Re: Hitting CheckRelationLockedByMe() ASSERT with force_generic_plan

On Fri, Nov 23, 2018 at 3:33 AM David Rowley <david.rowley@2ndquadrant.com> wrote:

On Thu, 22 Nov 2018 at 22:33, Rushabh Lathia <rushabh.lathia@gmail.com> wrote:
> CREATE TABLE foo (x int primary key);
> INSERT INTO foo VALUES (1), (2), (3), (4), (5);
>
> CREATE OR REPLACE FUNCTION f1(a int) RETURNS int
> AS $$
> BEGIN
>  DELETE FROM foo where x = a;
>  return 0;
> END;
> $$ LANGUAGE plpgsql;
>
> postgres@100858=#set plan_cache_mode = force_generic_plan;
> SET
> postgres@100858=#select f1(4);
>  f1
> ----
>   0
> (1 row)
>
> postgres@100858=#select f1(4);
> server closed the connection unexpectedly


> Now, to fix this issue either we need to hold proper lock before reaching
> to ExecInitIndexScan() or teach ExecInitIndexScan() to take AccessShareLock
> on the scan coming from CMD_DELETE.

I'd say the following comment and code in nodeIndexscan.c is outdated:

/*
* Open the index relation.
*
* If the parent table is one of the target relations of the query, then
* InitPlan already opened and write-locked the index, so we can avoid
* taking another lock here.  Otherwise we need a normal reader's lock.
*/
relistarget = ExecRelationIsTargetRelation(estate, node->scan.scanrelid);
indexstate->iss_RelationDesc = index_open(node->indexid,
  relistarget ? NoLock : AccessShareLock);

Despite what the above comment claims, these indexes have not been
opened in InitPlan since 389af951552ff2. As you mentioned, they're
opened in nodeModifyTable.c for non-delete statements.

+1.

I tried the same approach and with further testing I haven't found

any issues.

 

I think we either need to update the above code to align it to what's
now in nodeModifyTable.c, or just obtain an AccessShareLock
regardless. I kinda think we should just take the lock regardless as
determining if the relation is a result relation may be much more
expensive than just taking another lower-level lock on the index.

Anyway. I've attached a small patch to update the above fragment.

--
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

--

Rushabh Lathia