MSC3998: Add timestamp massaging to /join and /knock

[MadLittleMods]

MSC3998: Add timestamp massaging to /join?ts=123 and /knock?ts=123

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Server implementations:

• Implementations needed

Other related references:

• Original MSC where timestamp massaging was introduced: MSC3316
  • add ?ts massaging for ASes synapse#2754
  • Add query parameter ts to allow appservices set the origin_server_ts for state events synapse#11866
• Related MSC3997 to add timestamp massaging to /createRoom
  • Synapse issue tracking /createRoom timestamp massaging proposed in this MSC and mentions the /join problem: Add timestamp massaging to /createRoom synapse#15346

[MadLittleMods]

cc @ara4n as the original ?ts proposer from the old version of MSC2716

cc @tulir as an interested party in timestamp massaging (author of MSC3316 and one of the main consumers of the ?ts API)

[turt2live]

It's not entirely clear to me why /createRoom is isolated from these endpoints at the moment. To ease discussion, it might make sense to merge the MSCs into a single place to catch what other endpoints might need this behaviour then break it up (if needed) prior to FCP.

[MadLittleMods]

They need timestamp massaging for different reasons and alternatives vary. I think it makes more sense to split them up but it does make sense to keep the bigger picture in mind.

[MadLittleMods]

Add this context to the MSC: It also mattered to us in real life with historical Gitter rooms where we imported history and ran into these messy client issues:

• Scrolling over membership in Gitter historical room jumps calendar around because those events weren't timestamp massaged matrix-viewer#182
• Prevent date headers from appearing for rogue events that appear in the future matrix-viewer#198