Remove sentinel logcontext where we log in setup, start and exit (#18870)

Remove `sentinel` logcontext where we log in `setup`, `start`, and exit.

Instead of having one giant PR that removes all places we use `sentinel`
logcontext, I've decided to tackle this more piece-meal. This PR covers
the parts if you just startup Synapse and exit it with no requests or
activity going on in between.

Part of https://github.com/element-hq/synapse/issues/18905 (Remove
`sentinel` logcontext where we log in Synapse)

Prerequisite for https://github.com/element-hq/synapse/pull/18868.
Logging with the `sentinel` logcontext means we won't know which server
the log came from.



### Why


9cc4001778/docs/log_contexts.md (L71-L81)

(docs updated in https://github.com/element-hq/synapse/pull/18900)


### Testing strategy

1. Run Synapse normally and with `daemonize: true`: `poetry run
synapse_homeserver --config-path homeserver.yaml`
 1. Execute some requests
 1. Shutdown the server
 1. Look for any bad log entries in your homeserver logs:
    - `Expected logging context sentinel but found main`
    - `Expected logging context main was lost`
    - `Expected previous context`
    - `utime went backwards!`/`stime went backwards!`
- `Called stop on logcontext POST-0 without recording a start rusage`
 1. Look for any logs coming from the `sentinel` context


With these changes, you should only see the following logs (not from
Synapse) using the `sentinel` context if you start up Synapse and exit:

`homeserver.log`
```
2025-09-10 14:45:39,924 - asyncio - 64 - DEBUG - sentinel - Using selector: EpollSelector

2025-09-10 14:45:40,562 - twisted - 281 - INFO - sentinel - Received SIGINT, shutting down.

2025-09-10 14:45:40,562 - twisted - 281 - INFO - sentinel - (TCP Port 9322 Closed)
2025-09-10 14:45:40,563 - twisted - 281 - INFO - sentinel - (TCP Port 8008 Closed)
2025-09-10 14:45:40,563 - twisted - 281 - INFO - sentinel - (TCP Port 9093 Closed)
2025-09-10 14:45:40,564 - twisted - 281 - INFO - sentinel - Main loop terminated.
```

changelog.d/18870.misc

@@ -0,0 +1 @@
+Remove `sentinel` logcontext usage where we log in `setup`, `start` and exit.

synapse/app/_base.py

@@ -72,7 +72,7 @@ from synapse.events.auto_accept_invites import InviteAutoAccepter
 from synapse.events.presence_router import load_legacy_presence_router
 from synapse.handlers.auth import load_legacy_password_auth_providers
 from synapse.http.site import SynapseSite
-from synapse.logging.context import PreserveLoggingContext
+from synapse.logging.context import LoggingContext, PreserveLoggingContext
 from synapse.logging.opentracing import init_tracer
 from synapse.metrics import install_gc_manager, register_threadpool
 from synapse.metrics.background_process_metrics import run_as_background_process
@@ -183,25 +183,23 @@ def start_reactor(
         if gc_thresholds:
             gc.set_threshold(*gc_thresholds)
         install_gc_manager()
-        run_command()
 
-    # make sure that we run the reactor with the sentinel log context,
-    # otherwise other PreserveLoggingContext instances will get confused
-    # and complain when they see the logcontext arbitrarily swapping
-    # between the sentinel and `run` logcontexts.
-    #
-    # We also need to drop the logcontext before forking if we're daemonizing,
-    # otherwise the cputime metrics get confused about the per-thread resource usage
-    # appearing to go backwards.
-    with PreserveLoggingContext():
-        if daemonize:
-            assert pid_file is not None
+        # Reset the logging context when we start the reactor (whenever we yield control
+        # to the reactor, the `sentinel` logging context needs to be set so we don't
+        # leak the current logging context and erroneously apply it to the next task the
+        # reactor event loop picks up)
+        with PreserveLoggingContext():
+            run_command()
 
-            if print_pidfile:
-                print(pid_file)
+    if daemonize:
+        assert pid_file is not None
 
