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

Dear Hou,

Thanks for making the patch. Followings are my comments for v54-0003 and 0004.

0003

pa_free_worker()

+ /* Unlink any files that were needed to serialize partial changes. */
+ if (winfo->serialize_changes)
+ stream_cleanup_files(MyLogicalRepWorker->subid, winfo->shared->xid);
+

I think this part is not needed, because the LA cannot reach here if winfo->serialize_changes is true. Moreover stream_cleanup_files() is done in pa_free_worker_info().

LogicalParallelApplyLoop()

The parallel apply worker wakes up every 0.1s even if we are in the PARTIAL_SERIALIZE mode. Do you have idea to reduce that?

```
+ pa_spooled_messages();
```

Comments are needed here, like "Changes may be serialize...".

pa_stream_abort()

```
+ /*
+ * Reopen the file and set the file position to the saved
+ * position.
+ */
+ if (reopen_stream_fd)
+ {
+ char path[MAXPGPATH];
+
+ changes_filename(path, MyLogicalRepWorker->subid, xid);
+ stream_fd = BufFileOpenFileSet(&MyParallelShared->fileset,
+ path, O_RDONLY, false);
+ BufFileSeek(stream_fd, fileno, offset, SEEK_SET);
+ }
```

MyParallelShared->serialize_changes may be used instead of reopen_stream_fd.

worker.c

```
-#include "storage/buffile.h"
```

I think this include should not be removed.

handle_streamed_transaction()

```
+ if (apply_action == TRANS_LEADER_SEND_TO_PARALLEL)
+ pa_send_data(winfo, s->len, s->data);
+ else
+ stream_write_change(action, &original_msg);
```

Comments are needed here, 0001 has that bu removed in 0002.
There are some similar lines.

```
+ /*
+ * It is possible that while sending this change to parallel apply
+ * worker we need to switch to serialize mode.
+ */
+ if (winfo->serialize_changes)
+ pa_set_fileset_state(winfo->shared, FS_READY);
```

There are three same parts in the code, can we combine them to common part?

apply_spooled_messages()

```
+ /*
+ * Break the loop if the parallel apply worker has finished applying
+ * the transaction. The parallel apply worker should have closed the
+ * file before committing.
+ */
+ if (am_parallel_apply_worker() &&
+ MyParallelShared->xact_state == PARALLEL_TRANS_FINISHED)
+ goto done;
```

I thnk pfree(buffer) and pfree(s2.data) should not be skippied.
And this part should be at below "nchanges++;"

0004

set_subscription_retry()

```
+ LockSharedObject(SubscriptionRelationId, MySubscription->oid, 0,
+ AccessShareLock);
+
```

I think AccessExclusiveLock should be aquired instead of AccessShareLock.
In AlterSubscription(), LockSharedObject(AccessExclusiveLock) seems to be used.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED