Merge branch 'hannes-fix-cache-race' into develop

PR #197.

* hannes-fix-cache-race:
  Add bug fix to changelog
  Update test to ensure it fails
  Fix race condition in cache eviction

Author: jamesls

CHANGELOG.rst

@@ -3,6 +3,8 @@ Next Release (TBD)
 
 * Python 2.6 and 3.3 have reached end-of-life and have been deprecated.
   (`issue 175 <https://github.com/jmespath/jmespath.py/issues/175>`__)
+* Fix race condition when clearing cached parsed expressions.
+  (`issue 197 <https://github.com/jmespath/jmespath.py/pull/197>`__)
 
 
 0.9.5

jmespath/parser.py

@@ -490,7 +490,7 @@ def _raise_parse_error_maybe_eof(self, expected_type, token):
 
     def _free_cache_entries(self):
         for key in random.sample(self._CACHE.keys(), int(self._MAX_SIZE / 2)):
-            del self._CACHE[key]
+            self._CACHE.pop(key, None)
 
     @classmethod
     def purge(cls):

tests/test_parser.py

@@ -1,6 +1,8 @@
 #!/usr/bin/env python
-
 import re
+import random
+import string
+import threading
 from tests import unittest, OrderedDict
 
 from jmespath import parser
@@ -322,6 +324,30 @@ def test_cache_purge(self):
         self.assertEqual(first.parsed,
                          cached.parsed)
 
+    def test_thread_safety_of_cache(self):
+        errors = []
+        expressions = [
+            ''.join(random.choice(string.ascii_letters) for _ in range(3))
+            for _ in range(2000)
+        ]
+        def worker():
+            p = parser.Parser()
+            for expression in expressions:
+                try:
+                    p.parse(expression)
+                except Exception as e:
+                    errors.append(e)
+
+        threads = []
+        for i in range(10):
+            threads.append(threading.Thread(target=worker))
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+
+        self.assertEqual(errors, [])
+
 
 class TestParserAddsExpressionAttribute(unittest.TestCase):
     def test_expression_available_from_parser(self):