Make nbconvert root handler asynchronous

jupyter_server/services/nbconvert/handlers.py

@@ -1,5 +1,11 @@
 import json
 
+try:
+    from anyio.to_thread import run_sync
+except ImportError:
+    # fallback on anyio v2 for python version < 3.7
+    from anyio import run_sync_in_worker_thread as run_sync
+
 from tornado import web
 
 from ...base.handlers import APIHandler
@@ -8,16 +14,18 @@
 class NbconvertRootHandler(APIHandler):
 
     @web.authenticated
-    def get(self):
+    async def get(self):
         try:
             from nbconvert.exporters import base
         except ImportError as e:
             raise web.HTTPError(500, "Could not import nbconvert: %s" % e) from e
         res = {}
-        exporters = base.get_export_names()
+        # Some exporters use the filesystem when instantiating, delegate that
+        # to a thread so we don't block the event loop for it.
+        exporters = await run_sync(base.get_export_names)
         for exporter_name in exporters:
             try:
-                exporter_class = base.get_exporter(exporter_name)
+                exporter_class = await run_sync(base.get_exporter, exporter_name)
             except ValueError:
                 # I think the only way this will happen is if the entrypoint
                 # is uninstalled while this method is running