BUG #15791: WalSndCtl->sync_standbys_defined modified too late

The following bug has been logged on the website:

Bug reference: 15791
Logged by: DamionZ Zhao
Email address: zhq651@126.com
PostgreSQL version: 11.2
Operating system: linux
Description:

1.when commit transaction, we need to kwow it is sync or async . There is a
step : call function SyncRepWaitForLSN in function
RecordTransactionCommit.

void SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
{
....
if (!WalSndCtl->sync_standbys_defined ||
lsn <= WalSndCtl->lsn[mode] )
{
...
return;
}
}

2.when modify WalSndCtl->sync_standbys_defined?

2.1 in function SyncRepUpdateSyncStandbysDefined,
WalSndCtl->sync_standbys_defined will be modified.
----------------------------------------------------
void SyncRepUpdateSyncStandbysDefined(void)
{
bool sync_standbys_defined = SyncStandbysDefined();

if (sync_standbys_defined != WalSndCtl->sync_standbys_defined)
{

WalSndCtl->sync_standbys_defined = sync_standbys_defined;
LWLockRelease(SyncRepLock);
}
}

2.2 SyncRepUpdateSyncStandbysDefined call stack is:
------------------------------------------
(1)CheckpointerMain（start checkpoint）-
UpdateSharedMemoryConfig
SyncRepUpdateSyncStandbysDefined
(2)CheckPointGuts -- Flush all data in shared memory to disk, and fsync
CheckPointBuffers --Flush all dirty blocks in buffer pool to disk at
checkpoint time.
BufferSync -- Write out all dirty buffers in the pool.
CheckpointWriteDelay
UpdateSharedMemoryConfig
SyncRepUpdateSyncStandbysDefined

2.3 when any checkpoint situation happened, WalSndCtl->sync_standbys_defined
will be modified.

3.my question:
----------------

when modify synchronous_standby_names in postgres.conf, and no checkpoint
happenes, will synchronous_standby_names take effect?