Date: Sat, 3 Aug 2019 00:22:54 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Ian Barwick <ian.barwick@2ndquadrant.com>,
	Stephen Frost <sfrost@snowman.net>, Tom Lane <tgl@sss.pgh.pa.us>,
	Robert Haas <robertmhaas@gmail.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: [PATCH] Stop ALTER SYSTEM from making bad assumptions
Message-ID: <20190802222254.qsvqjtkwte464lu3@development>
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Hi,

This thread discusses an issue that's tracked as an open item for pg12,
but it's been quiet for the last ~1 month. I think it's probably time to
decide what to do with it. The thread is a bit long, so let me sum what
the issue is and what options we have.

The problem is that ALTER SYSTEM does not handle duplicate entries in
postgresql.auto.conf file correctly, because it simply modifies the
first item, but the value is then overridden by the duplicate items.
This contradicts the idea that duplicate GUCs are allowed, and that we
should use the last item.

This bug seems to exist since ALTER SYSTEM was introduced, so it's not
a clear PG12 item. But it was made more prominent by the removal of
recovery.conf in PG12, because pg_basebackup now appends stuff to
postgresql.auto.conf and may easily create duplicate items.


There seems to be a consensus that this this not a pg_basebackup issue
(i.e. duplicate values don't make the file invalid), and it should be
handled in ALTER SYSTEM.

The proposal seems to be to run through the .auto.conf file, remove any
duplicates, and append the new entry at the end. That seems reasonable.

There was a discussion whether to print warnings about the duplicates. I
personally see not much point in doing that - if we consider duplicates
to be expected, and if ALTER SYSTEM has the license to rework the config
file any way it wants, why warn about it?

The main issue however is that no code was written yet.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services