Re: Minimal logical decoding on standbys

On Wed, Apr 5, 2023 at 3:58 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Apr 5, 2023 at 2:41 PM Drouvot, Bertrand
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> minor nitpick:
> +
> + /* Intentional fall through to session cancel */
> + /* FALLTHROUGH */
>
> Do we need to repeat fall through twice in different ways?
>

Few minor comments on 0003:
========================
1.
+ case XLOG_PARAMETER_CHANGE:
+ {
+ xl_parameter_change *xlrec =
+ (xl_parameter_change *) XLogRecGetData(buf->record);
+
+ /*
+ * If wal_level on primary is reduced to less than logical,
+ * then we want to prevent existing logical slots from being
+ * used. Existing logical slots on standby get invalidated
+ * when this WAL record is replayed; and further, slot
+ * creation fails when the wal level is not sufficient; but
+ * all these operations are not synchronized, so a logical
+ * slot may creep in while the wal_level is being reduced.
+ * Hence this extra check.
+ */
+ if (xlrec->wal_level < WAL_LEVEL_LOGICAL)
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("logical decoding on standby requires wal_level to be at
least logical on the primary server")));

By looking at this change, it is not very clear that this can occur
only on standby. I understand that on primary, we will not allow
restarting the server after changing wal_level if there is a
pre-existing slot but still this looks a bit odd. Shall we have an
Assert to indicate that this will occur only on standby?

2.
/*
- * Since logical decoding is only permitted on a primary server, we know
- * that the current timeline ID can't be changing any more. If we did this
- * on a standby, we'd have to worry about the values we compute here
- * becoming invalid due to a promotion or timeline change.
+ * Since logical decoding is also permitted on a standby server, we need
+ * to check if the server is in recovery to decide how to get the current
+ * timeline ID (so that it also cover the promotion or timeline change
+ * cases).
*/
+
+ /* make sure we have enough WAL available */
+ flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
+
+ /* the standby could have been promoted, so check if still in recovery */
+ am_cascading_walsender = RecoveryInProgress();

The first part of the comment explains why it is important to check
RecoveryInProgress() and then immediately after that, the patch
invokes WalSndWaitForWal(). It may be better to move the comment after
WalSndWaitForWal() invocation. Also, it will be better to write a
comment as to why you need to do WalSndWaitForWal() before retrieving
the current timeline as previously that was done afterward.

--
With Regards,
Amit Kapila.