ci: test downstream libs against this version of nokogiri

- loofah
- rails-html-sanitizer
- sanitize
- signer
- Viewpoint
- ActiveSupport::XmlMini

See #2293

.github/workflows/downstream.yml

@@ -0,0 +1,68 @@
+name: downstream
+concurrency:
+  group: "${{github.workflow}}-${{github.ref}}"
+  cancel-in-progress: true
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 8 * * 1,3,5" # At 08:00 on Monday, Wednesday, and Friday # https://crontab.guru/#0_8_*_*_1,3,5
+  push:
+    branches:
+      - main
+      - v*.*.x
+    tags:
+      - v*.*.*
+  pull_request:
+    types: [opened, synchronize]
+    branches:
+      - '*'
+
+jobs:
+  downstream:
+    name: downstream-${{matrix.name}}
+    needs: ["basic"]
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - url: https://github.com/flavorjones/loofah
+            name: loofah
+            command: "bundle exec rake test"
+          - url: https://github.com/rails/rails-html-sanitizer
+            name: rails-html-sanitizer
+            command: "bundle exec rake test"
+          - url: https://github.com/rgrove/sanitize
+            name: sanitize
+            command: "bundle exec rake test"
+          - url: https://github.com/ebeigarts/signer
+            name: signer
+            command: "bundle exec rake spec"
+          - url: https://github.com/WinRb/Viewpoint
+            name: viewpoint
+            command: "bundle exec rspec spec"
+          - url: https://github.com/rails/rails
+            name: xmlmini
+            command: "cd activesupport && bundle exec rake test TESTOPTS=-n/XmlMini/"
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/sparklemotion/nokogiri-test:mri-3.1
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+      - uses: actions/cache@v2
+        with:
+          path: ports
+          key: ports-ubuntu-${{hashFiles('dependencies.yml', 'patches/**/*.patch')}}
+      - run: bundle install --local || bundle install
+      - run: bundle exec rake compile
+      - run: |
+          git clone --depth=1 ${{matrix.url}} ${{matrix.name}}
+          cd ${{matrix.name}}
+          if grep nokogiri Gemfile ; then
+            sed -i 's/\(.*nokogiri.*\)/\1, path: ".."/' Gemfile
+          else
+            echo "gem 'nokogiri', path: '..'" >> Gemfile
+          fi
+          bundle install --local || bundle install
+          ${{matrix.command}}