Adding prelaunch hook for kernel instances

voila/app.py

@@ -37,7 +37,7 @@
 
 from traitlets.config.application import Application
 from traitlets.config.loader import Config
-from traitlets import Unicode, Integer, Bool, Dict, List, default
+from traitlets import Unicode, Integer, Bool, Dict, List, Any, default
 
 from jupyter_server.services.kernels.kernelmanager import MappingKernelManager
 from jupyter_server.services.kernels.handlers import KernelHandler, ZMQChannelsHandler
@@ -241,6 +241,11 @@ class Voila(Application):
                                  cannot be determined reliably by the Jupyter notebook server (proxified
                                  or containerized setups for example)."""))
 
+    prelaunch_hook = Any(default_value=None, allow_none=True,
+                         help=_("""A function that is called prior to the launch of a new kernel instance
+                                   when a user visits the voila webpage. Used for custom user authorization
+                                   or any other necessary pre-launch functions."""))
+
     @property
     def display_url(self):
         if self.custom_display_url:
@@ -487,7 +492,8 @@ def start(self):
                     'notebook_path': os.path.relpath(self.notebook_path, self.root_dir),
                     'nbconvert_template_paths': self.nbconvert_template_paths,
                     'config': self.config,
-                    'voila_configuration': self.voila_configuration
+                    'voila_configuration': self.voila_configuration,
+                    'prelaunch_hook': self.prelaunch_hook
                 }
             ))
         else:

voila/handler.py

@@ -29,6 +29,7 @@ def initialize(self, **kwargs):
         self.nbconvert_template_paths = kwargs.pop('nbconvert_template_paths', [])
         self.traitlet_config = kwargs.pop('config', None)
         self.voila_configuration = kwargs['voila_configuration']
+        self.prelaunch_hook = kwargs.get('prelaunch_hook')
         # we want to avoid starting multiple kernels due to template mistakes
         self.kernel_started = False
 
@@ -59,6 +60,14 @@ async def get(self, path=None):
             return
         self.cwd = os.path.dirname(notebook_path)
 
+        if self.prelaunch_hook:
+            # Allow for preprocessing the notebook.
+            # Can be used to add auth, do custom formatting/standardization
+            # of the notebook, raise exceptions, etc
+            self.prelaunch_hook(self,
+                                notebook=self.notebook,
+                                cwd=self.cwd)
+
         # render notebook to html
         resources = {
             'base_url': self.base_url,