check for traefik entrypoint on startup

Informative error instead of hang in case of misconfigured values

jupyterhub_traefik_proxy/proxy.py

@@ -311,11 +311,16 @@ async def _traefik_api_request(self, path):
             self.log.debug("%s GET %s", resp.code, url)
         return resp
 
+    _static_config_awaited = False
+
     async def _wait_for_static_config(self):
         async def _check_traefik_static_conf_ready():
             """Check if traefik loaded its static configuration yet"""
             try:
                 await self._traefik_api_request("/api/overview")
+                await self._traefik_api_request(
+                    f"/api/entrypoints/{self.traefik_entrypoint}"
+                )
             except ConnectionRefusedError:
                 self.log.debug(
                     f"Connection Refused waiting for traefik at {self.traefik_api_url}. It's probably starting up..."
@@ -328,9 +333,14 @@ async def _check_traefik_static_conf_ready():
                     )
                     return False
                 if e.code == 404:
-                    self.log.debug(
-                        f"traefik api at {e.response.request.url} overview not ready yet"
-                    )
+                    if "/entrypoints/" in e.response.request.url:
+                        self.log.warning(
+                            f"c.{self.__class__.__name__}.traefik_entrypoint={self.traefik_entrypoint!r} not found in traefik. Is it correct?"
+                        )
+                    else:
+                        self.log.debug(
+                            f"traefik api at {e.response.request.url} overview not ready yet"
+                        )
                     return False
                 # unexpected
                 self.log.error(f"Error checking for traefik static configuration {e}")
@@ -343,6 +353,7 @@ async def _check_traefik_static_conf_ready():
             "Traefik static configuration not available",
             timeout=self.check_route_timeout,
         )
+        self._static_config_awaited = True
 
     def _stop_traefik(self):
         self.log.info("Cleaning up traefik proxy [pid=%i]...", self.traefik_process.pid)
@@ -461,6 +472,14 @@ async def start(self):
         self._start_traefik()
         await self._wait_for_static_config()
 
+    async def check_routes(self, *args, **kwargs):
+        # make sure we've loaded static config
+        # this won't have happened automatically
+        # if not self.should_start
+        if not self._static_config_awaited:
+            await self._wait_for_static_config()
+        return await super().check_routes(*args, **kwargs)
+
     async def stop(self):
         """Stop the proxy.