Fixed #18481 -- Wrapped request.FILES read error in UnreadablePostError

Thanks KyleMac for the report, André Cruz for the initial patch and
Hiroki Kiyohara for the tests.

django/http/request.py

@@ -238,11 +238,17 @@ class HttpRequest(object):
 
     def read(self, *args, **kwargs):
         self._read_started = True
-        return self._stream.read(*args, **kwargs)
+        try:
+            return self._stream.read(*args, **kwargs)
+        except IOError as e:
+            six.reraise(UnreadablePostError, UnreadablePostError(*e.args), sys.exc_info()[2])
 
     def readline(self, *args, **kwargs):
         self._read_started = True
-        return self._stream.readline(*args, **kwargs)
+        try:
+            return self._stream.readline(*args, **kwargs)
+        except IOError as e:
+            six.reraise(UnreadablePostError, UnreadablePostError(*e.args), sys.exc_info()[2])
 
     def xreadlines(self):
         while True:

tests/requests/tests.py

@@ -659,6 +659,24 @@ class RequestsTests(SimpleTestCase):
         with self.assertRaises(UnreadablePostError):
             request.body
 
+    def test_FILES_connection_error(self):
+        """
+        If wsgi.input.read() raises an exception while trying to read() the
+        FILES, the exception should be identifiable (not a generic IOError).
+        """
+        class ExplodingBytesIO(BytesIO):
+            def read(self, len=0):
+                raise IOError("kaboom!")
+
+        payload = b'x'
+        request = WSGIRequest({'REQUEST_METHOD': 'POST',
+                               'CONTENT_TYPE': 'multipart/form-data; boundary=foo_',
+                               'CONTENT_LENGTH': len(payload),
+                               'wsgi.input': ExplodingBytesIO(payload)})
+
+        with self.assertRaises(UnreadablePostError):
+            request.FILES
+
 
 @skipIf(connection.vendor == 'sqlite'
         and connection.settings_dict['NAME'] in ('', ':memory:'),