Optimize thumbnail file handling and add tests for state events

changelog.d/20100.bugfix

@@ -0,0 +1 @@
+Improve server concurrency by opening local media thumbnails asynchronously. Contributed by @guillemo12.

synapse/media/thumbnailer.py

         ) 
 if self._is_mine_server_name(server_name): 
     if self.dynamic_thumbnails: 
         await self.thumbnailer.select_or_generate_local_thumbnail( 
             request, 
             media_id, 
             width, 
             height, 
             method, 
             m_type, 
             max_timeout_ms, 
             False, 
             animated=animated, 
         ) 
     else: 
         await self.thumbnailer.respond_local_thumbnail( 
             request, 
             media_id, 
             width, 
             height, 
             method, 
             m_type, 
             max_timeout_ms, 
             False, 
         ) 
@@ -31,6 +31,7 @@
 from synapse.config.repository import THUMBNAIL_SUPPORTED_MEDIA_FORMAT_MAP
 from synapse.http.server import respond_with_json
 from synapse.http.site import SynapseRequest
+from synapse.logging.context import defer_to_thread
 from synapse.logging.opentracing import trace
 from synapse.media._base import (
     FileInfo,
@@ -533,10 +534,11 @@ async def select_or_generate_local_thumbnail(
             assert file_info.thumbnail is not None
 
             if for_federation:
+                f = await defer_to_thread(self.hs.get_reactor(), open, file_path, "rb")
                 await respond_with_multipart_responder(
                     self.hs.get_clock(),
                     request,
-                    FileResponder(self.hs, open(file_path, "rb")),
+                    FileResponder(self.hs, f),
                     file_info.thumbnail.type,
                     file_info.thumbnail.length,
                     None,