Makefile: Add build target for libgit2

This adds a build target for libgit2 as a prerequisite dependency for
Git2Go. The build is configured as a static archive with minimal
dependencies such that the only required shared library is the system's
libc.

Instead of building libgit2 manually, we could've also gone the way of
fetching the Git2Go repository and using its scripts. This has multiple
problems though. First, the build scripts aren't flexible and will
default to auto-detection of system-provided libraries. As a result, the
generated archive will usually have more dependencies than only libc.
Second, we'd have to use a "replace" statement for our go.mod file.
While that's not a problem by itself, it will cause several targets like
e.g. linting, verification and others to depend on the presence of this
Git2Go repository.

The approach of building libgit2 manually is thus preferable, as it
gives us both more flexibility and less dependencies on libgit2 in our
build process.

4 files changed, 48 insertions, 7 deletions

diff --git a/Makefile b/Makefile
index cf9c95a43..5a34c1c70 100644
--- a/Makefile
+++ b/Makefile
@@ -56,6 +56,7 @@ GOLANGCI_LINT_VERSION ?= 1.27.0
 PROTOC_VERSION        ?= 3.6.1
 PROTOC_GEN_GO_VERSION ?= 1.3.2
 GIT_VERSION           ?= v2.27.0
+LIBGIT2_VERSION       ?= v1.0.1
 
 # Dependency downloads
 ifeq (${OS},Darwin)
@@ -89,6 +90,29 @@ ifeq (${GIT_BUILD_OPTIONS},)
     GIT_BUILD_OPTIONS += NO_R_TO_GCC_LINKER=YesPlease
 endif
 
+# libgit2 target
+LIBGIT2_REPO_URL    ?= https://gitlab.com/libgit2/libgit2
+LIBGIT2_SOURCE_DIR  ?= ${BUILD_DIR}/src/libgit2
+LIBGIT2_BUILD_DIR   ?= ${LIBGIT2_SOURCE_DIR}/build
+LIBGIT2_INSTALL_DIR ?= ${BUILD_DIR}/libgit2
+
+ifeq (${LIBGIT2_BUILD_OPTIONS},)
+    LIBGIT2_BUILD_OPTIONS += -DTHREADSAFE=ON
+    LIBGIT2_BUILD_OPTIONS += -DBUILD_CLAR=OFF
+    LIBGIT2_BUILD_OPTIONS += -DBUILD_SHARED_LIBS=OFF
+    LIBGIT2_BUILD_OPTIONS += -DCMAKE_C_FLAGS=-fPIC
+    LIBGIT2_BUILD_OPTIONS += -DCMAKE_BUILD_TYPE=Release
+    LIBGIT2_BUILD_OPTIONS += -DCMAKE_INSTALL_PREFIX=${LIBGIT2_INSTALL_DIR}
+    LIBGIT2_BUILD_OPTIONS += -DCMAKE_INSTALL_LIBDIR=lib
+    LIBGIT2_BUILD_OPTIONS += -DENABLE_TRACE=OFF
+    LIBGIT2_BUILD_OPTIONS += -DUSE_SSH=OFF
+    LIBGIT2_BUILD_OPTIONS += -DUSE_HTTPS=OFF
+    LIBGIT2_BUILD_OPTIONS += -DUSE_NTLMCLIENT=OFF
+    LIBGIT2_BUILD_OPTIONS += -DUSE_BUNDLED_ZLIB=ON
+    LIBGIT2_BUILD_OPTIONS += -DUSE_HTTP_PARSER=builtin
+    LIBGIT2_BUILD_OPTIONS += -DREGEX_BACKEND=builtin
+endif
+
 # These variables control test options and artifacts
 TEST_OPTIONS    ?=
 TEST_REPORT_DIR ?= ${BUILD_DIR}/reports
@@ -110,9 +134,10 @@ find_go_sources  = $(shell find ${SOURCE_DIR} -type d \( -name ruby -o -name ven
 find_go_packages = $(dir $(call find_go_sources, 's|[^/]*\.go||'))
 
 unexport GOROOT
-export GOBIN        = ${BUILD_DIR}/bin
-export GOPROXY     ?= https://proxy.golang.org
-export PATH        := ${BUILD_DIR}/bin:${PATH}
+export GOBIN            = ${BUILD_DIR}/bin
+export GOPROXY         ?= https://proxy.golang.org
+export PATH            := ${BUILD_DIR}/bin:${PATH}
+export PKG_CONFIG_PATH := ${LIBGIT2_INSTALL_DIR}/lib/pkgconfig
 
