Обсуждение: [HACKERS] [PATCH] Fix drop replication slot blocking instead of returning error

On Thu, Aug 24, 2017 at 03:38:20PM +0200, Simone Gotti wrote:
> I noticed that in postgres 10beta3, calling pg_drop_replication_slot on an
> active slot will block until it's released instead of returning an error
> like
> done in pg 9.6. Since this is a change in the previous behavior and the docs
> wasn't changed I made a patch to restore the previous behavior.
> 
> Thanks,
> 
> Simone.
> 
> --
> 
> after git commit 9915de6c1cb calls to pg_drop_replication_slot or the
> replication protocol DROP_REPLICATION_SLOT command will block when a
> slot is in use instead of returning an error as before (as the doc
> states).
> 
> This patch will set nowait to true to restore the previous
> behavior.

[Action required within three days.  This is a generic notification.]

The above-described topic is currently a PostgreSQL 10 open item.  Álvaro,
since you committed the patch believed to have created it, you own this open
item.  If some other commit is more relevant or if this does not belong as a
v10 open item, please let us know.  Otherwise, please observe the policy on
open item ownership[1] and send a status update within three calendar days of
this message.  Include a date for your subsequent status update.  Testers may
discover new open items at any time, and I want to plan to get them all fixed
well in advance of shipping v10.  Consequently, I will appreciate your efforts
toward speedy resolution.  Thanks.

[1] https://www.postgresql.org/message-id/20170404140717.GA2675809%40tornado.leadboat.com

Noah Misch wrote:
> On Thu, Aug 24, 2017 at 03:38:20PM +0200, Simone Gotti wrote:
> > I noticed that in postgres 10beta3, calling pg_drop_replication_slot on an
> > active slot will block until it's released instead of returning an error
> > like
> > done in pg 9.6. Since this is a change in the previous behavior and the docs
> > wasn't changed I made a patch to restore the previous behavior.

> > after git commit 9915de6c1cb calls to pg_drop_replication_slot or the
> > replication protocol DROP_REPLICATION_SLOT command will block when a
> > slot is in use instead of returning an error as before (as the doc
> > states).
> > 
> > This patch will set nowait to true to restore the previous
> > behavior.

> The above-described topic is currently a PostgreSQL 10 open item.

Looking at it now -- next report tomorrow.


-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Simone Gotti wrote:
> Hi all,
> 
> I noticed that in postgres 10beta3, calling pg_drop_replication_slot on an
> active slot will block until it's released instead of returning an error
> like
> done in pg 9.6. Since this is a change in the previous behavior and the docs
> wasn't changed I made a patch to restore the previous behavior.

Changing that behavior was the entire point of the cited commit.

A better fix, from my perspective, is to amend the docs as per the
attached patch.  This is what would be useful for logical replication,
which is what replication slots were invented for in the first place.
If you disagree, let's discuss what other use cases you have, and we can
come up with alternatives that satisfy both.  I think a decent answer,
but one which would create a bit of extra churn, would be to have an
optional boolean flag in the command/function for "nowait", instead of
hardcoding either behavior.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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On Tue, Aug 29, 2017 at 12:13 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
>

Hi Alvaro,

> Simone Gotti wrote:
> > Hi all,
> >
> > I noticed that in postgres 10beta3, calling pg_drop_replication_slot on an
> > active slot will block until it's released instead of returning an error
> > like
> > done in pg 9.6. Since this is a change in the previous behavior and the docs
> > wasn't changed I made a patch to restore the previous behavior.
>
> Changing that behavior was the entire point of the cited commit.

Sorry, I was thinking that the new behavior was needed for internal
future functions since the doc wasn't changed.

>
> A better fix, from my perspective, is to amend the docs as per the
> attached patch.  This is what would be useful for logical replication,
> which is what replication slots were invented for in the first place.

I don't know the reasons why the new behavior is better for logical
replication so I trust you. We are using repl slots for physical
replication.

