While locking heap pages (function RelationGetBufferForTuple() in file hio.c) we acquire locks in increasing block number order to avoid deadlock. In certain cases, we do have to drop and reacquire lock on heap pages (when we have to pin visibility map pages) to avoid doing IO while holding exclusive lock. The problem is we now acquire locks in bufferId order, which looks like a slip and the intention was to grab it in block number order. Since it is a trivial change, I am attaching a patch to correct it.
