build: check docs for edits of autogenerated sections

This adds a lint step which checks the top commit for edits to
autogenerated doc sections.

.github/workflows/build.yml

@@ -226,6 +226,8 @@ jobs:
 
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+            fetch-depth: 0
 
       - name: Install Go
         id: setup-go
@@ -289,6 +291,10 @@ jobs:
       - name: Scan for vulnerabilities
         run: govulncheck ./...
 
+      - name: Scan edits of autogenerated files
+        run: bin/check_autogenerated_edits.py
+        if: github.event_name == 'pull_request'
+
   android:
     if: inputs.manual || (github.repository == 'rclone/rclone' && (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name))
     timeout-minutes: 30

bin/check_autogenerated_edits.py

@@ -0,0 +1,133 @@
+#!/usr/bin/env python3
+"""
+This script checks for unauthorized modifications in autogenerated sections of markdown files.
+It is designed to be used in a GitHub Actions workflow or a local pre-commit hook.
+
+Features:
+- Detects markdown files changed in the last commit.
+- Identifies modified autogenerated sections marked by specific comments.
+- Reports violations using GitHub Actions error messages.
+- Exits with a nonzero status code if unauthorized changes are found.
+
+It currently only checks the last commit.
+"""
+
+import re
+import subprocess
+import sys
+
+def run_git(args):
+    """
+    Run a Git command with the provided arguments and return its output as a string.
+    """
+    return subprocess.run(["git"] + args, stdout=subprocess.PIPE, text=True, check=True).stdout.strip()
+
+def get_changed_files():
+    """
+    Retrieve a list of markdown files that were changed in the last commit.
+    """
+    files = run_git(["diff", "--name-only", "HEAD~1", "HEAD"]).splitlines()
+    return [f for f in files if f.endswith(".md")]
+
+def get_diff(file):
+    """
+    Get the diff of a given file between the last commit and the current version.
+    """
+    return run_git(["diff", "-U0", "HEAD~1", "HEAD", "--", file]).splitlines()
+
+def get_file_content(ref, file):
+    """
+    Retrieve the content of a file from a given Git reference.
+    """
+    try:
+        return run_git(["show", f"{ref}:{file}"]).splitlines()
+    except Exception:
+        return []
+
+def find_regions(lines):
+    """
+    Identify the start and end line numbers of autogenerated regions in a file.
+    """
+    regions = []
+    start = None
+    for i, line in enumerate(lines, 1):
+        if "rem autogenerated options start" in line:
+            start = i
+        elif "rem autogenerated options stop" in line and start is not None:
+            regions.append((start, i))
+            start = None
+    return regions
+
+def in_region(ln, regions):
+    """
+    Check if a given line number falls within an autogenerated region.
+    """
+    return any(start <= ln <= end for start, end in regions)
+
+def show_error(file_name, line, message):
+    """
+    Print an error message in a GitHub Actions-compatible format.
+    """
+    print(f"::error file={file_name},line={line}::{message} at {file_name} line {line}")
+
+def check_file(file):
+    """
+    Check a markdown file for modifications in autogenerated regions.
+    """
+    viol = False
+    new_lines = get_file_content("HEAD", file)
+    old_lines = get_file_content("HEAD~1", file)
+    
+    # Entire autogenerated file check.
+    if any("autogenerated - DO NOT EDIT" in l for l in new_lines[:10]):
+        if get_diff(file):
+            show_error(file, 1, "Autogenerated file modified")
+            return True
+        return False
+    
+    # Partial autogenerated regions.
+    regions_new = find_regions(new_lines)
+    regions_old = find_regions(old_lines)
+    diff = get_diff(file)
+    hunk_re = re.compile(r"^@@ -(\d+),?(\d*) \+(\d+),?(\d*) @@")
+    new_ln = old_ln = None
+    
+    for line in diff:
+        if line.startswith("@@"):
+            m = hunk_re.match(line)
+            if m:
+                old_ln = int(m.group(1))
+                new_ln = int(m.group(3))
+        elif new_ln is None:
+            continue
+        elif line.startswith("+"):
+            if in_region(new_ln, regions_new):
+                show_error(file, new_ln, "Autogenerated region of file modified")
+                viol = True
+            new_ln += 1
+        elif line.startswith("-"):
+            if in_region(old_ln, regions_old):
+                show_error(file, old_ln, "Autogenerated region of file modified")
+                viol = True
+            old_ln += 1
+        else:
+            new_ln += 1
+            old_ln += 1
+    
+    return viol
+
+def main():
+    """
+    Main function that iterates over changed files and checks them for violations.
+    """
+    found = False
+    for f in get_changed_files():
+        if check_file(f):
+            found = True
+    if found:
+        sys.exit(1)
+    print("No unauthorized edits found in autogenerated sections.")
+    sys.exit(0)
+
+if __name__ == "__main__":
+    main()