PYTHON-5002 Add guard to synchro hook to accidental overwrite (#2026)

.pre-commit-config.yaml

@@ -24,6 +24,7 @@ repos:
       entry: bash ./tools/synchro.sh
       language: python
       require_serial: true
+      fail_fast: true
       additional_dependencies:
         - ruff==0.1.3
         - unasync

CONTRIBUTING.md

@@ -246,6 +246,18 @@ you are attempting to validate new spec tests in PyMongo.
 
 Follow the [Python Driver Release Process Wiki](https://wiki.corp.mongodb.com/display/DRIVERS/Python+Driver+Release+Process).
 
+## Asyncio considerations
+
+PyMongo adds asyncio capability by modifying the source files in `*/asynchronous` to `*/synchronous` using
+[unasync](https://github.com/python-trio/unasync/) and some custom transforms.
+
+Where possible, edit the code in `*/asynchronous/*.py` and not the synchronous files.
+You can run `pre-commit run --all-files synchro` before running tests if you are testing synchronous code.
+
+To prevent the `synchro` hook from accidentally overwriting code, it first checks to see whether a sync version
+of a file is changing and not its async counterpart, and will fail.
+In the unlikely scenario that you want to override this behavior, first export `OVERRIDE_SYNCHRO_CHECK=1`.
+
 ## Converting a test to async
 The `tools/convert_test_to_async.py` script takes in an existing synchronous test file and outputs a
 partially-converted asynchronous version of the same name to the `test/asynchronous` directory.

tools/synchro.py

@@ -19,7 +19,9 @@ Used as part of our build system to generate synchronous code.
 
 from __future__ import annotations
 
+import os
 import re
+import sys
 from os import listdir
 from pathlib import Path
 
@@ -356,6 +358,19 @@ def unasync_directory(files: list[str], src: str, dest: str, replacements: dict[
 
 
 def main() -> None:
+    modified_files = [f"./{f}" for f in sys.argv[1:]]
+    errored = False
+    for fname in async_files + gridfs_files:
+        # If the async file was modified, we don't need to check if the sync file was also modified.
+        if str(fname) in modified_files:
+            continue
+        sync_name = str(fname).replace("asynchronous", "synchronous")
+        if sync_name in modified_files and "OVERRIDE_SYNCHRO_CHECK" not in os.environ:
+            print(f"Refusing to overwrite {sync_name}")
+            errored = True
+    if errored:
+        raise ValueError("Aborting synchro due to errors")
+
     unasync_directory(async_files, _pymongo_base, _pymongo_dest_base, replacements)
     unasync_directory(gridfs_files, _gridfs_base, _gridfs_dest_base, replacements)
     unasync_directory(test_files, _test_base, _test_dest_base, replacements)

tools/synchro.sh

@@ -1,5 +1,7 @@
-#!/bin/bash -eu
+#!/bin/bash
 
-python ./tools/synchro.py
+set -eu
+
+python ./tools/synchro.py "$@"
 python -m ruff check pymongo/synchronous/ gridfs/synchronous/ test/ --fix --silent
 python -m ruff format pymongo/synchronous/ gridfs/synchronous/ test/ --silent