pgsql: Fix hash index scans that start during a bucket split.

Fix hash index scans that start during a bucket split.

A hash index scan whose target bucket is still being populated by a
split must read both buckets of the split: forward scans read the bucket
being populated (skipping the tuples that the split copied there) and
then the bucket being split, and backward scans read them in the
opposite order. _hash_readnext and _hash_readprev make the transition
from the first bucket to the second, but _hash_next did not: on reaching
the end of the first bucket, it ended the scan, so every matching tuple
in the second bucket was overlooked.

To fix, teach _hash_next to make the same transition, using helper
functions shared with _hash_readnext and _hash_readprev.

_hash_next only missed the transition when the last page that it read in
the first bucket had a match (otherwise _hash_readpage's loop made the
required transition). Getting wrong answers also needed a matching
tuple to be inserted into the bucket being populated _after_ the split
began. Forward scans then lost the older tuples in the bucket being
split. An incomplete split would keep the buckets in this state until
an inserter finished the split at some indeterminate later time. Note
that wrong answers didn't require an incomplete split.

Although this fix treats the bug as the omission of a required step in
_hash_next, the real problem is the way that page stepping works in
general. The logic for stepping to the next page appears both in
high-level routines like _hash_next and in low-level routines like
_hash_readnext and _hash_readprev. If anything, _hash_next already knew
too much about page stepping (cf. nbtree's _bt_next function). All of
the logic around stepping really belongs in one routine, modeled on
nbtree's _bt_steppage function, but that approach is much too invasive
for a backpatched fix.

Claude Code found this problem. The committed test case is a simplified
version of the one that it wrote to demonstrate this bug.

Oversight in commit 7c75ef57, which introduced page-at-a-time processing
of hash index scans.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Author: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Vignesh C <vignesh21(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAH2-Wz=4CJK9ysZzbzxBGGTkgAg9ib8cJyCx=9q_d1r8tf0ang@mail.gmail.com
Backpatch-through: 14

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/173cf85a079516fdd2b09ed853702b05fbfcd15e

Modified Files
--------------
src/backend/access/hash/hashpage.c | 5 +
src/backend/access/hash/hashsearch.c | 136 +++++++++++++++------
src/test/modules/Makefile | 4 +-
src/test/modules/hash/.gitignore | 4 +
src/test/modules/hash/Makefile | 28 +++++
.../hash/expected/hash_incomplete_splits.out | 114 +++++++++++++++++
src/test/modules/hash/meson.build | 16 +++
.../modules/hash/sql/hash_incomplete_splits.sql | 53 ++++++++
src/test/modules/meson.build | 1 +
9 files changed, 322 insertions(+), 39 deletions(-)