> If you disagree, let's discuss what other use cases you have, and we can
> come up with alternatives that satisfy both.

I just faced the opposite problem, in stolon [1], we currently rely on
the previous behavior. i.e. we don't want to block waiting for a slot
to be released (that under some partitioning conditions could not
happen for a long time), but prefer to continue retrying the drop
later. Now we partially avoid blocking timing out the drop operation
after some seconds.
Another idea will be to query the slot status before doing the drop
but will lead to a race condition (probably the opposite that that
commit was fixing) if the slot is acquired between the query and the
drop.

> I think a decent answer,
> but one which would create a bit of extra churn, would be to have an
> optional boolean flag in the command/function for "nowait", instead of
> hardcoding either behavior.

I think that this will be the best fix. I'm not sure on the policy of
these commands and if backward compatibility will be better (in such
case the old behavior should be the default and a new "wait" flag
could be added).

If the default behavior is going to change we have to add different
code for postgres >= 10.


Thanks,
Simone.


[1] https://github.com/sorintlab/stolon

>
>
> --
> Álvaro Herrera                https://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Hi,

On 2017-08-29 13:42:05 +0200, Simone Gotti wrote:
> On Tue, Aug 29, 2017 at 12:13 PM, Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
> >
> 
> Hi Alvaro,
> 
> > Simone Gotti wrote:
> > > Hi all,
> > >
> > > I noticed that in postgres 10beta3, calling pg_drop_replication_slot on an
> > > active slot will block until it's released instead of returning an error
> > > like
> > > done in pg 9.6. Since this is a change in the previous behavior and the docs
> > > wasn't changed I made a patch to restore the previous behavior.
> >
> > Changing that behavior was the entire point of the cited commit.
> 
> Sorry, I was thinking that the new behavior was needed for internal
> future functions since the doc wasn't changed.

FWIW, I also don't think it's ok to just change the behaviour
unconditionally and without a replacement for existing behaviour.


Greetings,

Andres Freund

Craig Ringer wrote:

> > FWIW, I also don't think it's ok to just change the behaviour
> > unconditionally and without a replacement for existing behaviour.
> 
> Seems like it just needs a new argument nowait DEFAULT false

I added a WAIT flag to DROP_REPLICATION_SLOT, preliminary patch
attached.  Running tests now.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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And another patch to restore behavior to replication origin drop.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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On Tue, Aug 29, 2017 at 12:04:33PM +0200, Alvaro Herrera wrote:
> Noah Misch wrote:
> > On Thu, Aug 24, 2017 at 03:38:20PM +0200, Simone Gotti wrote:
> > > I noticed that in postgres 10beta3, calling pg_drop_replication_slot on an
> > > active slot will block until it's released instead of returning an error
> > > like
> > > done in pg 9.6. Since this is a change in the previous behavior and the docs
> > > wasn't changed I made a patch to restore the previous behavior.
> 
> > > after git commit 9915de6c1cb calls to pg_drop_replication_slot or the
> > > replication protocol DROP_REPLICATION_SLOT command will block when a
> > > slot is in use instead of returning an error as before (as the doc
> > > states).
> > > 
> > > This patch will set nowait to true to restore the previous
> > > behavior.
> 
> > The above-described topic is currently a PostgreSQL 10 open item.
> 
> Looking at it now -- next report tomorrow.

This PostgreSQL 10 open item is past due for your status update.  Kindly send
a status update within 24 hours, and include a date for your subsequent status
update.  Refer to the policy on open item ownership:
https://www.postgresql.org/message-id/20170404140717.GA2675809%40tornado.leadboat.com

Both patches pushed, which should close the open item.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Fri, Sep 1, 2017 at 4:31 PM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> Both patches pushed, which should close the open item.

Hi Alvaro,

thanks a lot!

Cheers,
Simone.


>
> --
> Álvaro Herrera                https://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services