Homeserver ratelimit responses are not properly respected

[nexy7574]

When running as a bot user account (with ratelimits), some actions (such as mass-redactions and unban re-inviting) may fail as the current ratelimit handler does not respect the ratelimit response. As a result, the ratelimiter has a low chance of success, as repeated ratelimited requests to Synapse will increase the ratelimit exponentially, faster than Draupnir's fallback does.

[nexy7574]

There are two issues here.

The first issue is about to be fixed in an upcoming pull request, where the rate-limit response is incorrectly handled due to a check for an always undefined property in the response error object.

The second issue is that rate-limits are difficult to predict and categorise, meaning that when a rate-limit is encountered, it has to be waited out before additional actions can be taken (for example, if a redact batch succeeds in rooms A, B, and C, but gets rate-limited in D, rooms E, F, and G have to wait until D's rate-limit expires, even if they would not have been rate-limited themselves)