pgsql: Fix bug in nbtree array primitive scan scheduling.

Fix bug in nbtree array primitive scan scheduling.

A bug in nbtree's handling of primitive index scan scheduling could lead
to wrong answers when a scrollable cursor was used with an index scan
that had a SAOP index qual. Wrong answers were only possible when the
scan direction changed after a primitive scan was scheduled, but before
_bt_next was asked to fetch the next tuple in line (i.e. for things to
break, _bt_next had to be denied the opportunity to step off the page in
the same direction as the one used when the primscan was scheduled).
Furthermore, the issue only occurred when the page in question happened
to be the first page to be visited by the entire top-level scan; the
issue hinged upon the cursor backing up to the absolute beginning of the
key space that it returns tuples from (fetching in the opposite scan
direction across a "primitive scan boundary" always worked correctly).

To fix, make _bt_next unset the "needs primitive index scan" flag when
it detects that the current scan direction is not the one that was used
by _bt_readpage back when the primitive scan in question was scheduled.
This fixes the cases that are known to be faulty, and also seems like a
good idea on general robustness grounds.

Affected scrollable cursor cases now avoid a spurious primitive index
scan when they fetch backwards to the absolute start of the key space to
be visited by their cursor. Fetching backwards now only returns those
tuples at the start of the scan, as expected. It'll also be okay to
once again fetch forwards from the start at that point, since the scan
will be left in a state that's exactly consistent with the state it was
in before any tuples were ever fetched, as expected.

Oversight in commit 5bf748b8, which enhanced nbtree ScalarArrayOp
execution.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://postgr.es/m/CAH2-Wznv49bFsE2jkt4GuZ0tU2C91dEST=50egzjY2FeOcHL4Q@mail.gmail.com
Backpatch: 17-, where commit 5bf748b8 first appears.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/763d65ae254569e0bdbe2cbb27b184112765f24f

Modified Files
--------------
src/backend/access/nbtree/nbtsearch.c | 15 +++++++++++++--
src/backend/access/nbtree/nbtutils.c | 13 ++++++++-----
src/include/access/nbtree.h | 1 -
3 files changed, 21 insertions(+), 8 deletions(-)