[PATCH v1] Add vacuum_delay_point() to GiST empty-page deletion pass

Hi,

While comparing the vacuum loops of the various index AMs I noticed that
gistvacuum_delete_empty_pages() -- the second pass of a GiST vacuum that
unlinks empty leaf pages -- does not call vacuum_delay_point(), even
though it reads a buffer and issues WAL-logged page deletions for every
internal page it revisits. The other page-scanning loops in
gistvacuum.c, gistvacuumscan() and the recursion in gistvacuumpage(),
both call it.

As a result this phase of a GiST vacuum ignores the cost-based vacuum
delay entirely, and only reacts to query cancellation when a buffer read
happens to perform I/O. On a large GiST index with many emptied leaf
pages this loop can end up walking every internal page of the index.

The attached patch adds a single vacuum_delay_point(false) at the top of
the loop, where no buffer lock is held, matching the sibling idiom. The
function has lacked the call since it was introduced in 7df159a620b.

This is long-standing rather than a recent regression, so I'll leave the
question of back-patching to a committer. Note that on branches before
18 vacuum_delay_point() takes no argument (the bool was added by
e5b0b0ce150), so a back-patch would drop the "false".

Thanks,
Paul Kim