Improve GridFS backend thread safety

requests_cache/backends/gridfs.py

diff --git a/requests_cache/backends/gridfs.py b/requests_cache/backends/gridfs.py
index ca370f6..418dba6 100644
--- a/requests_cache/backends/gridfs.py
+++ b/requests_cache/backends/gridfs.py
@@ -11,13 +11,19 @@
    :classes-only:
    :nosignatures:
 """
+from logging import getLogger
+from threading import RLock
+
 from gridfs import GridFS
+from gridfs.errors import FileExists
 from pymongo import MongoClient
 
 from .._utils import get_valid_kwargs
 from .base import BaseCache, BaseStorage
 from .mongodb import MongoDict
 
+logger = getLogger(__name__)
+
 
 class GridFSCache(BaseCache):
     """GridFS cache backend.
@@ -56,27 +62,32 @@
         self.connection = connection or MongoClient(**connection_kwargs)
         self.db = self.connection[db_name]
         self.fs = GridFS(self.db)
+        self._lock = RLock()
 
     def __getitem__(self, key):
-        result = self.fs.find_one({'_id': key})
-        if result is None:
-            raise KeyError
-        return self.serializer.loads(result.read())
+        with self._lock:
+            result = self.fs.find_one({'_id': key})
+            if result is None:
+                raise KeyError
+            return self.serializer.loads(result.read())
 
     def __setitem__(self, key, item):
-        try:
-            self.__delitem__(key)
-        except KeyError:
-            pass
         value = self.serializer.dumps(item)
         encoding = None if isinstance(value, bytes) else 'utf-8'
-        self.fs.put(value, encoding=encoding, **{'_id': key})
+
+        with self._lock:
+            try:
+                self.fs.delete(key)
+                self.fs.put(value, encoding=encoding, **{'_id': key})
+            except FileExists as e:
+                logger.warning(e, exc_info=True)
 
     def __delitem__(self, key):
-        res = self.fs.find_one({'_id': key})
-        if res is None:
-            raise KeyError
-        self.fs.delete(res._id)
+        with self._lock:
+            res = self.fs.find_one({'_id': key})
+            if res is None:
+                raise KeyError
+            self.fs.delete(res._id)
 
     def __len__(self):
         return self.db['fs.files'].estimated_document_count()

tests/integration/base_cache_test.py

diff --git a/tests/integration/base_cache_test.py b/tests/integration/base_cache_test.py
index 750c31f..13e2ede 100644
--- a/tests/integration/base_cache_test.py
+++ b/tests/integration/base_cache_test.py
@@ -87,7 +87,7 @@
             pytest.skip(f'Dependencies not installed for {serializer}')
 
         session = self.init_session(serializer=serializer)
-        # Temporary workaround for this issue: https://github.com/kevin1024/pytest-httpbin/issues/60
+        # Workaround for this issue: https://github.com/kevin1024/pytest-httpbin/issues/60
         if response_format == 'json' and USE_PYTEST_HTTPBIN:
             session.allowable_codes = (200, 404)