Re: ALTER TABLE ALTER COLUMN SET TYPE crash

On Tue, Aug 25, 2020 at 03:05:11PM +0900, Michael Paquier wrote:
> On Tue, Aug 25, 2020 at 02:14:06PM +1000, Robins Tharakan wrote:
> > Unlike a recently reported similar issue, executing the following ALTER
> > TABLE on the regression database crashes Postgres (master).
> >
> > Admittedly it doesn't do anything constructive (and am new to the tool),
> > but do let me know if such reports are interesting and / or if you need
> > more details for reproduction.
>
> Such reports are constructive! I can reproduce the crash here down to
> 9.5. From what I can see, the problem comes from ATRewriteTable() ->
> ExecEvalExpr() when we evaluate expressions with inputs coming from
> the old tuple. It looks like a memory corruption issue or a context
> issue at quick glance, and I cannot get a clean backtrace.

OK, I have generated a attached backtrace by manually walking down
through the stack using break points. Notice len2 on the top line:

#0 varstr_cmp (arg1=0x55e0ef64262c "\252\061\306BT\026", len1=21,
arg2=0x7f274db17ebc "-\317\303=T\026", len2=-4, collid=100) at
varlena.c:1633 -------

-4 is not a good len. I will do more research to see why that -4 len is
happening.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee