[YSQL] Enable Batch Transaction on COPY FROM STDIN

[emhna]

Background

In Postgres, there is a flag called ybDataSent in the TransactionState structure.
When output buffer is flushed, the flag gets set to true, marking that we cannot do a transparent restart anymore.

In CopyFrom() method, prior to copying, ybDataSent flag is also used to verify if the current query is inside another transaction block (ie. nested transaction).
If so, an error will be thrown to disable rows_per_transaction option from being used.
This is to prevent previous transactions to get committed while running batch commits.

Problem

Today, when COPY FROM query is sourced from STDIN, it runs ReceiveCopyBegin() function to flush output buffer and ensure front-end knows it can send.
While flushing, 'ybDataSent' flag also gets set to true even though it is not inside a nested transaction.

Eg.

create table t (a int);
copy t from stdin with (rows_per_transaction 10); 
>> ERROR: ROWS_PER_TRANSACTION option is not supported in nested transaction

Solution

The flag should be explicitly turned off when it is not inside a nested transaction.
Before executing ReceiveCopyBegin() function, we should store the current state of ybDataSent to determine if inside a nested transaction.
After executing the flush,

• switch off flag (false) if inside transaction block
• keep the flag turned on (true) if outside transaction block.