On Jun 6 (http://archives.postgresql.org/pgsql-hackers/2011-06/msg00272.php), Pavel discovered an issue with PQsetvalue that could cause libpq to wander off into unallocated memory that was present in 9.0.x. A fairly uninteresting fix was quickly produced, but Tom indicated during subsequent review that he was not happy with the behavior of the function. Everyone was busy with the beta wrap at the time so I didn't press the issue.
A little history here: PQsetvalue's (http://www.postgresql.org/docs/9.0/static/libpq-misc.html) main reason to exist is to allow creating a PGresult out of scratch data on a result created via PQmakeEmptyResult(). This behavior works as intended and is not controversial...it was initially done to support libpqtypes but has apparently found use in other places like ecpg.
PQsetvalue *also* allows you to mess with results returned by the server using standard query methods for any tuple, not just the one you are creating as you iterate through. This behavior was unnecessary in terms of what libpqtypes and friends needed and may (as Tom suggested) come back to bite us at some point. As it turns out, PQsetvalue's operation on results that weren't created via PQmakeEmptyResult was totally busted because of the bug, so we have a unique opportunity to tinker with libpq here: you could argue that you have a window of opportunity to change the behavior here since we know it isn't being usefully used. I think it's too late for that but it's if it had to be done the time is now.
Pavel actually has been requesting to go further with being able to mess around with PGresults (see: http://archives.postgresql.org/pgsql-interfaces/2011-06/msg00000.php), such that the result would start to have some 'recordset' features you find in various other libraries. Maybe it's not the job of libpq to do that, but I happen to think it's pretty cool. Anyways, something has to be done -- I hate to see an unpatched known 9.0 issue remain in the wild.
+1
Exactly at this moment I am thinking about using modifiable (via PQsetvalue) PGresult instead of std::map in my C++ library for store parameters for binding to executing command. I am already designed how to implement it, and I supposed that PQsetvalue is intended to work with any PGresult and not only with those which has been created via PQmakeEmptyResult... So, I am absolutely sure, that PQsetvalue should works with any PGresult.
