BUG #14279: Logical decoding misses a transaction completely
The following bug has been logged on the website:
Bug reference: 14279
Logged by: Marko Tiikkaja
Email address: marko@joh.to
PostgreSQL version: 9.5.3
Operating system: Linux
Description:
Hi,
The following transaction does not get decoded at all in logical decoding:
BEGIN;
INSERT INTO foo VALUES ('bar');
SAVEPOINT s;
SELECT 1 FROM foo FOR UPDATE;
RELEASE SAVEPOINT s;
COMMIT;
The problem seems to be that ReorderBufferCommitChild() overwrites the main
transaction's base_snapshot with a NULL because it thinks the
subtransaction's base_snapshot_lsn=0 is older than the main transaction's
actual snapshot, which in ReorderBufferCommit:
/*
* If this transaction didn't have any real changes in our database,
it's
* OK not to have a snapshot. Note that ReorderBufferCommitChild will
have
* transferred its snapshot to this transaction if it had one and the
* toplevel tx didn't.
*/
if (txn->base_snapshot == NULL)
{
Assert(txn->ninvalidations == 0);
ReorderBufferCleanupTXN(rb, txn);
return;
}
causes the entire transaction to be skipped. I didn't debug further.
(Thanks to Andres Freund and Andrew Gierth for helping me track this problem
down!)
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"marko" == marko <marko@joh.to> writes:
marko> The problem seems to be that ReorderBufferCommitChild()
marko> overwrites the main transaction's base_snapshot with a NULL
marko> because it thinks the subtransaction's base_snapshot_lsn=0 is
marko> older than the main transaction's actual snapshot
i.e. here in ReorderBufferCommitChild:
/*
* Pass the our base snapshot to the parent transaction if it doesn't have
* one, or ours is older. That can happen if there are no changes in the
* toplevel transaction but in one of the child transactions. This allows
* the parent to simply use it's base snapshot initially.
*/
if (txn->base_snapshot == NULL ||
txn->base_snapshot_lsn > subtxn->base_snapshot_lsn)
{
it's possible for subtxn to exist, but for subtxn->base_snapshot to be
NULL and base_snapshot_lsn to be 0, and obviously propagating this to
txn is wrong.
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Andrew (irc:RhodiumToad)
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Ping. This is a very serious issue and it would be really good to
include it in the minor releases. I've attached a patch for 9.5 with a
test case and verified that it fixes the problem with our application.
.m
Attachments:
bug_14279.patchtext/x-patch; name=bug_14279.patchDownload+46-13
On Sun, Aug 7, 2016 at 7:49 PM, Marko Tiikkaja <marko@joh.to> wrote:
Ping. This is a very serious issue and it would be really good to include
it in the minor releases. I've attached a patch for 9.5 with a test case
and verified that it fixes the problem with our application.
Right. That's up to Robert or Andres to pick up that at this point. I
just added them in CC for awareness.
--
Michael
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Michael Paquier <michael.paquier@gmail.com> writes:
On Sun, Aug 7, 2016 at 7:49 PM, Marko Tiikkaja <marko@joh.to> wrote:
Ping. This is a very serious issue and it would be really good to include
it in the minor releases. I've attached a patch for 9.5 with a test case
and verified that it fixes the problem with our application.
Right. That's up to Robert or Andres to pick up that at this point. I
just added them in CC for awareness.
Andres is on vacation, and if Robert is around it'd be better for him to
be looking at the toast snapshot problem. I can pick this up, probably.
regards, tom lane
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On Mon, Aug 8, 2016 at 1:14 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Michael Paquier <michael.paquier@gmail.com> writes:
On Sun, Aug 7, 2016 at 7:49 PM, Marko Tiikkaja <marko@joh.to> wrote:
Ping. This is a very serious issue and it would be really good to include
it in the minor releases. I've attached a patch for 9.5 with a test case
and verified that it fixes the problem with our application.Right. That's up to Robert or Andres to pick up that at this point. I
just added them in CC for awareness.Andres is on vacation, and if Robert is around it'd be better for him to
be looking at the toast snapshot problem. I can pick this up, probably.
I was going to review that, until I noticed that it has already been
addressed by bcbecbc. Thanks!
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Michael
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On 2016-08-07 12:14:02 -0400, Tom Lane wrote:
Michael Paquier <michael.paquier@gmail.com> writes:
On Sun, Aug 7, 2016 at 7:49 PM, Marko Tiikkaja <marko@joh.to> wrote:
Ping. This is a very serious issue and it would be really good to include
it in the minor releases. I've attached a patch for 9.5 with a test case
and verified that it fixes the problem with our application.Right. That's up to Robert or Andres to pick up that at this point. I
just added them in CC for awareness.Andres is on vacation, and if Robert is around it'd be better for him to
be looking at the toast snapshot problem. I can pick this up, probably.
Thanks for that. I was too tired to address this before the vacation,
sorry for that.
Andres
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