[1.7.x] Fixed #24193 -- Prevented unclosed file warnings in static.serve()

This regression was caused by 818e59a3f0. The patch is a partial
backport of the new FileResponse class available in later Django
versions.
Thanks Raphaël Hertzog for the report, and Tim Graham and Collin
Anderson for the reviews.

django/http/__init__.py

@@ -1,7 +1,8 @@
 from django.http.cookie import SimpleCookie, parse_cookie
 from django.http.request import (HttpRequest, QueryDict,
     RawPostDataException, UnreadablePostError, build_request_repr)
-from django.http.response import (HttpResponse, StreamingHttpResponse,
+from django.http.response import (
+    HttpResponse, StreamingHttpResponse, FileResponse,
     HttpResponseRedirect, HttpResponsePermanentRedirect,
     HttpResponseNotModified, HttpResponseBadRequest, HttpResponseForbidden,
     HttpResponseNotFound, HttpResponseNotAllowed, HttpResponseGone,
@@ -16,5 +17,5 @@ __all__ = [
     'HttpResponseBadRequest', 'HttpResponseForbidden', 'HttpResponseNotFound',
     'HttpResponseNotAllowed', 'HttpResponseGone', 'HttpResponseServerError',
     'Http404', 'BadHeaderError', 'fix_location_header', 'JsonResponse',
-    'conditional_content_removal',
+    'FileResponse', 'conditional_content_removal',
 ]

django/http/response.py

@@ -382,6 +382,9 @@ class StreamingHttpResponse(HttpResponseBase):
 
     @streaming_content.setter
     def streaming_content(self, value):
+        self._set_streaming_content(value)
+
+    def _set_streaming_content(self, value):
         # Ensure we can never iterate on "value" more than once.
         self._iterator = iter(value)
         if hasattr(value, 'close'):
@@ -391,6 +394,21 @@ class StreamingHttpResponse(HttpResponseBase):
         return self.streaming_content
 
 
+class FileResponse(StreamingHttpResponse):
+    """
+    A streaming HTTP response class optimized for files.
+    """
+    block_size = 4096
+
+    def _set_streaming_content(self, value):
+        if hasattr(value, 'read'):
+            self._iterator = iter(lambda: value.read(self.block_size), b'')
+            if hasattr(value, 'close'):
+                self._closable_objects.append(value)
+        else:
+            super(FileResponse, self)._set_streaming_content(value)
+
+
 class HttpResponseRedirectBase(HttpResponse):
     allowed_schemes = ['http', 'https', 'ftp']