Ensure fallback returns something even if it has an error.

julianstirling · julianstirling · commit dc56665712d9 · 2026-01-09T11:58:41.000Z
diff --git a/src/labthings_fastapi/server/fallback.py b/src/labthings_fastapi/server/fallback.py
@@ -11,6 +11,7 @@
 import json
 from dataclasses import dataclass
 from pathlib import Path
+import logging
 from traceback import format_exception
 from typing import Any, TYPE_CHECKING
 
@@ -24,6 +25,8 @@
 if TYPE_CHECKING:
     from . import ThingServer
 
+LOGGER = logging.getLogger(__name__)
+
 _TEMPLATE_PATH = Path(__file__).with_name("fallback.html.jinja")
 
 
@@ -46,6 +49,19 @@ class FallbackContext:
     """Any logging history to show."""
 
 
+LAST_RESORT_PAGE = """
+<html>
+<head lang="en">
+  <title>LabThings Internal Error</title>
+</head>
+<body>
+  <h1>LabThings Internal Error</h1>
+  <p>Couldn't start LabThings Server.</p>
+  <p>A further error occurred when gathering context.</p>
+</body>
+"""
+
+
 class FallbackApp(FastAPI):
     """A basic FastAPI application to serve a LabThings error page."""
 
@@ -59,13 +75,17 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
         :param \**kwargs: is passed to `fastapi.FastAPI.__init__`\ .
         """
         super().__init__(*args, **kwargs)
-        # Handle dictionary config here for legacy reasons.
-        self._context: FallbackContext | None = None
-        self._env = Environment(
-            loader=BaseLoader(),
-            autoescape=select_autoescape(["html", "xml"]),
-        )
-        self.set_template_str(_TEMPLATE_PATH.read_text(encoding="utf-8"))
+        try:
+            # Handle dictionary config here for legacy reasons.
+            self._context: FallbackContext | None = None
+            self._env = Environment(
+                loader=BaseLoader(),
+                autoescape=select_autoescape(["html", "xml"]),
+            )
+            self.set_template_str(_TEMPLATE_PATH.read_text(encoding="utf-8"))
+        except BaseException as e:
+            # Catch any error and continue or there is no fallback server
+            LOGGER.exception(e)
         self.html_code = 500
 
     def set_context(self, context: FallbackContext) -> None:
@@ -101,26 +121,30 @@ def fallback_page(self) -> HTMLResponse:
         :return: The HTMLResponse for the fallback page.
         :raises RuntimeError: if the fallback context was never set.
         """
-        if self._context is None:
-            raise RuntimeError("Not context set for fallback server.")
-
-        error_message, error_w_trace = _format_error_and_traceback(self._context)
-        things = list(self._context.server.things) if self._context.server else []
-
-        if isinstance(self._context.config, ThingServerConfig):
-            conf_str = self._context.config.model_dump_json(indent=2)
-        else:
-            conf_str = json.dumps(self._context.config, indent=2)
-
-        content = app._template.render(
-            error_message=error_message,
-            things=things,
-            config=conf_str,
-            traceback=error_w_trace,
-            logginginfo=self._context.log_history,
-        )
-
-        return HTMLResponse(content=content, status_code=app.html_code)
+        try:
+            if self._context is None:
+                raise RuntimeError("Not context set for fallback server.")
+            error_message, error_w_trace = _format_error_and_traceback(self._context)
+            things = list(self._context.server.things) if self._context.server else []
+
+            if isinstance(self._context.config, ThingServerConfig):
+                conf_str = self._context.config.model_dump_json(indent=2)
+            else:
+                conf_str = json.dumps(self._context.config, indent=2)
+
+            content = app._template.render(
+                error_message=error_message,
+                things=things,
+                config=conf_str,
+                traceback=error_w_trace,
+                logginginfo=self._context.log_history,
+            )
+
+            return HTMLResponse(content=content, status_code=app.html_code)
+        except BaseException as e:
+            # Catch any error and continue or there is no fallback server
+            LOGGER.exception(e)
+            return HTMLResponse(content=LAST_RESORT_PAGE, status_code=500)
 
 
 app = FallbackApp()
diff --git a/tests/test_fallback.py b/tests/test_fallback.py
@@ -5,6 +5,7 @@
 verifies that it works as expected.
 """
 
+import logging
 import re
 from html import unescape
 
@@ -13,7 +14,7 @@
 
 from fastapi.testclient import TestClient
 import labthings_fastapi as lt
-from labthings_fastapi.server.fallback import app, FallbackContext
+from labthings_fastapi.server.fallback import app, FallbackApp, FallbackContext
 from labthings_fastapi.example_things import ThingThatCantStart
 
 CONFIG_DICT = {
@@ -33,6 +34,32 @@ def reset_app_state():
     app._context = None
 
 
+def test_fallback_carries_on_even_with_init_error(mocker, caplog):
+    """Ensure fallback is created even if there is and error in __init__.
+
+    The exception should be raised.
+    """
+    # Force set_template_str to fail
+    mocker.patch.object(
+        FallbackApp,
+        "set_template_str",
+        side_effect=RuntimeError("Mocked failure"),
+    )
+    with caplog.at_level(logging.ERROR):
+        FallbackApp()
+    assert len(caplog.records) == 1
+    assert "Mocked failure" in caplog.records[0].message
+    assert caplog.records[0].exc_info is not None
+
+
+def test_fallback_without_context():
+    """Test fallback server errors if context is not set at all."""
+    with TestClient(app) as client:
+        response = client.get("/")
+        html = response.text
+        assert "LabThings Internal Error" in html
+
+
 def test_fallback_redirect():
     """Test that the redirect works."""
     # Need to set a context even if everything in it is empty.
