Bug in logical decoding of in-progress transactions

Hi,

There is a recent build farm failure [1] in one of the test_decoding
tests as pointed by Tom Lane [2]. The failure report is shown below:

@@ -71,6 +71,8 @@
data
------------------------------------------
opening a streamed block for transaction
+ closing a streamed block for transaction
+ opening a streamed block for transaction
streaming change for transaction
streaming change for transaction
streaming change for transaction
@@ -83,7 +85,7 @@
streaming change for transaction
closing a streamed block for transaction
committing streamed transaction
-(13 rows)
+(15 rows)

Here, the symptoms are quite similar to what we have fixed in commit
82a0ba7707 which is that an extra empty transaction is being decoded
in the test. It can happen even if have instructed the test to 'skip
empty xacts' for streaming transactions because the test_decoding
plugin APIs (related to streaming changes for in-progress xacts) makes
no effort to skip such empty xacts. It was kept intentionally like
that under the assumption that we would never try to stream empty
xacts but on closer inspection of the code, it seems to me that
assumption was not correct. Basically, we can pick to stream a
transaction that has change messages for
REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT and we don't send such
messages to downstream rather they are just to update the internal
state. So, in this particular failure, it is possible that autovacuum
transaction has got such a change message added by one of the other
committed xact and on trying to stream it we get such additional
messages. The fix is to skip empty xacts when indicated by the user in
streaming APIs of test_decoding.

Thoughts?

[1] - https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2020-09-09+03%3A42%3A19
[2] - https://www.postgresql.org/message-id/118303.1599691636%40sss.pgh.pa.us

--
With Regards,
Amit Kapila.