Fix: MVCC design on replica and replica stream isolation

[as51340]

Description

Jepsen found a few bugs in replication, this PR fixes two of them.

When the replica recovered completely, on the next txn replication, it expected replica_stream_ to be already destroyed. There MG_ASSERT failed. Now, the replica stream is created as a local variable for transactions.

Another bug is related to the MVCC of the replica. Because REPLICA’s transaction timestamps are contingent on MAIN’s timestamps, resulting in the transaction timestamp counter (used for obtaining the start transaction ID, and commit timestamp ID) staying the same when the READ transaction started, MVCC guarantees were broken. Any time the WRITE transaction would commit before the READ transaction would finish, GC could incorrectly unlink deltas as the WRITE transaction would mark its start timestamp as finished, not knowing that the READ transaction is still in the system. This would result in the replica’s READ query reading two different data snapshots during a single transaction.

[master < Task] PR

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  • Fix: MVCC design on replica and replica stream isolation

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[Ignition]

Few minor comments, otherwise very good

[andrejtonev]

Makes sense to me.

[sonarcloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.8% Duplication on New Code

See analysis details on SonarCloud

[kgolubic]

@as51340 I will use this as RN:

The replication stream is now handled as a local variable for transactions, preventing assertion failures during recovery.