Redirect stdout/stderr to logs after initialization (#19131)

This regressed in https://github.com/element-hq/synapse/pull/19121. I
moved things in https://github.com/element-hq/synapse/pull/19121 because
I thought that it made sense to redirect anything printed to
`stdout`/`stderr` to the logs as early as possible. But we actually want
to log any immediately apparent problems during initialization to
`stderr` in the terminal so that they are obvious and visible to the
operator.

Now, I've moved `redirect_stdio_to_logs()` back to where it was
previously along with some proper comment context for why we have it
there.

changelog.d/19131.misc

@@ -0,0 +1 @@
+Refactor and align app entrypoints (avoid `exit(1)` in our composable functions).

synapse/app/admin_cmd.py

@@ -369,6 +369,7 @@ async def start(admin_command_server: AdminCmdServer, args: argparse.Namespace)
 def main() -> None:
     homeserver_config, args = load_config(sys.argv[1:])
     with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
+        # Initialize and setup the homeserver
         admin_command_server = create_homeserver(homeserver_config)
         setup(admin_command_server)
 

synapse/app/generic_worker.py

@@ -450,16 +450,21 @@ def main() -> None:
     # Create a logging context as soon as possible so we can start associating
     # everything with this homeserver.
     with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
-        # redirect stdio to the logs, if configured.
-        if not homeserver_config.logging.no_redirect_stdio:
-            redirect_stdio_to_logs()
-
+        # Initialize and setup the homeserver
         hs = create_homeserver(homeserver_config)
         try:
             setup(hs)
         except Exception as e:
             handle_startup_exception(e)
 
+        # For problems immediately apparent during initialization, we want to log to
+        # stderr in the terminal so that they are obvious and visible to the operator.
+        #
+        # Now that we're past the initialization stage, we can redirect anything printed
+        # to stdio to the logs, if configured.
+        if not homeserver_config.logging.no_redirect_stdio:
+            redirect_stdio_to_logs()
+
         # Register a callback to be invoked once the reactor is running
         register_start(hs, start, hs)
 

synapse/app/homeserver.py

@@ -479,16 +479,21 @@ def main() -> None:
     # Create a logging context as soon as possible so we can start associating
     # everything with this homeserver.
     with LoggingContext(name="main", server_name=homeserver_config.server.server_name):
-        # redirect stdio to the logs, if configured.
-        if not homeserver_config.logging.no_redirect_stdio:
-            redirect_stdio_to_logs()
-
+        # Initialize and setup the homeserver
         hs = create_homeserver(homeserver_config)
         try:
             setup(hs)
         except Exception as e:
             handle_startup_exception(e)
 
+        # For problems immediately apparent during initialization, we want to log to
+        # stderr in the terminal so that they are obvious and visible to the operator.
+        #
+        # Now that we're past the initialization stage, we can redirect anything printed
+        # to stdio to the logs, if configured.
+        if not homeserver_config.logging.no_redirect_stdio:
+            redirect_stdio_to_logs()
+
         # Register a callback to be invoked once the reactor is running
         register_start(hs, start, hs)