pgsql: Fix bug in following update chain when locking a heap tuple

Fix bug in following update chain when locking a heap tuple

After waiting for a concurrent updater to finish, heap_lock_tuple()
followed the update chain to lock all tuple versions. However, when
stepping from the initial tuple to the next one, it failed to check
that the next tuple's XMIN matches the initial tuple's XMAX. That's an
important check whenever following an update chain, and the recursive
part that follows the chain did it, but the initial step missed it.
Without the check, if the updating transaction aborts, the updated
tuple is vacuumed away and replaced by an unrelated tuple, the
unrelated tuple might get incorrectly locked.

Author: Jasper Smit <jasper.smit@servicenow.com>
Discussion: https://www.postgresql.org/message-id/CAOG+RQ74x0q=kgBBQ=mezuvOeZBfSxM1qu_o0V28bwDz3dHxLw@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b6201c76eba4ff01bcdaccfd5324b3e82045f229

Modified Files
--------------
src/backend/access/heap/heapam.c | 46 +++++++++++++++++++++++++++-------------
1 file changed, 31 insertions(+), 15 deletions(-)