Add adapters.get_segment_adapter to set the segment adapter on the request

object.

Previously the segment adapter was set globally with the request as
attribute. This causes thread safety issues. Instead we now initialise
the segment adapter for each request and set it as attribute

src/wagtail_personalisation/adapters.py

@@ -1,6 +1,8 @@
 from __future__ import absolute_import, unicode_literals
 
+from django.conf import settings
 from django.db.models import F
+from django.utils.module_loading import import_string
 
 from wagtail_personalisation.models import Segment
 from wagtail_personalisation.rules import AbstractBaseRule
@@ -10,6 +12,15 @@ from wagtail_personalisation.utils import create_segment_dictionary
 class BaseSegmentsAdapter(object):
     """Base segments adapter."""
 
+    def __init__(self, request):
+        """Prepare the request session for segment storage.
+
+        :param request: The http request
+        :type request: django.http.HttpRequest
+
+        """
+        self.request = request
+
     def setup(self):
         """Prepare the adapter for segment storage."""
         return None
@@ -64,15 +75,8 @@ class BaseSegmentsAdapter(object):
 class SessionSegmentsAdapter(BaseSegmentsAdapter):
     """Segment adapter that uses Django's session backend."""
 
-    def setup(self, request):
-        """Prepare the request session for segment storage.
-
-        :param request: The http request
-        :type request: django.http.HttpRequest
-
-        """
-        self.request = request
-
+    def __init__(self, request):
+        super(SessionSegmentsAdapter, self).__init__(request)
         self.request.session.setdefault('segments', [])
 
     def get_all_segments(self):
@@ -166,3 +170,18 @@ class SessionSegmentsAdapter(BaseSegmentsAdapter):
         self.request.session['segments'] = new_segments
 
         self.update_visit_count()
+
+
+SEGMENT_ADAPTER_CLASS = import_string(getattr(
+    settings,
+    'PERSONALISATION_SEGMENTS_ADAPTER',
+    'wagtail_personalisation.adapters.SessionSegmentsAdapter'))
+
+
+def get_segment_adapter(request):
+    """Return the Segment Adapter for the given request"""
+    try:
+        return request.segment_adapter
+    except AttributeError:
+        request.segment_adapter = SEGMENT_ADAPTER_CLASS(request)
+        return request.segment_adapter