Cherry-pick: Fix MVCC bug with prepared xact with subxacts on standby (v16) #447
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This went unnoticed, because only a few existing callers of BackgroundPsql->query used the result, and the ones that did were not bothered by an extra newline. I noticed because I was about to add a new test that checks the result. Discussion: https://www.postgresql.org/message-id/6b852e98-2d49-4ca1-9e95-db419a2696e0%40iki.fi NEON: cherry-picked from the upstream thread ahead of time, to unblock neondatabase/neon#7288. I expect this to be committed to upstream in the next few days, superseding this.
We did not recover the subtransaction IDs of prepared transactions when starting a hot standby from a shutdown checkpoint. As a result, such subtransactions were considered as aborted, rather than in-progress. That would lead to hint bits being set incorrectly, and the subtransactions suddenly becoming visible to old snapshots when the prepared transaction was committed. To fix, update pg_subtrans with prepared transactions's subxids when starting hot standby from a shutdown checkpoint. The snapshots taken from that state need to be marked as "suboverflowed", so that we also check the pg_subtrans. Discussion: https://www.postgresql.org/message-id/6b852e98-2d49-4ca1-9e95-db419a2696e0%40iki.fi NEON: cherry-picked from the upstream thread ahead of time, to unblock neondatabase/neon#7288. I expect this to be committed to upstream in the next few days, superseding this.
hlinnaka
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knizhnik
approved these changes
Jun 25, 2024
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See neondatabase/neon#8152 for the main PR in the neon repo