Don't accept threaded receipts for reactions to/replies to/edits of a thread root

[andybalaam]

Description

Previously, the spec implied that threaded receipts were acceptable when referring to thread roots, or reactions to them etc. but this was a spec bug, which is fixed in matrix-org/matrix-spec#1677 . See matrix-org/matrix-spec-proposals#4037 for more detail.

Synapse's code at

synapse/synapse/rest/client/receipts.py

Lines 124 to 162 in 930dc9e

	async def _is_event_in_thread(self, event_id: str, thread_id: str) -> bool:
	"""
	The event must be related to the thread ID (in a vague sense) to ensure
	clients aren't sending bogus receipts.
	A thread ID is considered valid for a given event E if:
	1. E has a thread relation which matches the thread ID;
	2. E has another event which has a thread relation to E matching the
	thread ID; or
	3. E is recursively related (via any rel_type) to an event which
	satisfies 1 or 2.
	Given the following DAG:
	A <---[m.thread]-- B <--[m.annotation]-- C
	^
	|--[m.reference]-- D <--[m.annotation]-- E
	It is valid to send a receipt for thread A on A, B, C, D, or E.
	It is valid to send a receipt for the main timeline on A, D, and E.
	Args:
	event_id: The event ID to check.
	thread_id: The thread ID the event is potentially part of.
	Returns:
	True if the event belongs to the given thread, otherwise False.
	"""
	# If the receipt is on the main timeline, it is enough to check whether
	# the event is directly related to a thread.
	if thread_id == MAIN_TIMELINE:
	return MAIN_TIMELINE == await self._main_store.get_thread_id(event_id)
	# Otherwise, check if the event is directly part of a thread, or is the
	# root message (or related to the root message) of a thread.
	return thread_id == await self._main_store.get_thread_id_for_receipts(event_id)

reflects the spec bug, accepting incorrect receipts. It should not accept threaded receipts for events that are not in the thread.

Steps to reproduce

• Create some messages in a room, including a thread
• Create a reaction to the thread root
• Send a threaded receipt for the reaction, that gives the thread_id as the ID of the thread root event, implying that the reaction is in the thread
• Note that Synapse accepts this receipt

Synapse should reject this receipt since a reaction to a thread root is not in the thread.

Homeserver

matrix.org

Synapse Version

1.98.0rc1 (b=matrix-org-hotfixes,c2deef254a)

Installation Method

I don't know

Database

Workers

Multiple workers

Platform

Configuration

Relevant log output

-

Anything else that would be useful to know?