Fix lsn for KEEPALIVE action.

[shubhamdhama]

It fixes an issue under the following conditions,

1. Source doesn't have any activity.
2. endpos is set to current LSN.
3. The action that crosses the endpos is KEEPALIVE.

in this case, the written lsn that corresponds to KEEPALIVE isn't correct, i.e. less than endpos. This ultimately leads to Apply process waiting for a statement >= endpos which never happens, hence it never terminates.

[shubhamdhama]

Draft as I will write a unit test when no activity on source happens (sort of like edge case).
I'm not able to come up with a test case where I can deterministically set the endpos right after a K action because we also have B/C action due to activity on the sentinel table. So I was thinking the best we can do is create a test case where we have no activity for 10 seconds, prefetch, and put a timeout on apply for 10s so that migration should finish quickly.

Also, I have a question about some other tests (cdc/*) I see that Keepalive is in the expected JSON. How is every test run emitting the Keepalive at the same position relative to other actions?

[dimitri]

My reading of the Postgres source code makes me think that this PR is okay, we actually want to advance to WalEnd when receiving a keepalive message. See function WalSndUpdateProgress in src/backend/replication/walsender.c:

	 * When skipping empty transactions in synchronous replication, we send a
	 * keepalive message to avoid delaying such transactions.

About the test cases and the Keepalive (and other messages) position, we just skip comparing them because each run may provide a different one...

[shubhamdhama]

Thanks for confirming that this fix is correct.

I've published the PR since this change is important and I'm couldn't finish the unit test I mentioned. I will share them in a separate PR once I am done.

[dimitri]

It seems you have added an extra KeepAlive message in some of the tests output, and the expected file has not been updated by your PR so tests are failing. Please see about updating the expected tests results.

[shubhamdhama]

Oh right, I have updated the PR.