Makefile: Allow building Gitaly with bundled Git binaries

In order to allow the use of feature flags to toggle different Git
versions at runtime, Gitaly needs to take much more ownership of
binaries such that it knows where to find the Git binaries without
having the admin to always set up different paths whenever there are two
different versions of Git. As a first step towards taking more
ownership, this commit introduces the new `WITH_BUNDLED_GIT` variable to
our Makefile: if set, we will build and install required Git binaries.
In order to not accidentally overwrite any preexisting binaries, the
required binaries all have a "gitaly-" prefix.

Note that ideally, we'd want to install only the "git" binary, which
nowadays contains almost all of the functionality required by us. One
exception though is cURL-based remote helpers, which are still split out
into a separate binary "git-remote-http". Given that we do interact with
HTTP-based Git servers we thus have to build and install both binaries.

This target is currently completely optional, and we want to continue to
support running Gitaly with an external Git executable not built by
Gitaly. It may eventually replace the current "make git" target though,
which sets up a complete Git installation for us.

1 files changed, 33 insertions, 2 deletions

diff --git a/Makefile b/Makefile
index 408659d07..1a0869b1c 100644
--- a/Makefile
+++ b/Makefile
@@ -303,6 +303,23 @@ install: build
 	${Q}mkdir -p ${INSTALL_DEST_DIR}
 	install $(call find_command_binaries) ${INSTALL_DEST_DIR}
 
+ifdef WITH_BUNDLED_GIT
+GIT_EXECUTABLES += git
+GIT_EXECUTABLES += git-remote-http
+GIT_EXECUTABLES += git-http-backend
+
+build: $(patsubst %,${BUILD_DIR}/bin/gitaly-%,${GIT_EXECUTABLES})
+
+install: $(patsubst %,${INSTALL_DEST_DIR}/gitaly-%,${GIT_EXECUTABLES})
+
+${BUILD_DIR}/bin/gitaly-%: ${GIT_SOURCE_DIR}/% | ${BUILD_DIR}/bin
+	${Q}install $< $@
+
+${INSTALL_DEST_DIR}/gitaly-%: ${BUILD_DIR}/bin/gitaly-%
+	${Q}mkdir -p $(@D)
+	${Q}install $< $@
+endif
+
 .PHONY: prepare-tests
 prepare-tests: git libgit2 prepare-test-repos ${SOURCE_DIR}/.ruby-bundle
 
@@ -521,7 +538,13 @@ ${LIBGIT2_INSTALL_DIR}/lib/libgit2.a: ${DEPENDENCY_DIR}/libgit2.version
 	${Q}CMAKE_BUILD_PARALLEL_LEVEL=$(shell nproc) cmake --build ${LIBGIT2_BUILD_DIR} --target install
 	go install -a github.com/libgit2/git2go/${GIT2GO_VERSION}
 
-${GIT_INSTALL_DIR}/bin/git: ${DEPENDENCY_DIR}/git.version
+# This target is responsible for checking out Git sources. In theory, we'd only
+# need to depend on the source directory. But given that the source directory
+# always changes when anything inside of it changes, like when we for example
+# build binaries inside of it, we cannot depend on it directly or we'd
+# otherwise try to rebuild all targets depending on it whenever we build
+# something else. We thus depend on the Makefile instead.
+${GIT_SOURCE_DIR}/Makefile: ${DEPENDENCY_DIR}/git.version
 	${Q}${GIT} -c init.defaultBranch=master init ${GIT_QUIET} ${GIT_SOURCE_DIR}
 	${Q}${GIT} -C "${GIT_SOURCE_DIR}" config remote.origin.url ${GIT_REPO_URL}
 	${Q}${GIT} -C "${GIT_SOURCE_DIR}" config remote.origin.tagOpt --no-tags
@@ -539,9 +562,17 @@ ifdef GIT_EXTRA_VERSION
 else
 	${Q}rm -f "${GIT_SOURCE_DIR}"/version
 endif
+	${Q}touch $@
+
+${GIT_SOURCE_DIR}/%: ${GIT_SOURCE_DIR}/Makefile
+	${Q}env -u PROFILE -u MAKEFLAGS -u GIT_VERSION ${MAKE} -C ${GIT_SOURCE_DIR} -j$(shell nproc) prefix=${GIT_PREFIX} ${GIT_BUILD_OPTIONS} $(notdir $@)
+	${Q}touch $@
+
+${GIT_INSTALL_DIR}/bin/git: ${GIT_SOURCE_DIR}/Makefile
 	${Q}rm -rf ${GIT_INSTALL_DIR}
 	${Q}mkdir -p ${GIT_INSTALL_DIR}
-	env -u PROFILE -u MAKEFLAGS -u GIT_VERSION ${MAKE} -C ${GIT_SOURCE_DIR} -j$(shell nproc) prefix=${GIT_PREFIX} ${GIT_BUILD_OPTIONS} install
+	${Q}env -u PROFILE -u MAKEFLAGS -u GIT_VERSION ${MAKE} -C ${GIT_SOURCE_DIR} -j$(shell nproc) prefix=${GIT_PREFIX} ${GIT_BUILD_OPTIONS} install
+	${Q}touch $@
 
 ${TOOLS_DIR}/protoc.zip: TOOL_VERSION = ${PROTOC_VERSION}
 ${TOOLS_DIR}/protoc.zip: ${TOOLS_DIR}/protoc.version