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

Dear Hou,

Thanks for updating the patch!
I tested the case whether the deadlock caused by foreign key constraint could be
detected, and it worked well.

Followings are my review comments. They are basically related with 0001, but
some contents may be not. It takes time to understand 0002 correctly...

01. typedefs.list

LeaderFileSetState should be added to typedefs.list.

02. 032_streaming_parallel_apply.pl

As I said in [1]: the test name may be not matched. Do you have reasons to
revert the change?

03. 032_streaming_parallel_apply.pl

The test does not cover the case that the backend process relates with the
deadlock. IIUC this is another motivation to use a stream/transaction lock.
I think it should be added.

04. log output

While being applied spooled changes by PA, there are so many messages like
"replayed %d changes from file..." and "applied %u changes...". They comes from
apply_handle_stream_stop() and apply_spooled_messages(). They have same meaning,
so I think one of them can be removed.

05. system_views.sql

In the previous version you modified pg_stat_subscription system view. Why do you revert that?

06. interrupt.c - SignalHandlerForShutdownRequest()

In the comment atop SignalHandlerForShutdownRequest(), some processes that assign the function
except SIGTERM are clarified. We may be able to add the parallel apply worker.

07. proto.c - logicalrep_write_stream_abort()

We may able to add assertions for abort_lsn and abort_time, like xid and subxid.

08. guc_tables.c - ConfigureNamesInt

```
&max_sync_workers_per_subscription,
+ 2, 0, MAX_PARALLEL_WORKER_LIMIT,
+ NULL, NULL, NULL
+ },
```

The upper limit for max_sync_workers_per_subscription seems to be wrong, it should
be used for max_parallel_apply_workers_per_subscription.

10. worker.c - maybe_reread_subscription()

```
+ if (am_parallel_apply_worker())
+ ereport(LOG,
+ /* translator: first %s is the name of logical replication worker */
+ (errmsg("%s for subscription \"%s\" will stop because of a parameter change",
+ get_worker_name(), MySubscription->name)));
```

I was not sure get_worker_name() is needed. I think "logical replication apply worker"
should be embedded.

11. worker.c - ApplyWorkerMain()

```
+ (errmsg_internal("%s for subscription \"%s\" two_phase is %s",
+ get_worker_name(),
```

The message for translator is needed.

[1]: https://www.postgresql.org/message-id/TYAPR01MB58666A97D40AB8919D106AD5F5709%40TYAPR01MB5866.jpnprd01.prod.outlook.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED