[patch] CLUSTER blocks scanned progress reporting

Hi,

The pg_stat_progress_cluster view can report incorrect
heap_blks_scanned values when synchronize_seqscans is enabled, because
it allows the sequential heap scan to not start at block 0. This can
result in wraparounds in the heap_blks_scanned column when the table
scan wraps around, and starting the next phase with heap_blks_scanned
!= heap_blks_total. This issue was introduced with the
pg_stat_progress_cluster view.

The attached patch fixes the issue by accounting for a non-0
heapScan->rs_startblock and calculating the correct number with a
non-0 heapScan->rs_startblock in mind.

The issue is reproducible starting from PG12 by stopping a
CLUSTER-command while it is sequential-scanning the table (seqscan
enforceable through enable_indexscan = off), and then re-starting the
seqscanning CLUSTER command (without other load/seq-scans on the
table).

Any thoughts?

Matthias van de Meent