regr_test.py: improve several concurrency details (#11984)

tests/regr_test.py

@@ -13,8 +13,8 @@ import subprocess
 import sys
 import tempfile
 import threading
-from collections.abc import Callable
-from contextlib import ExitStack, suppress
+from collections.abc import Callable, Iterator
+from contextlib import ExitStack, contextmanager, suppress
 from dataclasses import dataclass
 from enum import IntEnum
 from functools import partial
@@ -145,8 +145,12 @@ def setup_testcase_dir(package: DistributionTests, tempdir: Path, verbosity: Ver
     if requirements.external_pkgs:
         venv_location = str(tempdir / VENV_DIR)
         subprocess.run(["uv", "venv", venv_location], check=True, capture_output=True)
-        # Use --no-cache-dir to avoid issues with concurrent read/writes to the cache
-        uv_command = ["uv", "pip", "install", get_mypy_req(), *requirements.external_pkgs, "--no-cache-dir"]
+        uv_command = ["uv", "pip", "install", get_mypy_req(), *requirements.external_pkgs]
+        if sys.platform == "win32":
+            # Reads/writes to the cache are threadsafe with uv generally...
+            # but not on old Windows versions
+            # https://github.com/astral-sh/uv/issues/2810
+            uv_command.append("--no-cache-dir")
         if verbosity is Verbosity.VERBOSE:
             verbose_log(f"{package.name}: Setting up venv in {venv_location}. {uv_command=}\n")
         try:
@@ -309,6 +313,19 @@ def concurrently_run_testcases(
     if not to_do:
         return []
 
+    @contextmanager
+    def cleanup_threads(
+        event: threading.Event, printer_thread: threading.Thread, executor: concurrent.futures.ThreadPoolExecutor
+    ) -> Iterator[None]:
+        try:
+            yield
+        except:
+            _PRINT_QUEUE.put("Shutting down worker threads...")
+            event.set()
+            printer_thread.join()
+            executor.shutdown(cancel_futures=True)
+            raise
+
     event = threading.Event()
     printer_thread = threading.Thread(target=print_queued_messages, args=(event,))
     printer_thread.start()
@@ -321,10 +338,14 @@ def concurrently_run_testcases(
             executor.submit(setup_testcase_dir, package, tempdir, verbosity)
             for package, tempdir in packageinfo_to_tempdir.items()
         ]
-        concurrent.futures.wait(testcase_futures)
+
+        with cleanup_threads(event, printer_thread, executor):
+            concurrent.futures.wait(testcase_futures)
 
         mypy_futures = [executor.submit(task) for task in to_do]
-        results = [future.result() for future in mypy_futures]
+
+        with cleanup_threads(event, printer_thread, executor):
+            results = [future.result() for future in mypy_futures]
 
     event.set()
     printer_thread.join()