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Background update in 'Add a column participant to room_memberships table #18068' is not rollback-safe, or rolling-upgrade safe #18356

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@reivilibre

#18068

Since this background update takes the maximum stream ordering on its first iteration (assuming that rows after that will be populated at insertion time), it's not safe to either roll back, or to start the background update before all other writing workers have been upgraded.

This goes against our rollback policy — the SCHEMA_COMPAT_VERSION should be adjusted to reflect this, but even still, as a matter of principle we 'shouldn't' ship a version without at least one version of rollback ability.

This also goes against our (unstated, but relied upon) policy of letting you upgrade workers one by one. We usually require upgrade notes to be written when we can't meet this aspect.

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