrap-lib $v
 
 	$i "Add my_dep to the list of dependencies"
-# We use force_rebuild_dep to ensure it gets rebuilt even on Windows.
-	$t perl -ni.bak -e "print;if ($$.==1) {print \"DEPS = my_dep\ndep_my_dep = ln $(CURDIR)/$(APP)/my_dep/\nforce_rebuild_dep = test \";print '\$$1';print \" = $(CURDIR)/$(APP)/deps/my_dep\n\"}" $(APP)/Makefile
+# We use FORCE_REBUILD to ensure it gets rebuilt even on Windows.
+	$t perl -ni.bak -e "print;if ($$.==1) {print \"DEPS = my_dep\ndep_my_dep = ln $(CURDIR)/$(APP)/my_dep/\nFORCE_REBUILD = my_dep\n\"}" $(APP)/Makefile
 
 ifdef LEGACY
 	$i "Add my_dep to the applications key in the .app.src file"