add allow_implicit_spawn setting

- warn that there are known issues associated with enabling it
- it is inherently incompatible with named servers

jupyterhub/app.py

@@ -917,10 +917,37 @@ class JupyterHub(Application):
     def _authenticator_default(self):
         return self.authenticator_class(parent=self, db=self.db)
 
+    allow_implicit_spawn = Bool(
+        False,
+        help="""Allow implicit spawning
+
+        When a user visits a URL for a server that's not running,
+        instead of confirming the spawn request,
+        automatically begin the spawn process.
+
+        Warning: not compatible with named servers,
+        and known to cause issues with redirect loops,
+        server errors, and infinitely respawning servers.
+        """,
+    ).tag(config=True)
+
+    @validate('allow_implicit_spawn')
+    def _allow_named_changed(self, proposal):
+        if proposal.value and self.allow_named_servers:
+            self.log.warning("Implicit spawn cannot work with named servers")
+            return False
+        return proposal.value
+
     allow_named_servers = Bool(
         False, help="Allow named single-user servers per user"
     ).tag(config=True)
 
+    @observe('allow_named_servers')
+    def _allow_named_changed(self, change):
+        if change.new and self.allow_implicit_spawn:
+            self.log.warning("Implicit spawn cannot work with named servers")
+            self.allow_implicit_spawn = False
+
     named_server_limit_per_user = Integer(
         0,
         help="""
@@ -2158,6 +2185,7 @@ class JupyterHub(Application):
             subdomain_host=self.subdomain_host,
             domain=self.domain,
             statsd=self.statsd,
+            allow_implicit_spawn=self.allow_implicit_spawn,
             allow_named_servers=self.allow_named_servers,
             default_server_name=self._default_server_name,
             named_server_limit_per_user=self.named_server_limit_per_user,

jupyterhub/handlers/base.py

@@ -1426,11 +1426,17 @@ class UserUrlHandler(BaseHandler):
         # serve a page prompting for spawn and 503 error
         # visiting /user/:name no longer triggers implicit spawn
         # without explicit user action
-        self.set_status(503)
         spawn_url = url_concat(
             url_path_join(self.hub.base_url, "spawn", user.escaped_name, server_name),
             {"next": self.request.uri},
         )
+        if self.settings["allow_implicit_spawn"]:
+            self.log.warning("Allowing implicit spawn for %s", self.request.path)
+            self.redirect(spawn_url)
+            return
+        else:
+            self.set_status(503)
+
         auth_state = await user.get_auth_state()
         html = self.render_template(
             "not_running.html",