Put a command's #id on its answer and nothing else

Client.py

@@ -153,6 +153,20 @@ def set_msg_id(self, msg):
 		self.msg_id = '#%s ' % test
 		return (' '.join(msg.split(' ')[1:]))
 
+	def with_msg_id(self, callback):
+		# a reply sent after the handler returns, from a DB callback or the login queue, still
+		# answers the command that started it. Capture that command's id now and put it back
+		# on the client only while the callback runs.
+		msg_id = self.msg_id
+		def run(*args, **kwargs):
+			previous = self.msg_id
+			self.msg_id = msg_id
+			try:
+				return callback(*args, **kwargs)
+			finally:
+				self.msg_id = previous
+		return run
+
 	def setFlagByIP(self, ip, force=True):
 		cc = ip2country.lookup(ip)
 		if force or cc != '??':

DataHandler.py

@@ -210,7 +210,7 @@ def __init__(self):
 		# simultaneously backed up on their write buffers (the 2.3 producer signal), new
 		# logins are queued FIFO and drained by drain_login_queue() (a 1s LoopingCall) once
 		# the backpressure clears. Under normal load the queue stays empty.
-		self.login_queue = collections.deque() # (client, login_args) awaiting login under backpressure
+		self.login_queue = collections.deque() # (client, login_now carrying the LOGIN's msg_id, login_args) awaiting login under backpressure
 		self.login_backpressure_limit = 50     # paused-producer count above which login admission pauses
 
 		# rate limits
@@ -807,11 +807,11 @@ def drain_login_queue(self):
 		# shares login_queue with in_LOGIN without locking. Each login gets its own session
 		# commit/rollback/close, mirroring the per-request guards in dataReceived.
 		while self.login_queue and not self.login_backpressured():
-			client, args = self.login_queue.popleft()
+			client, login_now, args = self.login_queue.popleft()
 			if client.session_id not in self.clients:
 				continue # client disconnected while queued
 			try:
-				self.protocol.login_now(client, *args)
+				login_now(client, *args)
 				self.session_manager.commit_guard()
 			except:
 				logging.error(traceback.format_exc())

PROTOCOL.md

@@ -74,9 +74,13 @@ cannot drift. **[GAP]** the `@emits` convention does not exist yet — see
   argument may legitimately contain spaces (e.g. chat text, topics). This is the
   "sentence argument" behaviour. (`Protocol.py` `get_function_args`.)
 - **Message IDs.** A client may prefix a command with `#<id> ` to correlate a request
-  with the server's reply; the server echoes the id back on responses generated in that
-  command's handling. (`Client.py`.) **[GAP]** document exact echo semantics and which
-  responses carry the id vs which do not.
+  with the server's reply. The server puts the same `#<id> ` on every line it sends that
+  client as the answer to that command, including refusals (`SERVERMSG`, `FAILED`) and
+  replies that arrive later because the command waited on the database or the login queue.
+  Lines that are not an answer to the command carry no id, even when they arrive before
+  the answer: chat from other players, broadcasts, status changes, and offline messages
+  delivered after `LOGIN`. Lines sent to other clients as a result of the command carry no
+  id either. (`Client.py` `set_msg_id`, `with_msg_id`.)
 - **Tab-separated payloads.** Some structured fields (notably battle `script_tags`) use
   tab-separated `key=value` pairs. **[GAP]** enumerate every command that uses tab
   separation and the exact field grammar.