Add script to check that deleted slides have redirects (#3174)

Fixes #1417.

This adds a script and a CI job to ensure that slides that are deleted
have a matching redirect entry in `bool.toml`.

If a redirect entry is not found (for example, for
`src/hello-world.md`), the following is printed before exiting with a
status of 1:

```
The following deleted files have missing redirect entries in book.toml:                                                                                                       
src/hello-world.md
```

Co-authored-by: Brian Daniels <briandaniels@google.com>

Author: bridadan

.github/workflows/check-redirects.py

@@ -0,0 +1,121 @@
+# Copyright 2026 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Ensure all deleted files have a redirect entry.
+
+First, the code finds all deleted ".md" files under the "src/" directory between
+the current working copy and the specificed commit/branch.
+
+Next, for each deleted ".md" file it searches for a corresponding entry in
+"book.toml" under the "[output.html.redirect]" section. The ".md" extension
+should be swapped with a ".html" extension and a "src/" prefix add to it.
+Any deleted ".md" file that is not found is printed to stdout.
+
+If any deleted ".md" files are missing a redirect entry, the script exits with
+a value of 1. Otherwise, it exits with a value of 0.
+
+Usage:
+check-redirects.py <root directory of the repository> <commit to compare (e.g. origin/main)>
+"""
+
+import os
+import subprocess
+import sys
+
+
+def main():
+    if len(sys.argv) != 3:
+        print(
+            "Usage: check-redirects.py <root directory of the repository> <commit to compare (e.g. origin/main)>"
+        )
+        sys.exit(1)
+
+    repo_root = sys.argv[1]
+    commit = sys.argv[2]
+
+    if not os.path.isdir(repo_root):
+        print(f"Error: {repo_root} is not a directory")
+        sys.exit(1)
+
+    # Change to repo root to run git commands and find book.toml
+    os.chdir(repo_root)
+
+    # Get deleted .md files under src/
+    try:
+        diff_output = subprocess.check_output(
+            ["git", "diff", commit, "--diff-filter=D", "--name-only"],
+            text=True)
+    except subprocess.CalledProcessError as e:
+        print(f"Error running git diff: {e}")
+        sys.exit(1)
+
+    deleted_files = [
+        f for f in diff_output.splitlines()
+        if f.startswith("src/") and f.endswith(".md")
+    ]
+
+    if not deleted_files:
+        sys.exit(0)
+
+    # Read book.toml and extract redirects
+    book_toml_path = "book.toml"
+    if not os.path.exists(book_toml_path):
+        print(f"Error: {book_toml_path} not found in {repo_root}")
+        sys.exit(1)
+
+    with open(book_toml_path, "r") as f:
+        lines = f.readlines()
+
+    redirects = set()
+    in_redirect_section = False
+    for line in lines:
+        line = line.strip()
+        if not line or line.startswith("#"):
+            continue
+
+        if line.startswith("[") and line.endswith("]"):
+            if line == "[output.html.redirect]":
+                in_redirect_section = True
+            else:
+                in_redirect_section = False
+            continue
+
+        if in_redirect_section:
+            # Entry looks like "old.html" = "new.html" or "old.html" = "/new"
+            if "=" in line:
+                key = line.split("=")[0].strip().strip("\"'")
+                redirects.add(key)
+
+    missing_redirects = []
+    for md_file in deleted_files:
+        # Swap .md with .html and remove src/ prefix if it's relative to site root
+        # Usually redirects in book.toml for mdbook are relative to the output root.
+        # If the file is src/foo/bar.md, it becomes foo/bar.html in the output.
+        html_file = md_file.replace("src/", "", 1).replace(".md", ".html")
+        if html_file not in redirects:
+            missing_redirects.append(md_file)
+
+    if missing_redirects:
+        print(
+            "The following deleted files are missing a redirect entry in book.toml:"
+        )
+        for f in missing_redirects:
+            print(f)
+        sys.exit(1)
+
+    sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()

.github/workflows/check-redirects.yml

@@ -0,0 +1,18 @@
+name: Check that redirects are valid
+permissions:
+  contents: read
+
+on:
+  pull_request:
+
+jobs:
+  check-redirects:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Check redirects
+        run: python3 .github/workflows/check-redirects.py . origin/main