Overly strict validation of /sync filters

[squahtx]

This caused a nasty Element Android bug where initial syncs could not complete: element-hq/element-android#7516

For forward compatibility reasons, it's important for homeservers to ignore unrecognized fields in requests. Synapse does not do this for /sync filters, and returns an error to clients when there are unrecognized fields present.

Note that on the client side, clients are generally supposed to determine whether new fields are supported, based on the supported spec versions and unstable features in the /_matrix/client/versions response.

(thanks to richvdh and babolivier for clarifying how forward compatibility is supposed to work)

---

We'll want to set "additionalProperties": True in all of our filter schemas.

synapse/synapse/api/filtering.py

Lines 73 to 95 in a4ecb8e

	ROOM_EVENT_FILTER_SCHEMA = {
	"additionalProperties": False,
	"type": "object",
	"properties": {
	"limit": {"type": "number"},
	"senders": {"$ref": "#/definitions/user_id_array"},
	"not_senders": {"$ref": "#/definitions/user_id_array"},
	"types": {"type": "array", "items": {"type": "string"}},
	"not_types": {"type": "array", "items": {"type": "string"}},
	"rooms": {"$ref": "#/definitions/room_id_array"},
	"not_rooms": {"$ref": "#/definitions/room_id_array"},
	"contains_url": {"type": "boolean"},
	"lazy_load_members": {"type": "boolean"},
	"include_redundant_members": {"type": "boolean"},
	# Include or exclude events with the provided labels.
	# cf https://github.com/matrix-org/matrix-doc/pull/2326
	"org.matrix.labels": {"type": "array", "items": {"type": "string"}},
	"org.matrix.not_labels": {"type": "array", "items": {"type": "string"}},
	# MSC3440, filtering by event relations.
	"related_by_senders": {"type": "array", "items": {"type": "string"}},
	"related_by_rel_types": {"type": "array", "items": {"type": "string"}},
	},
	}