Add git-submodule fetch method

Initially submitted by Daniel White.

core/deps.mk

@@ -500,6 +500,10 @@ define dep_fetch_git
 	cd $(DEPS_DIR)/$(call dep_name,$(1)) && git checkout -q $(call dep_commit,$(1));
 endef
 
+define dep_fetch_git-submodule
+	git submodule update --init -- $(DEPS_DIR)/$1;
+endef
+
 define dep_fetch_hg
 	hg clone -q -U $(call dep_repo,$(1)) $(DEPS_DIR)/$(call dep_name,$(1)); \
 	cd $(DEPS_DIR)/$(call dep_name,$(1)) && hg update -q $(call dep_commit,$(1));

doc/src/guide/deps.asciidoc

@@ -162,6 +162,7 @@ The following table lists all existing methods:
 |===
 | Name           | Format          | Description
 | git            | git repo commit | Clone the Git repository and checkout the given version
+| git-submodule  | git-submodule   | Initialize and update the Git submodule
 | hg             | hg repo commit  | Clone the Mercurial repository and update to the given version
 | svn            | svn repo        | Checkout the given SVN repository
 | cp             | cp path/to/repo | Recursively copy a local directory
@@ -184,6 +185,13 @@ Or to fetch Ehsa tag 4.0.3 from Mercurial:
 [source,make]
 dep_ehsa = hg https://bitbucket.org/a12n/ehsa 4.0.3
 
+Git also comes with a concept of submodules. Erlang.mk can
+automatically initializes and updates submodules for dependencies,
+as long as they were added beforehand using `git submodule add`:
+
+[source,make]
+dep_cowboy = git-submodule
+
 The `svn` method only has a repository value, but that's
 simply because the SVN repository URL can also contain
 the path and commit.

test/core_deps.mk

@@ -1,6 +1,6 @@
 # Core: Packages and dependencies.
 
-CORE_DEPS_CASES = apps apps-conflict apps-deep-conflict apps-dir apps-new-app apps-new-lib apps-new-tpl apps-only build-c-8cc build-c-imagejs build-erl build-js dep-commit dir doc fetch-cp fetch-custom fetch-fail-bad fetch-fail-unknown fetch-git fetch-hex fetch-hg fetch-legacy fetch-svn ignore mv mv-rebar no-autopatch no-autopatch-erlang-mk no-autopatch-rebar order-first order-top otp pkg rel search shell skip test
+CORE_DEPS_CASES = apps apps-conflict apps-deep-conflict apps-dir apps-new-app apps-new-lib apps-new-tpl apps-only build-c-8cc build-c-imagejs build-erl build-js dep-commit dir doc fetch-cp fetch-custom fetch-fail-bad fetch-fail-unknown fetch-git fetch-git-submodule fetch-hex fetch-hg fetch-legacy fetch-svn ignore mv mv-rebar no-autopatch no-autopatch-erlang-mk no-autopatch-rebar order-first order-top otp pkg rel search shell skip test
 CORE_DEPS_TARGETS = $(addprefix core-deps-,$(CORE_DEPS_CASES))
 CORE_DEPS_CLEAN_TARGETS = $(addprefix clean-,$(CORE_DEPS_TARGETS))
 
@@ -710,6 +710,60 @@ endif
 		{ok, \"1.0.0\"} = application:get_key(cowboy, vsn), \
 		halt()"
 
+core-deps-fetch-git-submodule: build clean-core-deps-fetch-git-submodule
+
+	$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 "Bootstrap a new OTP library named my_dep inside $(APP)"
+	$t mkdir $(APP)/my_dep
+	$t cp ../erlang.mk $(APP)/my_dep/
+	$t $(MAKE) -C $(APP)/my_dep/ -f erlang.mk bootstrap-lib $v
+# Create an empty file so src/ gets committed.
+	$t touch $(APP)/my_dep/src/README
+	$t cd $(APP)/my_dep && \
+		git init -q && \
+		git config user.email "testsuite@erlang.mk" && \
+		git config user.name "test suite" && \
+		git add . && \
+		git commit -q -m "Tests"
+
+	$i "Add the submodule to my_dep"
+	$t mkdir $(APP)/deps
+	$t cd $(APP) && \
+		git init -q && \
+		git submodule -q add file://$(abspath $(APP)/my_dep) deps/my_dep && \
+		git config user.email "testsuite@erlang.mk" && \
+		git config user.name "test suite" && \
+		git add . && \
+		git commit -q -m "Tests"
+
+	$i "Distclean the application"
+	$t $(MAKE) -C $(APP) distclean $v
+
+	$i "Add my_dep to the list of dependencies"
+	$t perl -ni.bak -e 'print;if ($$.==1) {print "DEPS = my_dep\ndep_my_dep = git-submodule\n"}' $(APP)/Makefile
+
+ifdef LEGACY
+	$i "Add my_dep to the applications key in the .app.src file"
+	$t perl -ni.bak -e 'print;if ($$.==7) {print "\t\tmy_dep,\n"}' $(APP)/src/$(APP).app.src
+endif
+
+	$i "Build the application"
+	$t $(MAKE) -C $(APP) $v
+
+	$i "Check that all dependencies were fetched"
+	$t test -d $(APP)/deps/my_dep
+
+	$i "Check that the application was compiled correctly"
+	$t $(ERL) -pa $(APP)/ebin/ $(APP)/deps/*/ebin/ -eval " \
+		[ok = application:load(App) || App <- [$(APP), my_dep]], \
+		{ok, Deps} = application:get_key($(APP), applications), \
+		true = lists:member(my_dep, Deps), \
+		halt()"
+
 core-deps-fetch-hex: build clean-core-deps-fetch-hex
 
 	$i "Bootstrap a new OTP library named $(APP)"