[HACKERS] SPI: Correct way to rollback a subtransaction?

[Marko Kreen]

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

---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster