RE: Perform streaming logical transactions by background workers and parallel apply

Dear Hou-san,

> I will dig your patch more, but I send partially to keep the activity of the thread.

More minor comments about v28.

===
About 0002

For 015_stream.pl

14. check_parallel_log

```
+# Check the log that the streamed transaction was completed successfully
+# reported by parallel apply worker.
+sub check_parallel_log
+{
+ my ($node_subscriber, $offset, $is_parallel)= @_;
+ my $parallel_message = 'finished processing the transaction finish command';
+
+ if ($is_parallel)
+ {
+ $node_subscriber->wait_for_log(qr/$parallel_message/, $offset);
+ }
+}
```

I think check_parallel_log() should be called only when streaming = 'parallel' and if-statement is not needed

===
For 016_stream_subxact.pl

15. test_streaming

```
+ INSERT INTO test_tab SELECT i, md5(i::text) FROM generate_series( 3, 500) s(i);
```

" 3" should be "3".

===
About 0003

For applyparallelworker.c

16. parallel_apply_relation_check()

```
+ if (rel->parallel_apply_safe == PARALLEL_APPLY_SAFETY_UNKNOWN)
+ logicalrep_rel_mark_parallel_apply(rel);
```

I was not clear when logicalrep_rel_mark_parallel_apply() is called here.
IIUC parallel_apply_relation_check() is called when parallel apply worker handles changes,
but before that relation is opened via logicalrep_rel_open() and parallel_apply_safe is set here.
If it guards some protocol violation, we may use Assert().

===
For create_subscription.sgml

17.
The restriction about foreign key does not seem to be documented.

===
About 0004

For 015_stream.pl

18. check_parallel_log

I heard that the removal has been reverted, but in the patch
check_parallel_log() is removed again... :-(

===
About throughout

I checked the test coverage via `make coverage`. About appluparallelworker.c and worker.c, both function coverage is 100%, and
line coverages are 86.2 % and 94.5 %. Generally it's good.
But I read the report and following parts seems not tested.

In parallel_apply_start_worker():

```
if (tmp_winfo->error_mq_handle == NULL)
{
/*
* Release the worker information and try next one if the parallel
* apply worker exited cleanly.
*/
ParallelApplyWorkersList = foreach_delete_current(ParallelApplyWorkersList, lc);
shm_mq_detach(tmp_winfo->mq_handle);
dsm_detach(tmp_winfo->dsm_seg);
pfree(tmp_winfo);
}
```

In HandleParallelApplyMessage():

```
case 'X': /* Terminate, indicating clean exit */
{
shm_mq_detach(winfo->error_mq_handle);
winfo->error_mq_handle = NULL;
break;
}
```

Does it mean that we do not test the termination of parallel apply worker? If so I think it should be tested.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED