From 8a0f785cbc90e257cbf2380d8e13a0f292f772fb Mon Sep 17 00:00:00 2001
From: Andy Armstrong <andya@edx.org>
Date: Mon, 6 Jan 2014 18:00:32 -0500
Subject: [PATCH] Fix STUD-1129: handle out-of-range page numbers

---
 .../contentstore/tests/test_assets.py         |  5 +++
 cms/djangoapps/contentstore/views/assets.py   | 33 ++++++++++++++-----
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/cms/djangoapps/contentstore/tests/test_assets.py b/cms/djangoapps/contentstore/tests/test_assets.py
index 59c1cb068b..4bc14ad4a3 100644
--- a/cms/djangoapps/contentstore/tests/test_assets.py
+++ b/cms/djangoapps/contentstore/tests/test_assets.py
@@ -60,6 +60,11 @@ class AssetsToyCourseTestCase(CourseTestCase):
         self.assert_correct_asset_response(url + "?page_size=2", 0, 2, 3)
         self.assert_correct_asset_response(url + "?page_size=2&page=1", 2, 1, 3)
 
+        # Verify querying outside the range of valid pages
+        self.assert_correct_asset_response(url + "?page_size=2&page=-1", 0, 2, 3)
+        self.assert_correct_asset_response(url + "?page_size=2&page=2", 2, 1, 3)
+        self.assert_correct_asset_response(url + "?page_size=3&page=1", 0, 3, 3)
+
     def assert_correct_asset_response(self, url, expected_start, expected_length, expected_total):
         resp = self.client.get(url, HTTP_ACCEPT='application/json')
         json_response = json.loads(resp.content)
diff --git a/cms/djangoapps/contentstore/views/assets.py b/cms/djangoapps/contentstore/views/assets.py
index 556465c9d2..663cf22f64 100644
--- a/cms/djangoapps/contentstore/views/assets.py
+++ b/cms/djangoapps/contentstore/views/assets.py
@@ -27,7 +27,7 @@ from django.http import HttpResponseNotFound
 import json
 from django.utils.translation import ugettext as _
 from pymongo import DESCENDING
-
+import math
 
 __all__ = ['assets_handler']
 
@@ -91,17 +91,20 @@ def _assets_json(request, location):
     """
     requested_page = int(request.REQUEST.get('page', 0))
     requested_page_size = int(request.REQUEST.get('page_size', 50))
+    sort = [('uploadDate', DESCENDING)]
+
     current_page = max(requested_page, 0)
     start = current_page * requested_page_size
-
-    old_location = loc_mapper().translate_locator_to_location(location)
-
-    course_reference = StaticContent.compute_location(old_location.org, old_location.course, old_location.name)
-    assets, total_count = contentstore().get_all_content_for_course(
-        course_reference, start=start, maxresults=requested_page_size, sort=[('uploadDate', DESCENDING)]
-    )
+    assets, total_count = _get_assets_for_page(request, location, current_page, requested_page_size, sort)
     end = start + len(assets)
 
+    # If the query is beyond the final page, then re-query the final page so that at least one asset is returned
+    if requested_page > 0 and start >= total_count:
+        current_page = int(math.floor((total_count - 1) / requested_page_size))
+        start = current_page * requested_page_size
+        assets, total_count = _get_assets_for_page(request, location, current_page, requested_page_size, sort)
+        end = start + len(assets)
+
     asset_json = []
     for asset in assets:
         asset_id = asset['_id']
@@ -123,6 +126,20 @@ def _assets_json(request, location):
     })
 
 
+def _get_assets_for_page(request, location, current_page, page_size, sort):
+    """
+    Returns the list of assets for the specified page and page size.
+    """
+    start = current_page * page_size
+
+    old_location = loc_mapper().translate_locator_to_location(location)
+
+    course_reference = StaticContent.compute_location(old_location.org, old_location.course, old_location.name)
+    return contentstore().get_all_content_for_course(
+        course_reference, start=start, maxresults=page_size, sort=sort
+    )
+
+
 @require_POST
 @ensure_csrf_cookie
 @login_required