Add an option to the static() helper to switch on streaming for large files.

By default uses a 1M threshold.
ie. if the static file to serve is >= 1M it will stream the file.
This threshold value is configurable by passing an int instead of a bool to `stream_large_files` parameter of `static()`.

sanic/app.py

@@ -299,12 +299,14 @@ class Sanic:
 
     # Static Files
     def static(self, uri, file_or_directory, pattern=r'/?.+',
-               use_modified_since=True, use_content_range=False):
+               use_modified_since=True, use_content_range=False,
+               stream_large_files=False):
         """Register a root to serve files from. The input can either be a
         file or a directory. See
         """
         static_register(self, uri, file_or_directory, pattern,
-                        use_modified_since, use_content_range)
+                        use_modified_since, use_content_range,
+                        stream_large_files)
 
     def blueprint(self, blueprint, **options):
         """Register a blueprint on the application.

sanic/static.py

@@ -13,11 +13,12 @@ from sanic.exceptions import (
     InvalidUsage,
 )
 from sanic.handlers import ContentRangeHandler
-from sanic.response import file, HTTPResponse
+from sanic.response import file, file_stream, HTTPResponse
 
 
 def register(app, uri, file_or_directory, pattern,
-             use_modified_since, use_content_range):
+             use_modified_since, use_content_range,
+             stream_large_files):
     # TODO: Though sanic is not a file server, I feel like we should at least
     #       make a good effort here.  Modified-since is nice, but we could
     #       also look into etags, expires, and caching
@@ -34,6 +35,10 @@ def register(app, uri, file_or_directory, pattern,
                                server's
     :param use_content_range: If true, process header for range requests
                               and sends the file part that is requested
+    :param stream_large_files: If true, use the file_stream() handler rather
+                              than the file() handler to send the file
+                              If this is an integer, this represents the
+                              threshold size to switch to file_stream()
     """
     # If we're not trying to match a file directly,
     # serve from the folder
@@ -93,6 +98,16 @@ def register(app, uri, file_or_directory, pattern,
                     headers=headers,
                     content_type=guess_type(file_path)[0] or 'text/plain')
             else:
+                if stream_large_files:
+                    if isinstance(stream_large_files, int):
+                        threshold = stream_large_files
+                    else:
+                        threshold = 1024*1000
+
+                    if not stats:
+                        stats = await stat(file_path)
+                    if stats.st_size >= threshold:
+                        return await file_stream(file_path, headers=headers, _range=_range)
                 return await file(file_path, headers=headers, _range=_range)
         except ContentRangeError:
             raise