FileResponse works with files asynchronously (#3476)

* FileResponse works with files asynchronously

* FileResponse: the 'prepare' method works with files asynchronously

CHANGES/3313.feature

@@ -0,0 +1 @@
+FileResponse from web_fileresponse.py uses a ThreadPoolExecutor to work with files asynchronously.

CONTRIBUTORS.txt

@@ -82,6 +82,7 @@ Eric Sheng
 Erich Healy
 Eugene Chernyshov
 Eugene Naydenov
+Eugene Tolmachev
 Evert Lammerts
 FichteFoll
 Frederik Gladhorn

aiohttp/web_fileresponse.py

@@ -179,22 +179,24 @@ async def _sendfile_fallback(self, request: 'BaseRequest',
         # os.sendfile() system call. This should be used on systems
         # that don't support the os.sendfile().
 
-        # To avoid blocking the event loop & to keep memory usage low,
-        # fobj is transferred in chunks controlled by the
-        # constructor's chunk_size argument.
+        # To keep memory usage low,fobj is transferred in chunks
+        # controlled by the constructor's chunk_size argument.
 
         writer = await super().prepare(request)
         assert writer is not None
 
         chunk_size = self._chunk_size
+        loop = asyncio.get_event_loop()
 
-        chunk = fobj.read(chunk_size)
+        chunk = await loop.run_in_executor(None, fobj.read, chunk_size)
         while chunk:
             await writer.write(chunk)
             count = count - chunk_size
             if count <= 0:
                 break
-            chunk = fobj.read(min(chunk_size, count))
+            chunk = await loop.run_in_executor(
+                None, fobj.read, min(chunk_size, count)
+            )
 
         await writer.drain()
         return writer
@@ -218,7 +220,8 @@ async def prepare(
                 filepath = gzip_path
                 gzip = True
 
-        st = filepath.stat()
+        loop = asyncio.get_event_loop()
+        st = await loop.run_in_executor(None, filepath.stat)
 
         modsince = request.if_modified_since
         if modsince is not None and st.st_mtime <= modsince.timestamp():
@@ -334,8 +337,8 @@ async def prepare(
             self.headers[hdrs.CONTENT_RANGE] = 'bytes {0}-{1}/{2}'.format(
                 real_start, real_start + count - 1, file_size)
 
-        with filepath.open('rb') as fobj:
+        with (await loop.run_in_executor(None, filepath.open, 'rb')) as fobj:
             if start:  # be aware that start could be None or int=0 here.
-                fobj.seek(start)
+                await loop.run_in_executor(None, fobj.seek, start)
 
             return await self._sendfile(request, fobj, count)