 .NOTPARALLEL:
 
@@ -121,7 +146,7 @@ all: INSTALL_DEST_DIR = ${SOURCE_DIR}
 all: install
 
 .PHONY: build
-build: ${SOURCE_DIR}/.ruby-bundle
+build: ${SOURCE_DIR}/.ruby-bundle libgit2
 	go install ${GO_LDFLAGS} $(addprefix ${GITALY_PACKAGE}/cmd/, $(call find_commands))
 
 .PHONY: install
@@ -168,7 +193,7 @@ prepare-tests: ${GITLAB_SHELL_DIR}/config.yml ${TEST_REPO} ${TEST_REPO_GIT} ${SO
 test: test-go rspec rspec-gitlab-shell
 
 .PHONY: test-go
-test-go: prepare-tests ${GO_JUNIT_REPORT}
+test-go: prepare-tests ${GO_JUNIT_REPORT} libgit2
 	${Q}mkdir -p ${TEST_REPORT_DIR}
 	${Q}echo 0>${TEST_EXIT}
 	${Q}go test ${TEST_OPTIONS} -v -ldflags='${GO_TEST_LDFLAGS}' -count=1 $(call find_go_packages) 2>&1 | tee ${TEST_OUTPUT} || echo $$? >${TEST_EXIT}
@@ -207,7 +232,7 @@ check-mod-tidy:
 	${Q}${SOURCE_DIR}/_support/check-mod-tidy
 
 .PHONY: lint
-lint: ${GOLANGCI_LINT}
+lint: ${GOLANGCI_LINT} libgit2
 	${Q}${GOLANGCI_LINT} cache clean && ${GOLANGCI_LINT} run --out-format tab --config ${SOURCE_DIR}/.golangci.yml
 
 .PHONY: check-formatting
@@ -250,7 +275,7 @@ rubocop: ${SOURCE_DIR}/.ruby-bundle
 	${Q}cd ${GITALY_RUBY_DIR} && bundle exec rubocop --parallel
 
 .PHONY: cover
-cover: prepare-tests
+cover: prepare-tests libgit2
 	${Q}echo "NOTE: make cover does not exit 1 on failure, don't use it to check for tests success!"
 	${Q}mkdir -p "${COVERAGE_DIR}"
 	${Q}rm -f "${COVERAGE_DIR}/all.merged" "${COVERAGE_DIR}/all.html"
@@ -311,6 +336,9 @@ build-git:
 	${Q}mkdir -p ${GIT_INSTALL_DIR}
 	${MAKE} -C ${GIT_SOURCE_DIR} -j$(shell nproc) prefix=${GIT_PREFIX} ${GIT_BUILD_OPTIONS} install
 
+.PHONY: libgit2
+libgit2: ${LIBGIT2_INSTALL_DIR}/lib/libgit2.a
+
 # This file is used by Omnibus and CNG to skip the "bundle install"
 # step. Both Omnibus and CNG assume it is in the Gitaly root, not in
 # _build. Hence the '../' in front.
@@ -352,6 +380,13 @@ ${BUILD_DIR}/git_full_bins.tgz: | ${BUILD_DIR}
 	${Q}printf '${GIT_BINARIES_HASH}  $@.tmp' | shasum -a256 -c -
 	${Q}mv $@.tmp $@
 
