Re: assertion in 9.4 with wal_level=logical

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Steve Singer <steve(at)ssinger(dot)info>, PostgreSQL-development Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: assertion in 9.4 with wal_level=logical
Date: 2014-04-18 14:51:14
Message-ID: 20140418145114.GA1706@awork2.anarazel.de
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On 2014-04-18 16:44:55 +0200, Robert Haas wrote:
> On Thu, Apr 17, 2014 at 10:47 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > On 2014-04-17 17:40:01 -0300, Alvaro Herrera wrote:
> >> For once, this looks more like a problem in logical decoding, which is
> >> trying to assert about the tuple being updated; the assertion failing is
> >> the one added a week ago about not palloc'ing in a critical section.
> >
> > It's this (older) assertion in HeapTupleHeaderGetCmax():
> >
> > Assert(TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tup)));
> >
> > That can allocate memory if xmax is a multixact... Does anybody have a
> > better idea to solve this than adding a CritSectionCount == 0 && in
> > there?
>
> Blech. Isn't that just nerfing the assertion?

Not precicisely sure what you mean, but the only memory allocation in
HeapTupleHeaderGetCmax() and log_heap_new_cid() is that Assert(). And
that's the only "forbidden" thing in that codepath.
Now, we could alternatively restructure the codepaths so they pass in
xmax from outside the critical section, but I had a quick look and the
risk/complications from that seems bigger than the assertion buys us
there.
I don't have a better idea unfortunately :(

Greetings,

Andres Freund

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

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