SPI: Correct way to rollback a subtransaction?

I want to update one row from possibly several backends. In case
of SERIALIZABLE transactions, the update command may fail.  To
hide it from calling transaction, I use a subtransaction and try
to catch and hide the error.

With help of plpgsql source, I wrote following code that _seems_
to work.  But I have no idea if it's the correct way to do it:

/* store old state */MemoryContext oldcontext = CurrentMemoryContext;ResourceOwner oldowner = CurrentResourceOwner;
BeginInternalSubTransaction(NULL);res = SPI_connect();if (res < 0)    elog(ERROR, "cannot connect to SPI");
PG_TRY();{    res = SPI_execute("update one row", false, 0);    SPI_finish();
ReleaseCurrentSubTransaction();}PG_CATCH();{   SPI_finish();    RollbackAndReleaseCurrentSubTransaction();
FlushErrorState();   res = -1; /* remember failure */}PG_END_TRY(); 
/* restore old state */MemoryContextSwitchTo(oldcontext);CurrentResourceOwner = oldowner;


I am suspicious about the ..SubTransaction and SPI_* nesting
and resetting the error state.  Can anyone look if it's correct?

Goal of the exercise is to have 8-byte transaction ID's and snapshots
externally available.  (Port of xxid module from Slony).  Above code
does the update of the 'epoch' table that has only one row.

--
marko