Generate local thumbnails on a thread

synapse/rest/media/v1/base_resource.py

@@ -244,6 +244,9 @@ class BaseMediaResource(Resource):
             )
             return
 
+        local_thumbnails = []
+
+        def generate_thumbnails():
             scales = set()
             crops = set()
             for r_width, r_height, r_method, r_type in requirements:
@@ -262,9 +265,10 @@ class BaseMediaResource(Resource):
                 )
                 self._makedirs(t_path)
                 t_len = thumbnailer.scale(t_path, t_width, t_height, t_type)
-            yield self.store.store_local_thumbnail(
+
+                local_thumbnails.append((
                     media_id, t_width, t_height, t_type, t_method, t_len
-            )
+                ))
 
             for t_width, t_height, t_type in crops:
                 if (t_width, t_height, t_type) in scales:
@@ -278,9 +282,14 @@ class BaseMediaResource(Resource):
                 )
                 self._makedirs(t_path)
                 t_len = thumbnailer.crop(t_path, t_width, t_height, t_type)
-            yield self.store.store_local_thumbnail(
+                local_thumbnails.append((
                     media_id, t_width, t_height, t_type, t_method, t_len
-            )
+                ))
+
+        yield threads.deferToThread(generate_thumbnails)
+
+        for l in local_thumbnails:
+            yield self.store.store_local_thumbnail(*l)
 
         defer.returnValue({
             "width": m_width,