Re: [Postgres-xc-developers] a serious MVCC bug in Postgres-XC

[Koichi S. <koi...@gm...>]

The problem looks to need some careful review to dig into.   Please
allow me a bit more.

Regards;
---
Koichi Suzuki


2014-05-07 19:46 GMT+09:00 Wang Diancheng <dia...@gm...>:
> I wrote an extension 'xc2pctest' to debug and test internal 2PC in
> Postgres-XC, which need a small patch to master code. The patch and
> source of the extension attached.
>
> I add a hook at remote commit stage on coordinator when it do a 2PC
> commit, the extension xc2pctest will make one node fail to commit
> and the other node success, the whole cluster will go into partial
> commit state.
>
> following are reproduce steps for the MVCC bug:
>
> 1. patch the git master code using attached patch, recompile and
> reinstall the whole Postgres-XC, then compile and install
> the extension xc2pctest
>
> 2. create a cluster with 1 GTM, and 1 Coordinator named co1, and 2
> Datanodes named dn1 and dn2, and start it.
>
> 3. execute following SQL statement to create tables, load the extension
> and do some update to make cluster go into partial commit state:
>
> create table t1(a int, b int) to node (dn1);
> create table t2(a int, b int) to node (dn2);
> create extension xc2pctest;
> insert into t2 values(2,2);
> insert into t1 values(1,1);
> select install_xc2pctest_fail_hook();
> begin;
> update t1 set b = 11;
> update t2 set b = 22;
> commit;
> execute direct on (dn1) 'select * from pg_prepared_xact()';
> execute direct on (dn2) 'select * from pg_prepared_xact()';
>
> 4. the step 3, we created table 't1' on Datanode 'dn1', and table 't2'
> on Datanode 'dn2', and we *just inserted 1 row* for each table and
> updated them. The transaction of updtes will be failed due to the
> extension enabled. The last 2 statements in step 3 will tell us which
> node (which table) success to commit, it is the table 't1' on my machine.
>
> 5. now update the success updated table (it is 't1' in my test), and
> show the result, the output is following in my machine:
>
> postgres=# update t1 set b = 111 where a = 1;
> UPDATE 1
> postgres=# select xmin, xmax, * from t1;
>  xmin  | xmax  | a |  b
> -------+-------+---+-----
>  10018 | 10020 | 1 |   1
>  10096 | 10096 | 1 | 111
> (2 rows)
>
> 6. you can see 2 rows in step 5(there should be just 1 rows, and updates
> shoud be blocked under read committed isolation level).
>
> I think this is a serious MVCC bug in Postgres-XC.
>
>
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