Re: Hitting CheckRelationLockedByMe() ASSERT with force_generic_plan

On Thu, 22 Nov 2018 at 22:33, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)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(at)100858=#set plan_cache_mode = force_generic_plan;
> SET
> postgres(at)100858=#select f1(4);
> f1
> ----
> 0
> (1 row)
>
> postgres(at)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I'd say the following comment and code in nodeIndexscan.c is outdated: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.

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/
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