Re: more on out-of-memory

Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> So, the reason I started the thread about postmaster dying on OOM is
> that somebody asked me on IM what could have caused a backend to die
> with this backtrace:

[ of course you realize this is a backend, not the postmaster ]

> His question was: is it possible that we're handing a NULL pointer to a
> %s on fprintf? The involved code looks like this:
> ...
> And since this is being called from AllocSetAlloc, which is always
> handed a complete memory context (and not something that has only been
> partially set), I think the answer is that it's not possible, and that
> the bug must be on libc which is perhaps not handling out-of-memory very
> cleanly in its fprintf implementation.

Another theory is that the name pointer got clobbered by some sort of
memory-stomping bug. (We don't know from the available evidence that
it was NULL --- it could have been any garbage value that pointed
outside backend memory.) However, given that the context clearly
indicates being out-of-memory overall, your theory seems a bit more
probable.

The really odd thing is that the stack trace is so short; it seems
to have failed *very* early in query parsing, which is hard to credit
unless this person is in the habit of sending megabytes-long queries.
I guess if the system as a whole were under really severe memory
pressure, a backend could hit OOM without having eaten much itself.

What platform is this, and which PG version?

regards, tom lane