Use default kernel name for missing specs

tests/app/non_existing_kernel_test.py

@@ -0,0 +1,19 @@
+# test serving a notebook
+import pytest
+
+
+@pytest.fixture
+def non_existing_kernel_notebook(base_url):
+    return base_url + "/voila/render/non_existing_kernel.ipynb"
+
+
+@pytest.fixture
+def voila_args(notebook_directory, voila_args_extra):
+    return ['--VoilaTest.root_dir=%r' % notebook_directory] + voila_args_extra
+
+
+@pytest.mark.gen_test
+def test_non_existing_kernel(http_client, non_existing_kernel_notebook):
+    response = yield http_client.fetch(non_existing_kernel_notebook)
+    assert response.code == 200
+    assert 'non-existing kernel' in response.body.decode('utf-8')

tests/notebooks/non_existing_kernel.ipynb

@@ -0,0 +1,34 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "print('Executing with non-existing kernel')"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Xeus Python 2",
+   "language": "python",
+   "name": "xpython2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.14"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 2
+}

voila/app.py

@@ -42,6 +42,8 @@
 from jupyter_server.services.config import ConfigManager
 from jupyter_server.base.handlers import FileFindHandler
 
+from jupyter_client.kernelspec import KernelSpecManager
+
 from jupyter_core.paths import jupyter_config_path, jupyter_path
 
 from ipython_genutils.py3compat import getcwd
@@ -358,9 +360,14 @@ def start(self):
         self.log.info('Storing connection files in %s.' % self.connection_dir)
         self.log.info('Serving static files from %s.' % self.static_root)
 
+        self.kernel_spec_manager = KernelSpecManager(
+            parent=self
+        )
+
         self.kernel_manager = MappingKernelManager(
             parent=self,
             connection_dir=self.connection_dir,
+            kernel_spec_manager=self.kernel_spec_manager,
             allowed_message_types=[
                 'comm_msg',
                 'comm_info_request',
@@ -387,6 +394,7 @@ def start(self):
             base_url=self.base_url,
             server_url=self.server_url or self.base_url,
             kernel_manager=self.kernel_manager,
+            kernel_spec_manager=self.kernel_spec_manager,
             allow_remote_access=True,
             autoreload=self.autoreload,
             voila_jinja2_env=env,

voila/handler.py

@@ -51,7 +51,13 @@ def get(self, path=None):
             raise tornado.web.HTTPError(404, 'file not found')
 
         # Fetch kernel name from the notebook metadata
-        kernel_name = notebook.metadata.get('kernelspec', {}).get('name', self.kernel_manager.default_kernel_name)
+        default_kernel_name = self.kernel_manager.default_kernel_name
+        kernel_name = notebook.metadata.get('kernelspec', {}).get('name', default_kernel_name)
+
+        # Use the default kernel name if the kernel from the metadata is not in the kernel specs
+        all_kernel_specs = yield tornado.gen.maybe_future(self.kernel_spec_manager.get_all_specs())
+        if kernel_name not in all_kernel_specs:
+            kernel_name = default_kernel_name
 
         # Launch kernel and execute notebook
         cwd = os.path.dirname(notebook_path)