Sometimes get the "username already in use" message during registration

[lampholder]

Related: #7314

I got a similar experience twice in a row.

• First time, I:
  • registered with a username and password
  • went through three rounds of CAPTCHA
  • accepted the t's and c's
  • *was bounced back to the registration page with a 'This username is in use' error message at the top of the registration screen
  • checking the db, I had successfully registered the account
• Second time, I:
  • registered with a username and password
  • went through one or two rounds of CAPTCHA
  • accepted the t's and c's
  • saw a flash of "This username is in use' at the bottom of the registration screen
  • successfully entered the app

I had dev tools networking tab open for the second time through, here are some logs delivered in what is obviously the optimal format: anmiated gif:

[lampholder]

Oh and:

[lampholder]

This looks related to our polling /register every two seconds throughout the registration process, plus (it looks like to me) hitting /register again with every user-initiated progress through the registraiton process (clicking register, completing the CAPTCHA etc.). Also coupled with the HS responding to /register slowly, you can end up with multiple /register requests in flight. This seems to both trigger our old friend #9480 as well as exposes to registration races that sometimes result in the UI telling you that the mxid you've picked is already registered.

[lampholder]

That last (now hidden) comment was wrong. If you stop your client from being able to communicate with the IS (but the IS is still up), nothing bad happens at all - you can still invite people and you can still add your own additional threepid associations.

[jryans]

According to today's standup, this is believed to be fixed now. @lampholder, did you want to retest, or should we close?

[dbkr]

This is caused by matrix-org/synapse#5450

[dbkr]

But also, matrix-org/matrix-react-sdk#3101 means it would no longer be a problem for us.

[turt2live]

(closing - we believe we've fixed this on develop, therefore the latest RC, therefore the next release)