gitlab-ci.yml: Add check for issue or MR URL

This adds a pipeline stage for merge requests that checks that the
commit message contains an URL to either a issue or a merge request.
This means that for merge requests without corresponding issues will
always fail initially, as the merge request URL is not known until after
it is created. This is still arguably better than accidentally merging
merge requests without URLs.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/440

.gitlab-ci.yml

@@ -1,9 +1,17 @@
 image: registry.gitlab.gnome.org/gnome/mutter/master:v1
 
 stages:
+ - review
  - build
  - test
 
+check-commit-log:
+  stage: review
+  script:
+    - ./.gitlab-ci/check-commit-log.sh
+  only:
+    - merge_requests
+
 build-mutter:
   stage: build
   script:
@@ -14,6 +22,9 @@ build-mutter:
     expire_in: 1 day
     paths:
       - build
+  only:
+    - merge_requests
+    - /^.*$/
 
 test-mutter:
   stage: test
@@ -28,3 +39,6 @@ test-mutter:
     - >
       dbus-run-session -- xvfb-run -s '+iglx -noreset'
       meson test -C build --no-rebuild -t 10 --verbose --no-stdsplit --wrap catchsegv
+  only:
+    - merge_requests
+    - /^.*$/

.gitlab-ci/check-commit-log.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+if [ -z "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
+  echo Cannot review non-merge request
+  exit 1
+fi
+
+git fetch $CI_MERGE_REQUEST_PROJECT_URL.git $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+
+branch_point=$(git merge-base HEAD FETCH_HEAD)
+
+commits=$(git log --format='format:%H' $branch_point..$CI_COMMIT_SHA)
+
+if [ -z "$commits" ]; then
+  echo Commit range empty
+  exit 1
+fi
+
+function commit_message_has_url() {
+  commit=$1
+  commit_message=$(git show -s --format='format:%b' $commit)
+  echo "$commit_message" | grep -qe "\($CI_MERGE_REQUEST_PROJECT_URL/\(issues\|merge_requests\)/[0-9]\+\|https://bugzilla.gnome.org/show_bug.cgi?id=[0-9]\+\)"
+  return $?
+}
+
+for commit in $commits; do
+  if ! commit_message_has_url $commit; then
+    echo "Missing merge request or issue URL on commit $(echo $commit | cut -c -8)"
+    exit 1
+  fi
+done