Re: Getting rid of "tuple concurrently updated" elog()s withconcurrent DDLs (at least ALTER TABLE)

On Tue, Dec 26, 2017 at 4:14 AM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> On Tue, Dec 26, 2017 at 4:30 PM, Andres Freund <andres@anarazel.de> wrote:
>> You're proposing to lock the entire relation against many forms of concurrent DDL, just to get rid of that error?
Thatseems unacceptable.
 
>> Isn't the canonical way to solve this to take object locks?
>
> Sure. That's where things in lmgr.c come into play, like
> LockSharedObject(), and you could hold with an exclusive lock on a
> given object until the end of a transaction before opening the catalog
> relation with heap_open(), however with those you need to know the
> object OID before taking a lock on the parent relation, right? So you
> face problems with lock upgrades, or race conditions show up more
> easily. I have to admit that I have not dug much into the problem yet,
> it is easy enough to have isolation tests by the way, and I just
> noticed that ALTER DATABASE SET can equally trigger the error.

I don't understand what you mean by "you need to know the object OID
before taking a lock on the parent relation, right?".  That seems
wrong.

I think you might need something like what was done in
b3ad5d02c9cd8a4c884cd78480f221afe8ce5590; if, after we look up the
name and before we acquire a lock on the OID, we accept any
invalidation messages, recheck that the object we've locked is still
the one associated with that name.

I think Andres is certainly right that locking all of pg_authid is a nonstarter.

-- 
Robert Haas
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