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Re: Snapshot synchronization, again...

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Joachim Wieland <joe(at)mcknight(dot)de>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Snapshot synchronization, again...
Date: 2011-02-22 13:52:34
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
On Tue, Feb 22, 2011 at 8:01 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> This is hashing, not encryption, there is no key. The point is that even if
> the attacker has the hash value and knows the algorithm, he can not
> construct *another* snapshot that has the same hash.

What good does that do us?

> Yes. It would be good to perform those sanity checks anyway.

I don't think it's good; I think it's absolutely necessary.  Otherwise
someone can generate arbitrary garbage, hash it, and feed it to us.

> But even if we don't allow it, there's no harm in sending the whole snapshot
> to the client, anyway. Ie. instead of "1" as the identifier, use the
> snapshot itself. That leaves the door open for allowing it in the future,
> should we choose to do so.

The door is open either way, AFAICS: we could eventually allow:

and also
BEGIN TRANSACTION (SNAPSHOT '{xmin 123 xmax 456 xids 128 149 201}');

Robert Haas
The Enterprise PostgreSQL Company

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