ci: Add a job to verify commit messages on MRs

This uses [gitlint] and is based on their linting rules with some minor
config so it matches our style pretty well.

[gitlint]: https://jorisroovers.com/gitlint/#configuration

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3980>

.gitlab-ci.yml

@@ -34,6 +34,7 @@ variables:
 
   FEDORA_AMD64_SUFFIX: 'amd64/fedora'
   INDENT_AMD64_SUFFIX: 'amd64/gst-indent'
+  COMMITLINT_AMD64_SUFFIX: 'amd64/commitlint'
   WINDOWS_AMD64_SUFFIX: 'amd64/windows'
 
   WINDOWS_IMAGE: "$CI_REGISTRY_IMAGE/$WINDOWS_AMD64_SUFFIX:$WINDOWS_TAG-$GST_UPSTREAM_BRANCH"
@@ -154,6 +155,22 @@ gst-indent amd64 docker:
   # Do not depend on the trigger, as we want to run indent always
   needs: []
 
+.commitlint image:
+  variables:
+    FDO_DISTRIBUTION_VERSION: '37'
+    FDO_REPO_SUFFIX: "$COMMITLINT_AMD64_SUFFIX"
+    FDO_DISTRIBUTION_TAG: "$LINT_TAG-$GST_UPSTREAM_BRANCH"
+    FDO_DISTRIBUTION_PACKAGES: 'python3-pip git'
+    FDO_DISTRIBUTION_EXEC: 'ci/docker/lint/prepare.sh'
+
+commitlint docker:
+  extends:
+    - '.commitlint image'
+    - '.fdo.container-build@fedora'
+  stage: 'preparation'
+  # Do not depend on the trigger, as we want to run indent always
+  needs: []
+
 windows amd64 docker:
   stage: "preparation"
   needs:
@@ -205,6 +222,25 @@ gst indent:
     - if: '$CI_PROJECT_NAMESPACE != "gstreamer" || $CI_COMMIT_BRANCH != $GST_UPSTREAM_BRANCH'
       when: 'always'
 
+commitlint:
+  extends:
+    - '.commitlint image'
+    - '.fdo.suffixed-image@fedora'
+  stage: 'preparation'
+  variables:
+    GIT_STRATEGY: fetch
+    GIT_DEPTH: 100
+  needs:
+    - job: 'commitlint docker'
+      artifacts: false
+  script:
+    - echo $CI_MERGE_REQUEST_DIFF_BASE_SHA
+    - gitlint --commits $CI_MERGE_REQUEST_DIFF_BASE_SHA..HEAD
+  rules:
+    # Only run on MR as we need CI_MERGE_REQUEST_DIFF_BASE_SHA to know
+    # what commits to check
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+
 #
 # build setup templates
 #

.gitlab-image-tags.yml

@@ -7,4 +7,5 @@ variables:
   ###
   FEDORA_TAG:  '2022-12-10.0'
   INDENT_TAG: '2022-03-07.1'
+  LINT_TAG: '2023-02-16.5'
   WINDOWS_TAG: '2023-01-26.0'

.gitlint

@@ -0,0 +1,12 @@
+[title-max-length]
+line-length=100
+
+[general]
+ignore=body-is-missing
+
+[body-max-line-length]
+line-length=120
+
+# Ensure every title starts with a prefix
+[title-match-regex]
+regex=^\w+:.*

ci/docker/lint/prepare.sh

@@ -0,0 +1,4 @@
+set -eux
+
+python3 -m pip install gitlint
+

meson.build

@@ -33,6 +33,14 @@ endif
 # Install gst-indent pre-commit hook
 run_command(python3, '-c', 'import shutil; shutil.copy("scripts/git-hooks/multi-pre-commit.hook", ".git/hooks/pre-commit")', check: false)
 
+
+gitlint = find_program('gitlint', required: false)
+if gitlint.found()
+  run_command(gitlint, 'install-hook', check: false)
+else
+  message('gitlint not found, please install it with your package manager or `python3 -m pip install gitlint` to enable the commit message hook')
+endif
+
 # On macOS, you have to run "Install Certificates.command" otherwise Python
 # doesn't have access to the latest SSL CA Certificates, and Meson will fail to
 # download wrap files from websites that use, for example, Let's Encrypt.