Fallback server option

This adds the option to start a basic fallback server that shows an error page.
labthings · Aug 2, 2024 · a653f43 · a653f43
1 parent e519a6f
commit a653f43
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Showing 5 changed files with 125 additions and 7 deletions.
diff --git a/pyproject.toml b/pyproject.toml
@@ -31,6 +31,7 @@ dev = [
 ]
 server = [
   "fastapi[all]>=0.104.0", # NB must match FastAPI above
+  "zeroconf >=0.28.0",
 ]
 
 [project.urls]

diff --git a/src/labthings_fastapi/server/__init__.py b/src/labthings_fastapi/server/__init__.py
@@ -40,6 +40,7 @@ def __init__(self, settings_folder: Optional[str] = None):
         self.add_things_view_to_app()
         self._things: dict[str, Thing] = {}
         self.blocking_portal: Optional[BlockingPortal] = None
+        self.startup_status: dict[str, str | dict] = {"things": {}}
         global _thing_servers
         _thing_servers.add(self)
 
@@ -176,7 +177,10 @@ def server_from_config(config: dict) -> ThingServer:
                 f"specified as the class for {path}. The error is "
                 f"printed below:\n\n{e}"
             )
-        instance = cls(*thing.get("args", {}), **thing.get("kwargs", {}))
+        try:
+            instance = cls(*thing.get("args", {}), **thing.get("kwargs", {}))
+        except Exception as e:
+            raise e
         assert isinstance(instance, Thing), f"{thing['class']} is not a Thing"
         server.add_thing(instance, path)
     return server
diff --git a/src/labthings_fastapi/server/cli.py b/src/labthings_fastapi/server/cli.py
@@ -1,6 +1,9 @@
 from argparse import ArgumentParser, Namespace
 import json
 
+from labthings_fastapi.utilities.object_reference_to_object import (
+    object_reference_to_object,
+)
 import uvicorn
 
 from . import ThingServer, server_from_config
@@ -11,6 +14,11 @@ def parse_args(argv: list[str] | None = None) -> Namespace:
     parser = ArgumentParser()
     parser.add_argument("-c", "--config", type=str, help="Path to configuration file")
     parser.add_argument("-j", "--json", type=str, help="Configuration as JSON string")
+    parser.add_argument(
+        "--fallback",
+        action="store_true",
+        help="Serve an error page instead of exiting, if we can't start.",
+    )
     parser.add_argument(
         "--host", type=str, default="127.0.0.1", help="Bind socket to this host"
     )
@@ -46,9 +54,23 @@ def config_from_args(args: Namespace) -> dict:
 def serve_from_cli(argv: list[str] | None = None, dry_run=False):
     """Start the server from the command line"""
     args = parse_args(argv)
-    config = config_from_args(args)
-    server = server_from_config(config)
-    assert isinstance(server, ThingServer)
-    if dry_run:
-        return server
-    uvicorn.run(server.app, host=args.host, port=args.port)
+    try:
+        config, server = None, None
+        config = config_from_args(args)
+        server = server_from_config(config)
+        assert isinstance(server, ThingServer)
+        if dry_run:
+            return server
+        uvicorn.run(server.app, host=args.host, port=args.port)
+    except BaseException as e:
+        if args.fallback:
+            print(f"Error: {e}")
+            fallback_server = "labthings_fastapi.server.fallback:app"
+            print(f"Starting fallback server {fallback_server}.")
+            app = object_reference_to_object(fallback_server)
+            app.labthings_config = config
+            app.labthings_server = server
+            app.labthings_error = e
+            uvicorn.run(app, host=args.host, port=args.port)
+        else:
+            raise e
diff --git a/src/labthings_fastapi/server/fallback.py b/src/labthings_fastapi/server/fallback.py
@@ -0,0 +1,56 @@
+import json
+from fastapi import FastAPI
+from fastapi.responses import HTMLResponse
+from starlette.responses import RedirectResponse
+
+app = FastAPI()
+
+ERROR_PAGE = """
+<!DOCTYPE html>
+<html>
+<head>
+    <title>LabThings</title>
+</head>
+<body>
+    <h1>LabThings Could't Load</h1>
+    <p>Something went wrong when setting up your LabThings server.</p>
+    <p>Please check your configuration and try again.</p>
+    <p>More details may be shown below:</p>
+    <pre>{{error}}</pre>
+    <p>The following Things loaded successfuly:</p>
+    <ul>
+        {{things}}
+    </ul>
+    <p>Your configuration:</p>
+    <pre>
+        {{config}}
+    </pre>
+</body>
+</html>
+"""
+
+app.labthings_config = None
+app.labthings_server = None
+app.labthings_error = None
+
+
+@app.get("/")
+async def root():
+    error_message = f"{app.labthings_error!r}"
+    things = ""
+    if app.labthings_server:
+        for path, thing in app.labthings_server.things.items():
+            things += f"<li>{path}: {thing!r}</li>"
+
+    content = ERROR_PAGE
+    content = content.replace("{{error}}", error_message)
+    content = content.replace("{{things}}", things)
+    content = content.replace("{{config}}", json.dumps(app.labthings_config, indent=2))
+    return HTMLResponse(
+        content=ERROR_PAGE.replace("{{error}}", app.error_message), status_code=500
+    )
+
+
+@app.get("/{path:path}")
+async def redirect_to_root(path: str):
+    return RedirectResponse(url="/")
diff --git a/tests/test_server_from_config_and_cli.py → tests/test_server_cli.py b/tests/test_server_from_config_and_cli.py → tests/test_server_cli.py
@@ -119,5 +119,40 @@ def test_serve_with_no_config():
         check_serve_from_cli([])
 
 
+def test_invalid_thing_and_fallback():
+    """Check it fails for invalid things, and test the fallback option"""
+    config_json = json.dumps(
+        {
+            "things": {
+                "broken": "labthings_fastapi.example_things:MissingThing",
+            }
+        }
+    )
+    with raises(ImportError):
+        check_serve_from_cli(["-j", config_json])
+    ## the line below should start a dummy server with an error page -
+    ## it terminates happily once the server starts.
+    check_serve_from_cli(["-j", config_json, "--fallback"])
+
+
+def test_invalid_config():
+    """Check it fails for invalid config"""
+    with raises(FileNotFoundError):
+        check_serve_from_cli(["-c", "non_existent_file.json"])
+
+
+def test_thing_that_cant_start():
+    """Check it fails for a thing that can't start"""
+    config_json = json.dumps(
+        {
+            "things": {
+                "broken": "labthings_fastapi.example_things:ThingThatCantStart",
+            }
+        }
+    )
+    with raises(SystemExit):
+        check_serve_from_cli(["-j", config_json])
+
+
 if __name__ == "__main__":
     test_serve_from_cli_with_config_json()