From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Daniel Verite" <daniel@manitou-mail.org>
cc: "Rahila Syed" <rahilasyed90@gmail.com>,
        "Stephen Frost" <sfrost@snowman.net>,
        "Ashutosh Bapat" <ashutosh.bapat@enterprisedb.com>,
        "pgsql-hackers" <pgsql-hackers@postgresql.org>,
        "Andrew Dunstan" <andrew@dunslane.net>
Subject: Re: Improvements in psql hooks for variables
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Comments: In-reply-to "Daniel Verite" <daniel@manitou-mail.org>
	message dated "Tue, 31 Jan 2017 18:12:03 +0100"
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Date: Tue, 31 Jan 2017 13:07:22 -0500
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"Daniel Verite" <daniel@manitou-mail.org> writes:
> I notice that in the commited patch, you added the ability
> for DeleteVariable() to reject the deletion if the hook
> disagrees. 

Right.

> But this can't happen in practice because as mentioned just upthread
> the hook called with NULL doesn't know if the variable is getting unset
> or initialized, so rejecting on NULL is not an option.

It would have required the caller to set a value before installing the
hook, which would require some reshuffling of responsibility.  In the
draft patch I sent a little bit ago, this is handled by installing a
"substitution hook" first, and that hook transmogrifies NULL into an
allowed setting.  That gets to the same place in a slightly different
way, but it also covers allowing "\unset FOO", which inserting initial
values wouldn't.

> Attached is a proposed patch to add initial values to
> SetVariableAssignHook() to solve this problem, and an example for
> \unset AUTOCOMMIT being denied by the hook.

I think disallowing \unset is a nonstarter on compatibility grounds.
We should allow \unset but treat it like setting to "off" (or whatever
the default value is).

			regards, tom lane


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