+${LIBGIT2_INSTALL_DIR}/lib/libgit2.a:
+	${Q}rm -rf ${LIBGIT2_SOURCE_DIR}
+	git clone --depth 1 --branch ${LIBGIT2_VERSION} --quiet ${LIBGIT2_REPO_URL} ${LIBGIT2_SOURCE_DIR}
+	${Q}mkdir -p ${LIBGIT2_BUILD_DIR}
+	${Q}cd ${LIBGIT2_BUILD_DIR} && cmake ${LIBGIT2_SOURCE_DIR} ${LIBGIT2_BUILD_OPTIONS}
+	${Q}cmake --build ${LIBGIT2_BUILD_DIR} --target install
+
 ${GOIMPORTS}: ${BUILD_DIR}/go.mod | ${BUILD_DIR}/bin
 	${Q}cd ${BUILD_DIR} && go get golang.org/x/tools/cmd/goimports@2538eef75904eff384a2551359968e40c207d9d2
 
diff --git a/ctxt.BuildTags b/ctxt.BuildTags
index defa606e9..6deb1ea8a 100644
--- a/ctxt.BuildTags
+++ b/ctxt.BuildTags
@@ -1,3 +1,5 @@
 continuous_profiler_stackdriver
+static
+system_libgit2
 tracer_static
 tracer_static_jaeger
diff --git a/go.mod b/go.mod
index c9581053b..dfc6b0e6a 100644
--- a/go.mod
+++ b/go.mod
@@ -11,6 +11,7 @@ require (
 	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.0
 	github.com/kelseyhightower/envconfig v1.3.0
 	github.com/lib/pq v1.2.0
+	github.com/libgit2/git2go/v30 v30.0.5
 	github.com/olekukonko/tablewriter v0.0.2
 	github.com/otiai10/curr v1.0.0 // indirect
 	github.com/prometheus/client_golang v1.0.0
diff --git a/go.sum b/go.sum
index c91a01b46..5c96bf646 100644
--- a/go.sum
+++ b/go.sum
@@ -199,7 +199,10 @@ github.com/labstack/echo/v4 v4.1.11/go.mod h1:i541M3Fj6f76NZtHSj7TXnyM8n2gaodfvf
 github.com/labstack/gommon v0.3.0/go.mod h1:MULnywXg0yavhxWKc+lOruYdAhDwPK9wf0OL7NoOu+k=
 github.com/lib/pq v1.2.0 h1:LXpIM/LZ5xGFhOpXAQUIMM1HdyqzVYM13zNdjCEEcA0=
 github.com/lib/pq v1.2.0/go.mod h1:5WUZQaWbwv1U+lTReE5YruASi9Al49XbQIvNi/34Woo=
+github.com/libgit2/git2go v0.0.0-20190104134018-ecaeb7a21d47 h1:HDt7WT3kpXSHq4mlOuLzgXH9LeOK1qlhyFdKIAzxxeM=
 github.com/libgit2/git2go v0.0.0-20190104134018-ecaeb7a21d47/go.mod h1:4bKN42efkbNYMZlvDfxGDxzl066GhpvIircZDsm8Y+Y=
+github.com/libgit2/git2go/v30 v30.0.5 h1:gxKqXOslpvYDZNC62f8GV34TAk0qw4wZ++IdYw8V9I4=
+github.com/libgit2/git2go/v30 v30.0.5/go.mod h1:YReiQ7xhMoyAL4ISYFLZt+OGqn6xtLqvTC1xJ9oAH7Y=
 github.com/lightstep/lightstep-tracer-go v0.15.6 h1:D0GGa7afJ7GcQvu5as6ssLEEKYXvRgKI5d5cevtz8r4=
 github.com/lightstep/lightstep-tracer-go v0.15.6/go.mod h1:6AMpwZpsyCFwSovxzM78e+AsYxE8sGwiM6C3TytaWeI=
 github.com/magiconair/properties v1.8.0/go.mod h1:PppfXfuXeibc/6YijjN8zIbojt8czPbwD3XqdrwzmxQ=