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## Problem I discovered two bugs corresponding to safekeeper migration, which together might lead to a data loss during the migration. The second bug is from a hadron patch and might lead to a data loss during the safekeeper restore in hadron as well. 1. `switch_membership` returns the current `term` instead of `last_log_term`. It is used to choose the `sync_position` in the algorithm, so we might choose the wrong one and break the correctness guarantees. 2. The current `term` is used to choose the most advanced SK in `pull_timeline` with higher priority than `flush_lsn`. It is incorrect because the most advanced safekeeper is the one with the highest `(last_log_term, flush_lsn)` pair. The compute might bump term on the least advanced sk, making it the best choice to pull from, and thus making committed log entries "uncommitted" after `pull_timeline` Part of https://databricks.atlassian.net/browse/LKB-1017 ## Summary of changes - Return `last_log_term` in `switch_membership` - Use `(last_log_term, flush_lsn)` as a primary key for choosing the most advanced sk in `pull_timeline` and deny pulling if the `max_term` is higher than on the most advanced sk (hadron only) - Write tests for both cases - Retry `sync_safekeepers` in `compute_ctl` - Take into the account the quorum size when calculating `sync_position`