Add a multithreaded stress test (#199)

* Add a multithreaded stress test

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* add noqa

* add a docstring for the new test

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_password_hasher.py

@@ -1,5 +1,10 @@
 # SPDX-License-Identifier: MIT
 
+import secrets
+import sys
+import threading
+
+from concurrent.futures import ThreadPoolExecutor
 from unittest import mock
 
 import pytest
@@ -194,3 +199,38 @@ class TestPasswordHasher:
             hash = ph.hash("hello")
 
             assert ph.verify(hash, "hello") is True
+
+
+def test_multithreaded_hashing():
+    """
+    Hash passwords in a thread pool and check for thread safety
+    """
+    hasher = PasswordHasher(parallelism=2)
+
+    num_passwords = 100
+
+    passwords = [secrets.token_urlsafe(15) for _ in range(num_passwords)]
+
+    def closure(b, passwords):
+        b.wait()
+        for password in passwords:
+            assert hasher.verify(hasher.hash(password), password)
+
+    max_workers = 4
+
+    chunks = [passwords[i::max_workers] for i in range(max_workers)]
+    orig_interval = sys.getswitchinterval()
+
+    with ThreadPoolExecutor(max_workers=max_workers) as tpe:
+        barrier = threading.Barrier(max_workers)
+        futures = []
+        try:
+            sys.setswitchinterval(0.00001)
+            for chunk in chunks:
+                futures.append(tpe.submit(closure, barrier, chunk))  # noqa: PERF401
+        finally:
+            sys.setswitchinterval(orig_interval)
+            if len(futures) < max_workers:
+                barrier.abort()
+        for f in futures:
+            f.result()