Make a config option for whether to generate new thumbnail sizes dynamically

synapse/config/repository.py

@@ -22,6 +22,7 @@ class ContentRepositoryConfig(Config):
         self.max_image_pixels = self.parse_size(config["max_image_pixels"])
         self.media_store_path = self.ensure_directory(config["media_store_path"])
         self.uploads_path = self.ensure_directory(config["uploads_path"])
+        self.dynamic_thumbnails = config["dynamic_thumbnails"]
 
     def default_config(self, config_dir_path, server_name):
         media_store = self.default_path("media_store")
@@ -38,4 +39,11 @@ class ContentRepositoryConfig(Config):
 
         # Maximum number of pixels that will be thumbnailed
         max_image_pixels: "32M"
+
+        # Whether to generate new thumbnails on the fly to precisely match
+        # the resolution requested by the client. If true then whenever
+        # a new resolution is requested by the client the server will
+        # generate a new thumbnail. If false the server will pick a thumbnail
+        # from a precalcualted list.
+        dynamic_thumbnails: false
         """ % locals()

synapse/rest/media/v1/base_resource.py

@@ -69,6 +69,7 @@ class BaseMediaResource(Resource):
         self.filepaths = filepaths
         self.version_string = hs.version_string
         self.downloads = {}
+        self.dynamic_thumbnails = hs.config.dynamic_thumbnails
 
     def _respond_404(self, request):
         respond_with_json(

synapse/rest/media/v1/thumbnail_resource.py

@@ -43,14 +43,25 @@ class ThumbnailResource(BaseMediaResource):
         m_type = parse_string(request, "type", "image/png")
 
         if server_name == self.server_name:
-            yield self._select_or_generate_local_thumbnail(
-                request, media_id, width, height, method, m_type
-            )
+            if self.dynamic_thumbnails:
+                yield self._select_or_generate_local_thumbnail(
+                    request, media_id, width, height, method, m_type
+                )
+            else:
+                yield self._respond_local_thumbnail(
+                    request, media_id, width, height, method, m_type
+                )
         else:
-            yield self._select_or_generate_remote_thumbnail(
-                request, server_name, media_id,
-                width, height, method, m_type
-            )
+            if self.dynamic_thumbnails:
+                yield self._select_or_generate_remote_thumbnail(
+                    request, server_name, media_id,
+                    width, height, method, m_type
+                )
+            else:
+                yield self._respond_remote_thumbnail(
+                    request, server_name, media_id,
+                    width, height, method, m_type
+                )
 
     @defer.inlineCallbacks
     def _respond_local_thumbnail(self, request, media_id, width, height,