ERROR: Memory exhausted in AllocSetAlloc(909324558)

I saw someone else also saw this error.  I am seeing it in 7.1.2 and I
think I may have tracked it down.  It's after a different operation but
perhaps it is related.

I have written a trigger.  It dies with that error when I make the
following call.

SPI_modifytuple (rel,rettuple,1,&targ_att,&newval,NULL)

I created targ_att as follows.

targ_att = SPI_fnumber (tupdesc, args[0]

This returns 23 in my case which is the correct field that I want to
modify.  The existing value for that field is NULL.  I looked at the
length of that fields with rel->rd_att->attrs[23]->attlen and it is -1.
I assume that that is OK for a NULL value.

I eventually followed this call to heaptuple.c.  In heap_formtuple()
there is a call to ComputeDataSize() with the existing tuple, the value
I am changing to and the nulls.  In that function the length (-1) is
added to the total length.  This causes the problem I am seeing.

Am I misunderstanding the call?  I thought that the nulls parameter was
to map out which values you were changing to were NULL.  It appears
to be the value of the existing value.  Why would I care about that value?
Is it up to me to find all the NULLS in an existing tuple before calling SPI
functions?

I called this function that way in an earlier version with no problem.

-- 
D'Arcy J.M. Cain <darcy@{druid|vex}.net>   |  Democracy is three wolves
http://www.druid.net/darcy/                |  and a sheep voting on
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