[1.7.x] Fixed #22996 -- Prevented crash with unencoded query string

Thanks Jorge Carleitao for the report and Aymeric Augustin, Tim Graham
for the reviews.
Backport of fa02120d36 from master.

django/http/request.py

@@ -313,8 +313,12 @@ class QueryDict(MultiValueDict):
         self.encoding = encoding
         if six.PY3:
             if isinstance(query_string, bytes):
-                # query_string contains URL-encoded data, a subset of ASCII.
-                query_string = query_string.decode()
+                # query_string normally contains URL-encoded data, a subset of ASCII.
+                try:
+                    query_string = query_string.decode(encoding)
+                except UnicodeDecodeError:
+                    # ... but some user agents are misbehaving :-(
+                    query_string = query_string.decode('iso-8859-1')
             for key, value in parse_qsl(query_string or '',
                                         keep_blank_values=True,
                                         encoding=encoding):
@@ -322,8 +326,12 @@ class QueryDict(MultiValueDict):
         else:
             for key, value in parse_qsl(query_string or '',
                                         keep_blank_values=True):
+                try:
+                    value = value.decode(encoding)
+                except UnicodeDecodeError:
+                    value = value.decode('iso-8859-1')
                 self.appendlist(force_text(key, encoding, errors='replace'),
-                                force_text(value, encoding, errors='replace'))
+                                value)
         self._mutable = mutable
 
     @property

docs/releases/1.6.6.txt

@@ -40,3 +40,6 @@ Bugfixes
 * Fixed JavaScript errors while editing multi-geometry objects in the OpenLayers
   widget (`#23137 <https://code.djangoproject.com/ticket/23137>`_,
   `#23293 <https://code.djangoproject.com/ticket/23293>`_).
+
+* Prevented a crash on Python 3 with query strings containing unencoded
+  non-ASCII characters (`#22996 <http://code.djangoproject.com/ticket/22996>`_).

docs/releases/1.7.txt

@@ -1425,6 +1425,9 @@ Miscellaneous
   databases, use the :djadminopt:`--database` flag to get SQL for those
   models (previously they would always be included in the output).
 
+* Decoding the query string from URLs now fallbacks to the ISO-8859-1 encoding
+  when the input is not valid UTF-8.
+
 .. _deprecated-features-1.7:
 
 Features deprecated in 1.7

tests/handlers/tests.py

@@ -42,14 +42,30 @@ class HandlerTests(TestCase):
         self.assertEqual(response.status_code, 400)
 
     def test_non_ascii_query_string(self):
-        """Test that non-ASCII query strings are properly decoded (#20530)."""
+        """
+        Test that non-ASCII query strings are properly decoded (#20530, #22996).
+        """
         environ = RequestFactory().get('/').environ
-        raw_query_string = 'want=café'
+        raw_query_strings = [
+            b'want=caf%C3%A9', # This is the proper way to encode 'café'
+            b'want=caf\xc3\xa9', # UA forgot to quote bytes
+            b'want=caf%E9', # UA quoted, but not in UTF-8
+            b'want=caf\xe9', # UA forgot to convert Latin-1 to UTF-8 and to quote (typical of MSIE)
+        ]
+        got = []
+        for raw_query_string in raw_query_strings:
             if six.PY3:
-            raw_query_string = raw_query_string.encode('utf-8').decode('iso-8859-1')
+                # Simulate http.server.BaseHTTPRequestHandler.parse_request handling of raw request
+                environ['QUERY_STRING'] = str(raw_query_string, 'iso-8859-1')
+            else:
                 environ['QUERY_STRING'] = raw_query_string
             request = WSGIRequest(environ)
-        self.assertEqual(request.GET['want'], "café")
+            got.append(request.GET['want'])
+        if six.PY2:
+            self.assertListEqual(got, ['café', 'café', 'café', 'café'])
+        else:
+            # On Python 3, %E9 is converted to the unicode replacement character by parse_qsl
+            self.assertListEqual(got, ['café', 'café', 'caf\ufffd', 'café'])
 
     def test_non_ascii_cookie(self):
         """Test that non-ASCII cookies set in JavaScript are properly decoded (#20557)."""

tests/httpwrappers/tests.py

@@ -202,14 +202,14 @@ class QueryDictTests(unittest.TestCase):
         def test_invalid_input_encoding(self):
             """
             QueryDicts must be able to handle invalid input encoding (in this
-            case, bad UTF-8 encoding).
+            case, bad UTF-8 encoding), falling back to ISO-8859-1 decoding.
 
             This test doesn't apply under Python 3 because the URL is a string
             and not a bytestring.
             """
             q = QueryDict(str(b'foo=bar&foo=\xff'))
-            self.assertEqual(q['foo'], '\ufffd')
-            self.assertEqual(q.getlist('foo'), ['bar', '\ufffd'])
+            self.assertEqual(q['foo'], '\xff')
+            self.assertEqual(q.getlist('foo'), ['bar', '\xff'])
 
     def test_pickle(self):
         q = QueryDict(str(''))