Re: Single transaction in the tablesync worker?

Hi Amit.

PSA my v6 WIP patch for the Solution1.

This patch still applies onto the v30 patch set [1] from other 2PC thread:
[1] https://www.postgresql.org/message-id/CAFPTHDYA8yE6tEmQ2USYS68kNt%2BkM%3DSwKgj%3Djy4AvFD5e9-UTQ%40mail.gmail.com

(I understand you would like this to be delivered as a separate patch
independent of v30. I will convert it ASAP)

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Coded / WIP:

* tablesync slot is now permanent instead of temporary. The tablesync
slot name is no longer tied to the Subscription slot name.

* the tablesync slot cleanup (drop) code is added for DropSubscription
and for finish_sync_worker functions

* tablesync worked now allowing multiple tx instead of single tx

* a new state (SUBREL_STATE_COPYDONE) is persisted after a successful
copy_table in LogicalRepSyncTableStart.

* if a relaunched tablesync finds the state is SUBREL_STATE_COPYDONE
then it will bypass the initial copy_table phase.

* tablesync sets up replication origin tracking in
LogicalRepSyncTableStart (similar as done for the apply worker). The
origin is advanced when first created.

* tablesync replication origin tracking is cleaned up during
DropSubscription and/or process_syncing_tables_for_apply

TODO / Known Issues:

* Crashed tablesync workers may not be known to DropSubscription
current code. This might be a problem to cleanup slots and/or origin
tracking belonging to those unknown workers.

* There seems to be a race condition during DROP SUBSCRIPTION. It
manifests as the TAP test 007 hanging. Logging shows it seems to be
during replorigin_drop when called from DropSubscription. It is timing
related and quite rare - e.g. Only happens 1x every 10x running
subscription TAP tests.

* Help / comments / cleanup

* There is temporary "!!>>" excessive logging of mine scattered around
which I added to help my testing during development

* Address review comments

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Kind Regards,
Peter Smith.
Fujitsu Australia