Re: Disallow altering invalidated replication slots

Hi, here are some review comments for patch v1.

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Commit message

1.
ALTER_REPLICATION_SLOT on invalidated replication slots is unnecessary
as there is no way...

suggestion:
ALTER_REPLICATION_SLOT for invalid replication slots should not be
allowed because there is no way...

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2. Missing docs update

Should this docs page [1] be updated to say ALTER_REPLICATION_SLOT is
not allowed for invalid slots?

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src/backend/replication/slot.c

3.
+ if (MyReplicationSlot->data.invalidated != RS_INVAL_NONE)
+ ereport(ERROR,
+ errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("cannot alter replication slot \"%s\"", name),
+ errdetail("This replication slot was invalidated due to \"%s\".",
+ SlotInvalidationCauses[MyReplicationSlot->data.invalidated]));
+

I thought including the reason "invalid" (e.g. "cannot alter invalid
replication slot \"%s\"") in the message might be better, but OTOH I
see the patch message is the same as an existing one. Maybe see what
others think.

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src/test/recovery/t/035_standby_logical_decoding.pl

3.
There is already a comment about this test:
##################################################
# Recovery conflict: Invalidate conflicting slots, including in-use slots
# Scenario 1: hot_standby_feedback off and vacuum FULL
#
# In passing, ensure that replication slot stats are not removed when the
# active slot is invalidated.
##################################################

IMO we should update that "In passing..." sentence to something like:

In passing, ensure that replication slot stats are not removed when
the active slot is invalidated, and check that an error occurs when
attempting to alter the invalid slot.

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[1] docs - https://www.postgresql.org/docs/devel/protocol-replication.html

Kind Regards,
Peter Smith.
Fujitsu Austalia