If a user registers their account at a time when replication lag is high and the database is set to read-only mode, the user gets redirected to the Welcome Survey URL but the survey isn't rendered. Instead, the user gets an error message, an example shown below from Czech Wikipedia. The error message appears to only link to the Main page, the user does not get any information about the survey or what they might do to be able to get access to it.
|Resolved||MMiller_WMF||T206365 [EPIC] Growth: Welcome survey|
|Declined||None||T229054 Welcome Survey: not accessible when database is in read-only mode|
I'm not sure we have much control over what happens here, or if we did, that we should interfere with the default rendering when this issue occurs. @MMiller_WMF I'm going to mark this as Triaged but Future but @nettrom_WMF if it seems like this occurs regularly enough to warrant fixing please comment, and @MMiller_WMF of course feel free to re-prioritize.
In theory should not be hard to control - catch the DBReadOnlyError, throw a custom MWException instead, make it say whatever is preferred. I'd rather not mess with the rendering of extremely rare system errors though (what are the chances that the registration itself succeeds but the welcome survey doesn't?). The root issue will be solved by T212021: Welcome survey: allow users to complete the survey later eventually.
@kostajh : I'm not sure that it's possible for us to determine how often this happens, these users show up in the WelcomeSurvey data as having abandoned the survey (which kinda makes sense, except they never got to see it). Digging around in the database, I couldn't find a meaningful way to find them. As I suspect this is rare, I don't see it being a high priority to fix, and as @Tgr mentions, eventually these users will have a way to take the survey.