tests: Clean hashutil._blake2_hash_cache after mocking blake2 functions.

Depending on the order in which tests are run, these tests may insert
lambdas with mocked blake2 functions in their closure to be inserted in
hashutil._blake2_hash_cache; causing all future tests to fail.

While this does not happen with the default order of tests, it does when
using pytest-xdist.

swh/model/tests/test_hashutil.py

-# Copyright (C) 2015-2018  The Software Heritage developers
+# Copyright (C) 2015-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import contextlib
 import hashlib
 import io
 import os
@@ -14,6 +15,17 @@ from swh.model import hashutil
 from swh.model.hashutil import MultiHash
 
 
+@contextlib.contextmanager
+def patch_blake2(function_name):
+    try:
+        with patch(function_name) as mock:
+            yield mock
+    finally:
+        # mocking blake2 inserts mock objects in the cache; we need
+        # to clean it before the next test runs
+        hashutil._blake2_hash_cache.clear()
+
+
 class BaseHashutil(unittest.TestCase):
     def setUp(self):
         # Reset function cache
@@ -195,7 +207,7 @@ class Hashutil(BaseHashutil):
             if "blake2b" not in hashlib.algorithms_available:
                 self.skipTest("blake2b not built in")
 
-            with patch("hashlib.blake2b") as mock_blake2b:
+            with patch_blake2("hashlib.blake2b") as mock_blake2b:
                 mock_blake2b.return_value = sentinel = object()
 
                 h = hashutil._new_hash("blake2b512")
@@ -216,7 +228,7 @@ class Hashutil(BaseHashutil):
             if "blake2s" not in hashlib.algorithms_available:
                 self.skipTest("blake2s not built in")
 
-            with patch("hashlib.blake2s") as mock_blake2s:
+            with patch_blake2("hashlib.blake2s") as mock_blake2s:
                 mock_blake2s.return_value = sentinel = object()
 
                 h = hashutil._new_hash("blake2s256")
@@ -233,7 +245,7 @@ class Hashutil(BaseHashutil):
         if "blake2b" in hashlib.algorithms_available:
             self.skipTest("blake2b built in")
 
-        with patch("pyblake2.blake2b") as mock_blake2b:
+        with patch_blake2("pyblake2.blake2b") as mock_blake2b:
             mock_blake2b.return_value = sentinel = object()
 
             h = hashutil._new_hash("blake2b512")
@@ -247,7 +259,7 @@ class Hashutil(BaseHashutil):
         if "blake2s" in hashlib.algorithms_available:
             self.skipTest("blake2s built in")
 
-        with patch("pyblake2.blake2s") as mock_blake2s:
+        with patch_blake2("pyblake2.blake2s") as mock_blake2s:
             mock_blake2s.return_value = sentinel = object()
 
             h = hashutil._new_hash("blake2s256")