Re: hash join error improvement (old)

Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> I recently noticed this in a customer log file:
> ERROR: could not read from hash-join temporary file: Success
> The problem is we're reporting with %m when the problem is a partial
> read or write.
> I propose the attached patch to solve it: report "wrote only X of X
> bytes". This caused a lot of other trouble, the cause of which I've
> been unable to pinpoint as yet. But in the meantime, this is already a
> small improvement.

+1 for the idea, but there's still one small problem with what you did
here: errcode_for_file_access() looks at errno, which we can presume
will not have a relevant value in the "wrote only %d bytes" paths.

Most places that are taking pains to deal with this scenario
do something like

errno = 0;
if (write(fd, data, len, xlrec->offset) != len)
{
/* if write didn't set errno, assume problem is no disk space */
if (errno == 0)
errno = ENOSPC;
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not write to file \"%s\": %m", path)));
}

I don't mind if you want to extend that paradigm to also use "wrote only
%d bytes" wording, but the important point is to get the SQLSTATE set on
the basis of ENOSPC rather than whatever random value errno will have
otherwise.

regards, tom lane