Added streaming sendfile backend

The simple sendfile backend doesn't stream responses. This commit adds a backend that does.

wagtail/utils/sendfile_streaming_backend.py

@@ -0,0 +1,63 @@
+# Sendfile "streaming" backend
+# This is based on sendfiles builtin "simple" backend but uses a StreamingHttpResponse
+
+import os
+import stat
+import re
+try:
+    from email.utils import parsedate_tz, mktime_tz
+except ImportError:
+    from email.Utils import parsedate_tz, mktime_tz
+from wsgiref.util import FileWrapper
+
+from django.core.files.base import File
+from django.http import StreamingHttpResponse, HttpResponseNotModified
+from django.utils.http import http_date
+
+
+def sendfile(request, filename, **kwargs):
+    # Respect the If-Modified-Since header.
+    statobj = os.stat(filename)
+
+    if not was_modified_since(request.META.get('HTTP_IF_MODIFIED_SINCE'),
+                              statobj[stat.ST_MTIME], statobj[stat.ST_SIZE]):
+        return HttpResponseNotModified()
+
+    response = StreamingHttpResponse(FileWrapper(open(filename, 'rb')))
+
+    response["Last-Modified"] = http_date(statobj[stat.ST_MTIME])
+    return response
+
+
+def was_modified_since(header=None, mtime=0, size=0):
+    """
+    Was something modified since the user last downloaded it?
+
+    header
+      This is the value of the If-Modified-Since header.  If this is None,
+      I'll just return True.
+
+    mtime
+      This is the modification time of the item we're talking about.
+
+    size
+      This is the size of the item we're talking about.
+    """
+    try:
+        if header is None:
+            raise ValueError
+        matches = re.match(r"^([^;]+)(; length=([0-9]+))?$", header,
+                           re.IGNORECASE)
+        header_date = parsedate_tz(matches.group(1))
+        if header_date is None:
+            raise ValueError
+        header_mtime = mktime_tz(header_date)
+        header_len = matches.group(3)
+        if header_len and int(header_len) != size:
+            raise ValueError
+        if mtime > header_mtime:
+            raise ValueError
+    except (AttributeError, ValueError, OverflowError):
+        return True
+    return False
+

wagtail/wagtaildocs/views/serve.py

@@ -2,6 +2,7 @@ from django.shortcuts import get_object_or_404
 from django.conf import settings
 
 from wagtail.utils.sendfile import sendfile
+from wagtail.utils import sendfile_streaming_backend
 
 from wagtail.wagtaildocs.models import Document, document_served
 
@@ -15,7 +16,5 @@ def serve(request, document_id, document_filename):
     if hasattr(settings, 'SENDFILE_BACKEND'):
         return sendfile(request, doc.file.path, attachment=True, attachment_filename=doc.filename)
     else:
-        # Fallback to simple backend if user hasn't specified SENDFILE_BACKEND (will crash by default)
-        from sendfile.backends.simple import sendfile as simple_sendfile_backend
-
-        return sendfile(request, doc.file.path, attachment=True, attachment_filename=doc.filename, backend=simple_sendfile_backend)
+        # Fallback to streaming backend if user hasn't specified SENDFILE_BACKEND
+        return sendfile(request, doc.file.path, attachment=True, attachment_filename=doc.filename, backend=sendfile_streaming_backend.sendfile)