While creating an "export snapshot" I don't see any protection why the number of xids in the snapshot can not cross the "GetMaxSnapshotXidCount()"?.
Basically, while converting the HISTORIC snapshot to the MVCC snapshot in "SnapBuildInitialSnapshot()", we add all the xids between snap->xmin to snap->xmax to the MVCC snap->xip array (xids for which commit were not recorded). The problem is that we add both topxids as well as the subxids into the same array and expect that the "xid" count does not cross the "GetMaxSnapshotXidCount()". So it seems like an issue but I am not sure what is the fix for this, some options are a) Don't limit the xid count in the exported snapshot and dynamically resize the array b) Increase the limit to GetMaxSnapshotXidCount() + GetMaxSnapshotSubxidCount(). But in option b) there would still be a problem that how do we handle the overflowed subtransaction? I have locally, reproduced the issue, 1. Configuration max_connections= 5 autovacuum = off max_worker_processes = 0 2.Then from pgbench I have run the attached script (test.sql) from 5 clients. ./pgbench -i postgres ./pgbench -c4 -j4 -T 3000 -f test1.sql -P1 postgres 3. Concurrently, create replication slot, [dilipkumar@localhost bin]$ ./psql "dbname=postgres replication=database" postgres[7367]=# postgres[6463]=# CREATE_REPLICATION_SLOT "slot" LOGICAL "test_decoding"; ERROR: 40001: initial slot snapshot too large LOCATION: SnapBuildInitialSnapshot, snapbuild.c:597 postgres[6463]=# CREATE_REPLICATION_SLOT "slot" LOGICAL "test_decoding"; ERROR: XX000: clearing exported snapshot in wrong transaction state LOCATION: SnapBuildClearExportedSnapshot, snapbuild.c:690 I could reproduce this issue, at least once in 8-10 attempts of creating the replication slot. Note: After that issue, I have noticed one more issue "clearing exported snapshot in wrong transaction state", that is because the "ExportInProgress" is not cleared on the transaction abort, for this, a simple fix is we can clear this state on the transaction abort, maybe I will raise this as a separate issue? -- Regards, Dilip Kumar EnterpriseDB: http://www.enterprisedb.com
test.sql
Description: application/sql
