Parallel query fails on standby server

Hi All,

While testing a parallel scan feature on standby server, it is found that the parallel query fails with an error "ERROR:  failed to initialize transaction_read_only to 0".

Following are the steps used to reproduce the issue:

Master :-

edb=# create table ert(n int);
edb=# insert into  ert values (generate_series(1,5000000));
edb=# analyze ert;
edb=# vacuum ert;

Slave :-

edb=# set max_parallel_degree =5;
SET
edb=# explain analyze verbose  select * from ert where n<=1000;
ERROR:  failed to initialize transaction_read_only to 0
CONTEXT:  parallel worker, PID 26042

Root cause Analysis:  After debugging the worker, it is observed that in RestoreGUCState(), if a guc var can't be skipped it is Initialiazed with a default value and
in this process when a guc variable "transaction_read_only" is being Initialzed it calls a check_hook check_transaction_read_only() which eventually fails due to
below check which says the guc var "transaction_read_only" can't be set while recovery is in progress:

if (RecoveryInProgress())
{
GUC_check_errcode(ERRCODE_FEATURE_NOT_SUPPORTED);
GUC_check_errmsg("cannot set transaction read-write mode during recovery");
return false;
}

Solution: Make use of a global variable "InitializingParallelWorker" to protect the check for RecoveryInProgress() when Parallel Worker is being Initialsed.
PFA patch to fix the issue.