Graceful shutdown in case of cancellation

black.py

@@ -5,9 +5,11 @@
 from concurrent.futures import Executor, ProcessPoolExecutor
 from functools import partial, wraps
 import keyword
+import logging
 import os
 from pathlib import Path
 import tokenize
+import signal
 import sys
 from typing import (
     Callable,
@@ -167,7 +169,7 @@ def main(
                 )
             )
         finally:
-            loop.close()
+            shutdown(loop)
             ctx.exit(return_code)
 
 
@@ -192,6 +194,9 @@ async def schedule_formatting(
         )
         for src in sources
     }
+    _task_values = list(tasks.values())
+    loop.add_signal_handler(signal.SIGINT, cancel, _task_values)
+    loop.add_signal_handler(signal.SIGTERM, cancel, _task_values)
     await asyncio.wait(tasks.values())
     cancelled = []
     report = Report(check=not write_back)
@@ -200,13 +205,16 @@ async def schedule_formatting(
             report.failed(src, 'timed out, cancelling')
             task.cancel()
             cancelled.append(task)
+        elif task.cancelled():
+            cancelled.append(task)
         elif task.exception():
             report.failed(src, str(task.exception()))
         else:
             report.done(src, task.result())
     if cancelled:
-        await asyncio.wait(cancelled, timeout=2)
-    out('All done! ✨ 🍰 ✨')
+        await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
+    else:
+        out('All done! ✨ 🍰 ✨')
     click.echo(str(report))
     return report.return_code
 
@@ -1986,5 +1994,34 @@ def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     )
 
 
+def cancel(tasks: List[asyncio.Task]) -> None:
+    """asyncio signal handler that cancels all `tasks` and reports to stderr."""
+    err("Aborted!")
+    for task in tasks:
+        task.cancel()
+
+
+def shutdown(loop: BaseEventLoop) -> None:
+    """Cancel all pending tasks on `loop`, wait for them, and close the loop."""
+    try:
+        # This part is borrowed from asyncio/runners.py in Python 3.7b2.
+        to_cancel = [task for task in asyncio.Task.all_tasks(loop) if not task.done()]
+        if not to_cancel:
+            return
+
+        for task in to_cancel:
+            task.cancel()
+        loop.run_until_complete(
+            asyncio.gather(*to_cancel, loop=loop, return_exceptions=True)
+        )
+    finally:
+        # `concurrent.futures.Future` objects cannot be cancelled once they
+        # are already running. There might be some when the `shutdown()` happened.
+        # Silence their logger's spew about the event loop being closed.
+        cf_logger = logging.getLogger("concurrent.futures")
+        cf_logger.setLevel(logging.CRITICAL)
+        loop.close()
+
+
 if __name__ == '__main__':
     main()