gitlab: Add job to warn about "dead" branches

It is quite common for we developers (me included) to forget to
delete already merged branches. Let's fix this mess with a job.

.gitlab-ci.yml

@@ -695,6 +695,19 @@ clang-format:
       - fetch_origin.log
     expire_in: 2 days
 
+branches-check:
+  extends: .default
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "push" && $CI_OPEN_MERGE_REQUESTS == null && $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH'
+  stage: analysis
+  variables:
+    GIT_DEPTH: "0"
+  script:
+    - apt-get update -qq
+    - apt-get install -qq -y --no-install-recommends git
+    - sh .gitlab/check_dead_branches.sh
+  allow_failure: true
+
 cppcheck:
   extends: .default
   rules:

.gitlab/check_dead_branches.sh

@@ -0,0 +1,40 @@
+#!/bin/sh
+
+printf "\e[0Ksection_start:`date +%s`:branch_check[collapsed=false]\r\e[0KChecking for dead branches\n"
+git branch -r | grep -v 'origin/HEAD' | grep -v "origin/$CI_COMMIT_SHA" | while IFS= read remote_branch; do
+  remote_branch=$(printf "%s\n" "$remote_branch" | sed 's/^ *//;s/ *$//')
+  branch_name=$(printf "%s\n" "$remote_branch" | sed 's|origin/||')
+  
+  # NOT CHECKING
+  ## Skip old stable branches
+  if echo "$branch_name" | grep -q "^gimp-[0-9]-" || [ "$branch_name" = "gnome-2-2" ] || [ "$branch_name" = "gnome-2-4" ]; then
+    printf "\033[33m(SKIP)\033[0m: $branch_name is a snapshot of $CI_DEFAULT_BRANCH but no problem\n"
+    continue
+  fi
+  ## Skip recently created branches
+  if [ "$(git rev-parse "$remote_branch")" = "$(git rev-parse "$CI_COMMIT_SHA")" ]; then
+    printf "\033[33m(SKIP)\033[0m: $branch_name is identical to $CI_DEFAULT_BRANCH but no problem\n"
+    continue
+  fi
+
+  # CHECKING
+  ## Check: merge-base
+  if git merge-base --is-ancestor "$remote_branch" "$CI_COMMIT_SHA"; then
+    printf "\033[31m(ERROR)\033[0m: $branch_name is fully merged into $CI_DEFAULT_BRANCH (via git merge-base)\n"
+    touch 'dead_branch'
+    continue
+  fi
+  ## Fallback check: cherry
+  cherry_output=$(git cherry "$CI_COMMIT_SHA" "$remote_branch")
+  if [ -z "$(printf "%s\n" "$cherry_output" | grep '^+')" ]; then
+    printf "\033[31m(ERROR)\033[0m: $branch_name is fully merged into $CI_DEFAULT_BRANCH (via git cherry)\n"
+    touch 'dead_branch'
+    continue
+  fi
+done
+
+if [ -f "dead_branch" ]; then
+  printf "         Please delete the merged branches\n"
+  exit 1
+fi
+printf "\e[0Ksection_end:`date +%s`:branch_check\r\e[0K\n"