ci: Add a style check stage

Run clang-format-diff on the C sources, and fail if the coding style
isn't respected.

3 files changed, 68 insertions, 0 deletions

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index f0326ab..23a8e92 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,11 +1,26 @@
 variables:
 
 stages:
+  - style-check
   - build
   - analysis
   - docs
   - deploy
 
+.only-default:
+  only:
+    - branches
+  except:
+    - tags
+
+style-check-diff:
+  extends: .only-default
+  image: fedora:latest
+  stage: style-check
+  script:
+    - dnf install -y clang-tools-extra diffutils git
+    - sh -x ./.gitlab-ci/run-style-check.sh
+
 # Recipe for setting up the build
 # @PROJECT_DEPS: the dependencies of the project (on Fedora)
 # @MESON_VERSION: the version of Meson required by the project
diff --git a/.gitlab-ci/run-style-check.sh b/.gitlab-ci/run-style-check.sh
new file mode 100755
index 0000000..07b1242
--- /dev/null
+++ b/.gitlab-ci/run-style-check.sh
@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -e
+
+ancestor_horizon=28  # days (4 weeks)
+
+# Wrap everything in a subshell so we can propagate the exit status.
+(
+
+source .gitlab-ci/search-common-ancestor.sh
+
+git diff -U0 --no-color "${newest_common_ancestor_sha}" libgweather/*.c libgweather/tests/*.c libgweather/tools/*.c | clang-format-diff -p1
+
+)
+exit_status=$?
+
+exit ${exit_status}
diff --git a/.gitlab-ci/search-common-ancestor.sh b/.gitlab-ci/search-common-ancestor.sh
new file mode 100755
index 0000000..bac99d2
--- /dev/null
+++ b/.gitlab-ci/search-common-ancestor.sh
@@ -0,0 +1,36 @@
+#!/bin/bash
+
+set -e
+
+ancestor_horizon=28  # days (4 weeks)
+
+# We need to add a new remote for the upstream target branch, since this script
+# could be running in a personal fork of the repository which has out of date
+# branches.
+#
+# Limit the fetch to a certain date horizon to limit the amount of data we get.
+# If the branch was forked from origin/main before this horizon, it should
+# probably be rebased.
+if ! git ls-remote --exit-code upstream >/dev/null 2>&1 ; then
+    git remote add upstream https://gitlab.gnome.org/GNOME/${CI_PROJECT_NAME}.git
+fi
+git fetch --shallow-since="$(date --date="${ancestor_horizon} days ago" +%Y-%m-%d)" upstream
+
+# Work out the newest common ancestor between the detached HEAD that this CI job
+# has checked out, and the upstream target branch (which will typically be
+# `upstream/main` or `upstream/gnome-40`).
+# `${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}` or `${CI_MERGE_REQUEST_SOURCE_BRANCH_NAME}`
+# are only defined if we’re running in a merge request pipeline,
+# fall back to `${CI_DEFAULT_BRANCH}` or `${CI_COMMIT_BRANCH}` respectively
+# otherwise.
+
+source_branch="${CI_MERGE_REQUEST_SOURCE_BRANCH_NAME:-${CI_COMMIT_BRANCH}}"
+git fetch --shallow-since="$(date --date="${ancestor_horizon} days ago" +%Y-%m-%d)" origin "${source_branch}"
+
+newest_common_ancestor_sha=$(diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "upstream/${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_DEFAULT_BRANCH}}") <(git rev-list --first-parent "origin/${source_branch}") | head -1)
+if [ -z "${newest_common_ancestor_sha}" ]; then
+    echo "Couldn’t find common ancestor with upstream main branch. This typically"
+    echo "happens if you branched from main a long time ago. Please update"
+    echo "your clone, rebase, and re-push your branch."
+    exit 1
+fi