Re: handling out of memory conditions when fetching row descriptions

"'Isidor Zeuner'" <postgresql(at)quidecco(dot)de> writes:
> using the latest git source code, I found that libpq will let the
> connection stall when getRowDescriptions breaks on an out of memory
> condition. I think this should better be handled differently to allow
> application code to handle such situations gracefully.

The basic assumption in there is that if we wait and retry, eventually
there will be enough memory. I agree that that's not ideal, since the
application may not be releasing memory elsewhere. But what you propose
doesn't seem like an improvement: you're converting a maybe-failure into
a guaranteed-failure, and one that's much more difficult to recover from
than an ordinary query error.

Also, this patch breaks async operation, in which a failure return from
getRowDescriptions normally means that we have to wait for more data
to arrive. The test would really need to be inserted someplace else.

In any case, getRowDescriptions is really an improbable place for an
out-of-memory to occur: it would be much more likely to happen while
absorbing the body of a large query result. There already is some logic
in getAnotherTuple for dealing with that case, which I suggest is a
better model for what to do than "break the connection". But probably
making things noticeably better here would require going through all
the code to check for other out-of-memory cases, and developing some
more uniform method of representing an already-known-failed query
result. (For instance, it looks like getAnotherTuple might not work
very well if it fails to get memory for one tuple and then succeeds
on later ones. We probably ought to have some explicit state that
says "we are absorbing the remaining data traffic for a query result
that we already ran out of memory for".)

regards, tom lane