Cache lookup failed for relation X [was: DROP FUNCTION cache lookup failed for relation X]

Tom Lane wrote:
>
> Still in the think-about-it mode, personally ... my proposed fix is
> certainly much too invasive to consider back-patching, so unless someone
> comes up with a way-simpler idea, it's 8.3 material at best ...
>
>
I ran into a variant of this today - simply creating and dropping a
table repeatedly while doing \d from another session:

Session 1:

perl -e 'while (1) {print "drop table if exists z0; \n create table z0
(a int, b int);\n drop table z0;\n"}' | psql cache > z0.log 2>&1

Session 2:

psql cache
=# \d
ERROR: cache lookup failed for relation 945897 (in RelationIsVisible,
namespace.c:406)

The previous discussion centered around working on on locking in
dependency.c whilst dropping related objects - but does this apply when
there is just one? Anyway I tried to understand what was happening and
the attached rather hacky patch seems to cure the behaviour - So I've
submitted it as a discussion aid, rather than 'the way of fixing
this'... since I'm hoping there is a better way :-)

regards

Mark