RE: [PoC] pg_upgrade: allow to upgrade publisher node

Dear Amit,

Thank you for reviewing! PSA new version patch set.

> Few comments:
> 1. Why is the FPI record (XLOG_FPI_FOR_HINT) not considered a record
> to be ignored? This can be generated during reading system tables.

Oh, I just missed. Written in comments atop the function, but not added here.
Added to white-list.

> 2.
> +binary_upgrade_validate_wal_record_types_after_lsn(PG_FUNCTION_ARGS)
> {
> ...
> + if (initial_record)
> + {
> + /* Initial record must be XLOG_CHECKPOINT_SHUTDOWN */
> + if (!CHECK_WAL_RECORD(rmid, info, RM_XLOG_ID,
> +   XLOG_CHECKPOINT_SHUTDOWN))
> + result = false;
> ...
> + if (!CHECK_WAL_RECORD(rmid, info, RM_XLOG_ID,
> XLOG_CHECKPOINT_SHUTDOWN) &&
> + !CHECK_WAL_RECORD(rmid, info, RM_XLOG_ID,
> XLOG_CHECKPOINT_ONLINE) &&
> + !CHECK_WAL_RECORD(rmid, info, RM_STANDBY_ID,
> XLOG_RUNNING_XACTS) &&
> + !CHECK_WAL_RECORD(rmid, info, RM_HEAP2_ID, XLOG_HEAP2_PRUNE))
> + result = false;
> ...
> }
> 
> Isn't it better to immediately return false if any unexpected WAL is
> found? This will avoid reading unnecessary WAL

IIUC we can exit the loop of the result == false, so we do not have to read
unnecessary WALs. See the condition below. I used the approach because
private_data and xlogreader should be pfree()'d as cleanup.

```
	/* Loop until all WALs are read, or unexpected record is found */
	while (result && ReadNextXLogRecord(xlogreader))
	{
```

> 3.
> +Datum
> +binary_upgrade_validate_wal_record_types_after_lsn(PG_FUNCTION_ARGS)
> +{
> ...
> +
> + CHECK_IS_BINARY_UPGRADE;
> +
> + /* Quick exit if the given lsn is larger than current one */
> + if (start_lsn >= curr_lsn)
> + PG_RETURN_BOOL(true);
> 
> Why do you return true here? My understanding was if the first record
> is not a shutdown checkpoint record then it should fail, if that is
> not true then I think we need to explain the same in comments.

I wondered what should be because it is unexpected input for us (note that this 
unction could be used only for upgrade purpose). But yes, initially read WAL must
be XLOG_SHUTDOWN_CHECKPOINT,  so changed as you said.

Also, I did a self-reviewing again and reworded comments.

BTW, the 0002 ports some functions from pg_walinspect, it may be not elegant.
Coupling degree between core/extensions should be also lower. So I made another
patch which does not port anything and implements similar functionalities instead.
I called the patch 0003, but can be applied atop 0001 (not 0002). To make cfbot
happy, attached as txt file.
Could you please tell me which do you like 0002 or 0003?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED