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From: Tom Lane <[email protected]>
To: Andres Freund <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Justin Pryzby <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: Erik Rijkers <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: TRAP: FailedAssertion("HaveRegisteredOrActiveSnapshot()", File: "toast_internals.c", Line: 670, PID: 19403)
Date: Mon, 18 Apr 2022 10:51:08 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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Andres Freund <[email protected]> writes:
> On 2022-04-17 11:51:58 -0400, Tom Lane wrote:
>> The fact that we have a snapshot at the instant of fetch doesn't prove
>> that it existed continually since we fetched the toast reference,
>> which seems to be the condition we actually need to assure.

> Right.

>> (And TBH I see little reason to think that whether the snapshot is the
>> CatalogSnapshot or not changes things in any meaningful way.)

> It is a meaningful difference, see e.g. the bug referenced above.

Well, that's true given the current arrangements for managing
CatalogSnapshot; but that doesn't make the CatalogSnapshot any
less of a protection when it exists.  The direction I was vaguely
imagining is that we create some refcount-like infrastructure
directly ensuring that once a snapshot is used to read a toast
reference, it gets kept around until we dereference or discard
that reference.  With a scheme like that, there'd be no reason to
discriminate against a CatalogSnapshot as being the protective
snapshot.

(I hasten to add that I have no idea how to make this half-baked
plan work, and there may be better solutions anyway.)

>> While it's something to worry about, there's no reason to think
>> that v15 is any worse than prior versions in this area, is there?
>> So I'm inclined to remove this from the list of v15 open items,
>> or at least demote the remaining concern to "older bug" status.

> Yes.

OK, I'll update the open-items page.

			regards, tom lane

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