Use htmltest to check links in the Synapse documentation.

.github/workflows/docs-pr.yaml

@@ -4,6 +4,8 @@ on:
   pull_request:
     paths:
       - docs/**
+      - book.toml
+      - .github/workflows/docs-pr.yaml
 
 jobs:
   pages:
@@ -32,3 +34,27 @@ jobs:
           path: book
           # We'll only use this in a workflow_run, then we're done with it
           retention-days: 1
+
+  link-check:
+    name: Check links in documentation
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup mdbook
+        uses: peaceiris/actions-mdbook@adeb05db28a0c0004681db83893d56c0388ea9ea # v1.2.0
+        with:
+          mdbook-version: '0.4.17'
+
+      - name: Setup htmltest
+        run: |
+          wget https://github.com/wjdp/htmltest/releases/download/v0.17.0/htmltest_0.17.0_linux_amd64.tar.gz
+          echo '775c597ee74899d6002cd2d93076f897f4ba68686bceabe2e5d72e84c57bc0fb  htmltest_0.17.0_linux_amd64.tar.gz' | sha256sum -c
+          tar zxf htmltest_0.17.0_linux_amd64.tar.gz
+
+      - name: Test links with htmltest
+        # Build the book with `./` as the site URL (to make checks on 404.html possible)
+        # Then run htmltest (without checking external links since that involves the network and is slow).
+        run: |
+          MDBOOK_OUTPUT__HTML__SITE_URL="./" mdbook build
+          ./htmltest book --skip-external

changelog.d/14743.misc

@@ -0,0 +1 @@
+Use `htmltest` to check links in the Synapse documentation.