Обсуждение: Unreproducible bug in snapshot import code

Gurjeet Singh <singh.gurjeet@gmail.com> writes:
>     Starting from line 89 of terminal1.txt we see this snippet:

> postgres=# rollback;
> ROLLBACK
> postgres=# begin TRANSACTION ;
> BEGIN
> postgres=# set transaction snapshot '000002C8-1';
> ERROR:  SET TRANSACTION SNAPSHOT must be called before any query
> postgres=# rollback;
> ROLLBACK

>     As is evident from the rest of the lines in terminal1.txt, I have not
> been able to reproduce this error again; I tried even with a clean build of
> the sources.

Given the weird capitalization of the BEGIN command, I'm guessing that
you used tab-completion to enter it.  I wonder if this could have been
affected by the execution of some query in support of tab completion?
I couldn't reproduce it either on the basis of that guess, though.
        regards, tom lane

Excerpts from Gurjeet Singh's message of lun oct 24 13:55:44 -0300 2011:

>     Starting from line 89 of terminal1.txt we see this snippet:
> 
> postgres=# rollback;
> ROLLBACK
> postgres=# begin TRANSACTION ;
> BEGIN
> postgres=# set transaction snapshot '000002C8-1';
> ERROR:  SET TRANSACTION SNAPSHOT must be called before any query
> postgres=# rollback;
> ROLLBACK
> 
> 
>     As you can see the SET TRANSACTION SNAPSHOT was the first statement in
> that transaction, and yet the ERROR message says that it is not.

Maybe the tab-completion feature issued a query before the set
transaction command.

-- 
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

On Tue, Oct 25, 2011 at 2:41 AM, Gurjeet Singh <singh.gurjeet@gmail.com> wrote:
> On Mon, Oct 24, 2011 at 1:08 PM, Alvaro Herrera <alvherre@commandprompt.com>
> wrote:
>>
>> Excerpts from Gurjeet Singh's message of lun oct 24 13:55:44 -0300 2011:
>>
>> >     Starting from line 89 of terminal1.txt we see this snippet:
>> >
>> > postgres=# rollback;
>> > ROLLBACK
>> > postgres=# begin TRANSACTION ;
>> > BEGIN
>> > postgres=# set transaction snapshot '000002C8-1';
>> > ERROR:  SET TRANSACTION SNAPSHOT must be called before any query
>> > postgres=# rollback;
>> > ROLLBACK
>> >
>> >
>> >     As you can see the SET TRANSACTION SNAPSHOT was the first statement
>> > in
>> > that transaction, and yet the ERROR message says that it is not.
>>
>> Maybe the tab-completion feature issued a query before the set
>> transaction command.
>
> I have tried reproducing the bug starting from 1 and 2 transactions before
> the one shown in snippet, and I used tab-completion to get the same
> screen-output as termonal1.txt and yet it's not reproducible.

I could reproduce it when I typed TAB just after typing "set" in "set
transaction snapshot".
As Tom and Alvaro pointed out, the tab-completion issues a query and which
prevents the "set transaction snapshot" command.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

Gurjeet Singh wrote:
> > > I have tried reproducing the bug starting from 1 and 2 transactions
> > before
> > > the one shown in snippet, and I used tab-completion to get the same
> > > screen-output as termonal1.txt and yet it's not reproducible.
> >
> > I could reproduce it when I typed TAB just after typing "set" in "set
> > transaction snapshot".
> > As Tom and Alvaro pointed out, the tab-completion issues a query and which
> > prevents the "set transaction snapshot" command.
> >
> 
> Great! That settles it then. Reproducible, but not a bug.

Yes, it is only tabs that query the database for completion that cause
this.  Should this be documented somehow?  (No idea how.)

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +

On Fri, Oct 28, 2011 at 11:07:25AM -0400, Gurjeet Singh wrote:
> On Fri, Oct 28, 2011 at 10:11 AM, Bruce Momjian <bruce@momjian.us> wrote:
>
>     Gurjeet Singh wrote:
>     > > > I have tried reproducing the bug starting from 1 and 2 transactions
>     > > before
>     > > > the one shown in snippet, and I used tab-completion to get the same
>     > > > screen-output as termonal1.txt and yet it's not reproducible.
>     > >
>     > > I could reproduce it when I typed TAB just after typing "set" in "set
>     > > transaction snapshot".
>     > > As Tom and Alvaro pointed out, the tab-completion issues a query and
>     which
>     > > prevents the "set transaction snapshot" command.
>     > >
>     >
>     > Great! That settles it then. Reproducible, but not a bug.
>
>     Yes, it is only tabs that query the database for completion that cause
>     this.  Should this be documented somehow?  (No idea how.)
>
>
> If we have a doc section on psql's tab-completion, I think this needs to go
> there. A note like:
>
> "Trying to tab-complete on psql may send queries to the server, and depending
> on the transaction state, execution of these queries may lead to non-default/
> unexpected behaviour by the queries executed after tab-completion. For example,
> ..."

I have added the attached patch to document this limitation.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +