Fix nbconvert handler run_sync() (#667)

jupyter_server/nbconvert/handlers.py

@@ -119,7 +119,7 @@ async def get(self, format, path):
         # Exporting can take a while, delegate to a thread so we don't block the event loop
         try:
             output, resources = await run_sync(
-                exporter.from_notebook_node(nb, resources=resource_dict)
+                lambda: exporter.from_notebook_node(nb, resources=resource_dict)
             )
         except Exception as e:
             self.log.exception("nbconvert failed: %s", e)
@@ -146,22 +146,22 @@ class NbconvertPostHandler(JupyterHandler):
     SUPPORTED_METHODS = ("POST",)
 
     @web.authenticated
-    def post(self, format):
+    async def post(self, format):
         exporter = get_exporter(format, config=self.config)
 
         model = self.get_json_body()
         name = model.get("name", "notebook.ipynb")
         nbnode = from_dict(model["content"])
 
         try:
-            output, resources = exporter.from_notebook_node(
-                nbnode,
-                resources={
-                    "metadata": {
-                        "name": name[: name.rfind(".")],
+            output, resources = await run_sync(
+                lambda: exporter.from_notebook_node(
+                    nbnode,
+                    resources={
+                        "metadata": {"name": name[: name.rfind(".")]},
+                        "config_dir": self.application.settings["config_dir"],
                     },
-                    "config_dir": self.application.settings["config_dir"],
-                },
+                )
             )
         except Exception as e:
             raise web.HTTPError(500, "nbconvert failed: %s" % e) from e