-            daemonize_process(pid_file, logger)
-        run()
+        if print_pidfile:
+            print(pid_file)
+
+        daemonize_process(pid_file, logger)
+
+    run()
 
 
 def quit_with_error(error_string: str) -> NoReturn:
@@ -601,10 +599,12 @@ async def start(hs: "HomeServer") -> None:
     hs.get_datastores().main.db_pool.start_profiling()
     hs.get_pusherpool().start()
 
+    def log_shutdown() -> None:
+        with LoggingContext("log_shutdown"):
+            logger.info("Shutting down...")
+
     # Log when we start the shut down process.
-    hs.get_reactor().addSystemEventTrigger(
-        "before", "shutdown", logger.info, "Shutting down..."
-    )
+    hs.get_reactor().addSystemEventTrigger("before", "shutdown", log_shutdown)
 
     setup_sentry(hs)
     setup_sdnotify(hs)

synapse/app/generic_worker.py

@@ -355,7 +355,12 @@ def start(config_options: List[str]) -> None:
     except Exception as e:
         handle_startup_exception(e)
 
-    register_start(_base.start, hs)
+    async def start() -> None:
+        # Re-establish log context now that we're back from the reactor
+        with LoggingContext("start"):
+            await _base.start(hs)
+
+    register_start(start)
 
     # redirect stdio to the logs, if configured.
     if not hs.config.logging.no_redirect_stdio:

synapse/app/homeserver.py

@@ -377,15 +377,17 @@ def setup(config_options: List[str]) -> SynapseHomeServer:
         handle_startup_exception(e)
 
     async def start() -> None:
-        # Load the OIDC provider metadatas, if OIDC is enabled.
-        if hs.config.oidc.oidc_enabled:
-            oidc = hs.get_oidc_handler()
-            # Loading the provider metadata also ensures the provider config is valid.
-            await oidc.load_metadata()
+        # Re-establish log context now that we're back from the reactor
+        with LoggingContext("start"):
+            # Load the OIDC provider metadatas, if OIDC is enabled.
+            if hs.config.oidc.oidc_enabled:
+                oidc = hs.get_oidc_handler()
+                # Loading the provider metadata also ensures the provider config is valid.
+                await oidc.load_metadata()
 
-        await _base.start(hs)
+            await _base.start(hs)
 
-        hs.get_datastores().main.db_pool.updates.start_doing_background_updates()
+            hs.get_datastores().main.db_pool.updates.start_doing_background_updates()
 
     register_start(start)
 

synapse/util/daemonize.py

@@ -29,6 +29,11 @@ import sys
 from types import FrameType, TracebackType
 from typing import NoReturn, Optional, Type
 
+from synapse.logging.context import (
+    LoggingContext,
+    PreserveLoggingContext,
+)
+
 
 def daemonize_process(pid_file: str, logger: logging.Logger, chdir: str = "/") -> None:
     """daemonize the current process
@@ -64,8 +69,14 @@ def daemonize_process(pid_file: str, logger: logging.Logger, chdir: str = "/") -
             pid_fh.write(old_pid)
         sys.exit(1)
 
-    # Fork, creating a new process for the child.
-    process_id = os.fork()
+    # Stop the existing context *before* we fork the process. Otherwise the cputime
+    # metrics get confused about the per-thread resource usage appearing to go backwards
+    # because we're comparing the resource usage from the original process to the forked
+    # process. `PreserveLoggingContext` already takes care of restarting the original
+    # context *after* the block.
+    with PreserveLoggingContext():
+        # Fork, creating a new process for the child.
+        process_id = os.fork()
 
     if process_id != 0:
         # parent process: exit.
@@ -140,9 +151,10 @@ def daemonize_process(pid_file: str, logger: logging.Logger, chdir: str = "/") -
 
     # Cleanup pid file at exit.
     def exit() -> None:
-        logger.warning("Stopping daemon.")
-        os.remove(pid_file)
-        sys.exit(0)
+        with LoggingContext("atexit"):
+            logger.warning("Stopping daemon.")
+            os.remove(pid_file)
+            sys.exit(0)
 
     atexit.register(exit)