Added startup_timeout config

docs/customize.md

@@ -639,3 +639,17 @@ By using the [layout customization system](https://jupyterlab.readthedocs.io/en/
    }
 }
 ```
+
+## Custom kernel_spec_manager class
+
+By default, Voilà uses `jupyter_client`'s KernelSpecManager.  To change this class, add the following to your config like so:
+```py
+import CustomKernelSpecManager
+
+c.VoilaConfiguration.kernel_spec_manager_class = CustomKernelSpecManager
+```
+
+## Kernel startup_timeout
+By default, Voilà's grace period for kernel startup time is 60 seconds.  It can be adjusted as such, in seconds
+```sh
+voila --VoilaExecutor.startup_timeout=60

voila/app.py

@@ -168,7 +168,7 @@ class Voila(Application):
         "template": "VoilaConfiguration.template",
         "theme": "VoilaConfiguration.theme",
         "classic_tree": "VoilaConfiguration.classic_tree",
-        "kernel_spec_manager_class": "VoilaConfiguration.kernel_spec_manager_class"
+        "kernel_spec_manager_class": "VoilaConfiguration.kernel_spec_manager_class",
     }
     classes = [VoilaConfiguration, VoilaExecutor, VoilaExporter]
     connection_dir_root = Unicode(
@@ -544,7 +544,9 @@ def init_settings(self) -> Dict:
         # default server_url to base_url
         self.server_url = self.server_url or self.base_url
 
-        self.kernel_spec_manager = self.voila_configuration.kernel_spec_manager_class(parent=self)
+        self.kernel_spec_manager = self.voila_configuration.kernel_spec_manager_class(
+            parent=self
+        )
 
         # we create a config manager that load both the serverconfig and nbconfig (classical notebook)
         read_config_path = [

voila/configuration.py

@@ -163,7 +163,7 @@ def _valid_file_blacklist(self, proposal):
         default_value="jupyter_client.kernelspec.KernelSpecManager",
         klass="jupyter_client.kernelspec.KernelSpecManager",
         help="""The kernel spec manager class.  Allows for setting a custom kernel spec manager for finding and running kernels
-        """
+        """,
     )
 
     http_header_envs = List(

voila/execute.py

@@ -10,7 +10,7 @@
 from nbclient.client import NotebookClient
 from nbclient.exceptions import CellExecutionError
 from nbconvert.preprocessors.clearoutput import ClearOutputPreprocessor
-from traitlets import Bool, Unicode
+from traitlets import Bool, Unicode, Integer
 
 
 def strip_code_cell_warnings(cell):
@@ -50,6 +50,18 @@ class VoilaExecutor(NotebookClient):
         help=("Whether to send tracebacks to clients on exceptions."),
     )
 
+    startup_timeout: int = Integer(
+        60,
+        config=True,
+        help=(
+            """
+            The time to wait (in seconds) for the kernel to start.
+            If kernel startup takes longer, a RuntimeError is
+            raised.
+            """
+        ),
+    )
+
     def execute(self, nb, resources, km=None):
         try:
             result = super().execute()