pgsql: heapam: Track heap block in IndexFetchHeapData.

heapam: Track heap block in IndexFetchHeapData.

Add an explicit BlockNumber field (xs_blk) to IndexFetchHeapData that
tracks which heap block is currently pinned in xs_cbuf.

heapam_index_fetch_tuple now uses xs_blk to determine when buffer
switching is needed, replacing the previous approach that compared
buffer identities via ReleaseAndReadBuffer on every non-HOT-chain call.

This is preparatory work for an upcoming commit that will add index
prefetching using a read stream.  Delegating the release of a currently
pinned buffer to ReleaseAndReadBuffer won't work anymore -- at least not
when the next buffer that the scan needs to pin is one returned by
read_stream_next_buffer (not a buffer returned by ReadBuffer).

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wz=g=JTSyDB4UtB5su2ZcvsS7VbP+ZMvvaG6ABoCb+s8Lw@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/c7d09595e46ff513ae0a23ae80e58871a321174f

Modified Files
--------------
src/backend/access/heap/heapam_indexscan.c | 31 ++++++++++++++++++------------
src/include/access/heapam.h                |  5 +++--
2 files changed, 22 insertions(+), 14 deletions(-)