Draft: Ignore sending events to appservice when they come from our own homeserver

[MadLittleMods]

Ignore sending events to appservice when they come from our own homeserver

This is a rough patch for the Gitter homeserver so we can better cope with the massive import load. Since we're the ones who imported the messages and the only ones who send messages on the homeserver, we can safely ignore any events that come from us since we already know about the events on the Gitter side.

Currently, we import a bunch of messages Gitter -> Synapse and Synapse sends them back to us via the appservice where they are ignored on the Gitter bridge side via the suppressEcho option in matrix-appservice-bridge. But just the overhead of ignoring all of the transactions isn't fast enough to keep up. Instead of having the round-trip just to ignore, we can ignore directly in Synapse to not send to us via the appservice.

This logic only really applies to the Gitter situation but if we can write a more general patch in the future, it would be good to ignore events that come in through the appservice to not go back out the appservice that sent it. This probably requires a MSC to define the behavior in the AS registration config.

An alternative, is to something like #3237 which ignores any old events which for the Gitter import would be perfectly fine since we're only importing old messages. And on the Gitter side of the bridge, we already ignore any events older than 30 minutes anyway so a solution to that separate issue makes sense in any case.

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Context:

• Gitter sunset plan
• https://github.com/matrix-org/matrix-hosted/issues/6793
• Update: @turt2live shared a similar t2bot.io patch that they've been running for a long-time: t2bot@3c1cd9a

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[MadLittleMods]

Created #14776 to track adding this kind of functionality to Synapse itself

[MadLittleMods]

Update: @turt2live shared a similar t2bot.io patch that they've been running for a long-time: t2bot@3c1cd9a