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Re: Curious buildfarm failures (fwd)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: Sergey Koposov <koposov(at)ast(dot)cam(dot)ac(dot)uk>, pgsql-hackers(at)postgreSQL(dot)org, Andrew Dunstan <andrew(at)dunslane(dot)net>
Subject: Re: Curious buildfarm failures (fwd)
Date: 2013-01-15 22:56:40
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> I played a bit arround (thanks Sergey!) and it seems to be some rather
> strange optimization issue around the fsync request queue.

> Namely changing 
> 	request->rnode = rnode;
> into
> 	request->rnode.spcNode = rnode.spcNode;
> 	request->rnode.dbNode = rnode.dbNode;
> 	request->rnode.relNode = rnode.relNode;
> makes it pass reliably.

Jeez.  That's my candidate for weird compiler bug of the month.

> How the hell thats correlating with the elog changes I don't yet know.

There is an elog(ERROR) further up in the same function, but it's sure
not clear how that could cause the compiler to misimplement a struct
assignment.  Maybe the problem is not in those lines alone, but the fact
that rnode is a pass-by-value struct?  (That is, maybe it's the value of
the rnode local variable that's getting munged, somewhere up near the
elog call?)  We tend to not use pass-by-value struct params much, so we
might not have noticed a compiler bug associated with that.  Or IOW,
does changing ForwardFsyncRequest to use a "const RelFileNode *rnode"
parameter make it go away?

			regards, tom lane

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