Add workflow for checking for externally hosted images

.github/workflows/test-image-links.yml

@@ -0,0 +1,14 @@
+name: Test image links
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  test-summary:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+    - name: Run image link check
+      run: python tests/test_image_links.py

tests/test_image_links.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+
+# This script checks that no external images are linked.
+# Images should instead be self-hosted where possible.
+
+import re
+import sys
+from pathlib import Path
+from urllib.parse import urlparse
+
+REPO_ROOT = Path.cwd()
+
+# Folders to check
+DOCS_DIRS = [
+    REPO_ROOT / r"src/en",
+]
+
+# Configure allowed hosts (empty = only relative paths allowed)
+ALLOWED_HOSTS = [
+    # "example.com",
+]
+
+def main() -> int:
+	# regex explanation:
+	# links have the following format
+	# ![alt text](image-url)
+	# [^\]]* matches any number of characters that are not a ]
+	# ([^)])+ captures any text with at least one character until a ( appears
+
+	IMAGE_REGEX = re.compile(r'!\[[^\]]*\]\(([^)]+)\)')
+
+	docs = []
+
+	for docs_dir in DOCS_DIRS:
+		for md_file in docs_dir.rglob("*.md"):
+			docs.append(md_file)
+
+	print(f"found {len(docs)} .md files")
+
+	errors = []
+
+	for md_file in docs:
+		print(f"checking {md_file}")
+		content = md_file.read_text(encoding="utf-8")
+		for match in IMAGE_REGEX.findall(content):
+			url = match.strip() # remove whitespace
+
+			# Ignore anchors and mailto
+			if url.startswith("#") or url.startswith("mailto:"):
+				continue
+
+			parsed = urlparse(url)
+
+			# Relative paths are OK
+			if not parsed.scheme and not parsed.netloc:
+				continue
+
+			# Absolute URLs must be explicitly allowed
+			if parsed.netloc not in ALLOWED_HOSTS:
+				errors.append(f"{md_file}: found external image link: {url}")
+
+	if errors:
+		print("❌ External image links detected:\n")
+		print("\n".join(errors))
+		print("Make sure to self-host all images instead of using external hosts like imgur!")
+		print("Put them into the assets folder.")
+		return 1
+
+	print("✅ All image links are self-hosted.")
+	return 0
+
+exit(main())

tests/test_summary.py

@@ -1,5 +1,7 @@
 #!/usr/bin/env python3
 
+# This script checks if all documents in the repository are linked in the table of contents in SUMMARY.md so that they can be reached via the website.
+
 import re
 import sys
 from pathlib import Path