Speed up processing of federation stream RDATA rows.

[erikjohnston]

Instead of storing and sending an ACK for every single row we send
synchronously, we instead do it asynchronously while batching up
updates.

[clokep]

Suggested change

	# There are waiting deferreds only in the OrderedDict of deferreds is
	# There are waiting deferreds only if the OrderedDict of deferreds is

I don't think entry is an OrderedDict, isn't it a Sequence? I'm actually a little confused why this would check entry[1].

[erikjohnston]

synapse/synapse/util/async_helpers.py

Lines 220 to 226 in a6a40a1

	# key_to_defer is a map from the key to a 2 element list where
	# the first element is the number of things executing, and
	# the second element is an OrderedDict, where the keys are deferreds for the
	# things blocked from executing.
	self.key_to_defer = (
	{}
	) # type: Dict[str, Sequence[Union[int, Dict[defer.Deferred, int]]]]

explains it. The sequence is basically a data structure of "number of stuff current in flight" and "deferreds that are waiting".

[erikjohnston]

i.e. entry[1] is getting the OrderedDict of deferreds waiting.

[clokep]

Ah, I see. I only read half the comment. 🙄 Should this really be a Tuple[int, Dict[defer.Deferred, int]] I wonder?

[clokep]

We talked about this a bit...and it can't be a tuple cause it gets modified. Anyway, the type checking here is funky, but it is essentially a list of two items.