Makefile: Build Git with extra GitLab patch level version

With Gitaly now being the single source of truth for the Git version
deployed in production, we have started to make a lot more use of
applying patches on top of our self-built Git version. The policy is and
will remain to only ever apply patches which have been accepted
upstream, but it is sensible to allow for backporting patches such that
we can make use of them faster and test whether they have the expected
benefits.

This has uncovered a problem though: it's impossible for us to detect
which patches have been applied on top. This makes it hard for admins to
see whether they are running a custom version of Git or the mainline
version, and it is a problem for Gitaly because we cannot detect what
the current version supports in case we backport important fixes which
require changes both in Git and Gitaly.

Introduce a new Git extra version into our build process. This extra
version is a simple counter "gl${PATCHLEVEL}", where each newly added
set of Git patches must from now on increment the extra version. With
this version in place, we can fix both of above usecases.

When setting the Git version, we need to be careful to not break our
Git version parsing. But luckily, the current code is very lenient with
regards to parsing the fourth tuple: we only recognize the "rc" prefix,
but ignore everything else. Adding the GitLab patch level as this fourth
tuple is thus backwards compatible with older Gitaly versions, which
will simply ignore it. The end result is thus:

    $ git --version
    git version 2.33.0.gl1

It still needs an admin to correlate the set of patches with the patch
level, but this should be an easy enough task and is easier to maintain
on our side compared to somehow encoded "real" capabilities into the
version string.

Changelog: added

1 files changed, 17 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 54154c18b..dd0eebdc8 100644
--- a/Makefile
+++ b/Makefile
@@ -120,6 +120,14 @@ ifndef GIT_PATCHES
     GIT_PATCHES += 0004-revision-stop-retrieving-reference-twice.patch
     GIT_PATCHES += 0005-commit-graph-split-out-function-to-search-commit-pos.patch
     GIT_PATCHES += 0006-revision-avoid-hitting-packfiles-when-commits-are-in.patch
+
+    # This extra version has two intentions: first, it allows us to detect
+    # capabilities of the command at runtime. Second, it helps admins to
+    # discover which version is currently in use. As such, this version must be
+    # incremented whenever a new patch is added above. When no patches exist,
+    # then this should be undefined. Otherwise, it must be set to at least
+    # `gl1` given that `0` is the "default" GitLab patch level.
+    GIT_EXTRA_VERSION = gl1
 endif
 
 ifndef GIT_BUILD_OPTIONS
@@ -465,7 +473,7 @@ ${DEPENDENCY_DIR}: | ${BUILD_DIR}
 ${DEPENDENCY_DIR}/libgit2.version: dependency-version | ${DEPENDENCY_DIR}
 	${Q}[ x"$$(cat "$@" 2>/dev/null)" = x"${LIBGIT2_VERSION} ${LIBGIT2_BUILD_OPTIONS}" ] || >$@ echo -n "${LIBGIT2_VERSION} ${LIBGIT2_BUILD_OPTIONS}"
 ${DEPENDENCY_DIR}/git.version: dependency-version | ${DEPENDENCY_DIR}
-	${Q}[ x"$$(cat "$@" 2>/dev/null)" = x"${GIT_VERSION} ${GIT_BUILD_OPTIONS} ${GIT_PATCHES}" ] || >$@ echo -n "${GIT_VERSION} ${GIT_BUILD_OPTIONS} ${GIT_PATCHES}"
+	${Q}[ x"$$(cat "$@" 2>/dev/null)" = x"${GIT_VERSION}.${GIT_EXTRA_VERSION} ${GIT_BUILD_OPTIONS} ${GIT_PATCHES}" ] || >$@ echo -n "${GIT_VERSION}.${GIT_EXTRA_VERSION} ${GIT_BUILD_OPTIONS} ${GIT_PATCHES}"
 ${TOOLS_DIR}/%.version: dependency-version | ${TOOLS_DIR}
 	${Q}[ x"$$(cat "$@" 2>/dev/null)" = x"${TOOL_VERSION}" ] || >$@ echo -n "${TOOL_VERSION}"
 
@@ -491,6 +499,14 @@ ${GIT_INSTALL_DIR}/bin/git: ${DEPENDENCY_DIR}/git.version
 ifneq (${GIT_PATCHES},)
 	${Q}${GIT} -C "${GIT_SOURCE_DIR}" apply $(addprefix "${SOURCE_DIR}"/_support/git-patches/,${GIT_PATCHES})
 endif
+	# We're writing the version into the "version" file in Git's own source
+	# directory. If it exists, Git's Makefile will pick it up and use it as
+	# the version instead of auto-detecting via git-describe(1).
+ifneq (${GIT_EXTRA_VERSION},0)
+	${Q}echo ${GIT_VERSION}.${GIT_EXTRA_VERSION} >"${GIT_SOURCE_DIR}"/version
+else
+	${Q}rm -f "${GIT_SOURCE_DIR}"/version
+endif
 	${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