Problem returning strings with pgsql 8.3.x

While developing PL/LOLCODE, I've found something wrong with returning
strings from LOLCODE functions using 8.3.0 or greater. Using 8.4beta
from a few days ago, for instance, a function that should return "test
string" returns
"\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F" in
pgsql (sometimes the number of \x7F characters varies). In 8.2.4 it
works fine.

Here's the code involved, from pl_lolcode_call_handler, the call
handler function for PL/LOLCODE. First, the bit that finds the
FmgrInfo structure and typioparam for the result type:

procTup = SearchSysCache(PROCOID,
ObjectIdGetDatum(fcinfo->flinfo->fn_oid), 0, 0, 0);
if (!HeapTupleIsValid(procTup)) elog(ERROR, "Cache lookup
failed for procedure %u", fcinfo->flinfo->fn_oid);
procStruct = (Form_pg_proc) GETSTRUCT(procTup);

typeTup = SearchSysCache(TYPEOID,
ObjectIdGetDatum(procStruct->prorettype), 0, 0, 0);
if (!HeapTupleIsValid(typeTup)) elog(ERROR, "Cache lookup
failed for type %u", procStruct->prorettype);
typeStruct = (Form_pg_type) GETSTRUCT(typeTup);

resultTypeIOParam = getTypeIOParam(typeTup);
fmgr_info_cxt(typeStruct->typinput, &flinfo,
TopMemoryContext); /*CurTransactionContext); */
ReleaseSysCache(typeTup);

Here's the code that converts the return value into a Datum later on
in the function:

if (returnTypeOID != VOIDOID) {
if (returnVal != NULL) {
if (returnVal->type == ident_NOOB)
fcinfo->isnull = true;
else {
SPI_push();
if (returnTypeOID == BOOLOID)
retval =
InputFunctionCall(&flinfo, lolVarGetTroof(returnVal) == lolWIN ?
"TRUE" : "FALSE", resultTypeIOParam, -1);
else {
/* elog(NOTICE,
lolVarGetString(returnVal, true)); */
retval =
InputFunctionCall(&flinfo, lolVarGetString(returnVal, true),
resultTypeIOParam, -1);
}
SPI_pop();
}
}
else {
fcinfo->isnull = true;
}
}

SPI_finish();
/* elog(NOTICE, "PL/LOLCODE ending"); */

return retval;

returnVal is an instance of the struct PL/LOLCODE uses to store its
variables. The key line in this case is the one after the
commented-out call to elog. retval is a Datum type. lolVarGetString()
returns the string value the returnVal struct represents -- I'm
certain of that thanks to gdb and other testing. All other data types
PL/LOLCODE knows about internally seem to return just fine. I'm fairly
certain I'm screwing up memory somewhere, but I can't see what I've
done wrong.

I'm glad to provide further details, but those included above are all
the ones I thought were relevant. Thanks in advance for any help you
can provide.

- Josh / eggyknap

Note: The -hackers list seemed like the place for this post, but its
list description gives instructions to try another list first, hence
the post here.