Don't block the event loop when exporting with nbconvert

jupyter_server/nbconvert/handlers.py

@@ -10,6 +10,7 @@
 from nbformat import from_dict
 from tornado import web
 from tornado.log import app_log
+from anyio.to_thread import run_sync
 
 from ..base.handlers import FilesRedirectHandler
 from ..base.handlers import JupyterHandler
@@ -115,8 +116,10 @@ async def get(self, format, path):
         if ext_resources_dir:
             resource_dict["metadata"]["path"] = ext_resources_dir
 
+        # Some exporters use the filesystem, delegate that
+        # to a thread so we don't block the event loop for it.
         try:
-            output, resources = exporter.from_notebook_node(nb, resources=resource_dict)
+            output, resources = await run_sync(exporter.from_notebook_node(nb, resources=resource_dict))
         except Exception as e:
             self.log.exception("nbconvert failed: %s", e)
             raise web.HTTPError(500, "nbconvert failed: %s" % e) from e