pgsql: Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

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pgsql: Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

Álvaro Herrera
Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

Commit 866e24d47db1 added an assert that HEAP_XMAX_LOCK_ONLY and
HEAP_KEYS_UPDATED cannot appear together, on the faulty assumption that
the latter necessarily referred to an update and not a tuple lock; but
that's wrong, because SELECT FOR UPDATE can use precisely that
combination, as evidenced by the amcheck test case added here.

Remove the Assert(), and also patch amcheck's verify_heapam.c to not
complain if the combination is found.  Also, out of overabundance of
caution, update (across all branches) README.tuplock to be more explicit
about this.

Author: Julien Rouhaud <[hidden email]>
Reviewed-by: Mahendra Singh Thalor <[hidden email]>
Reviewed-by: Dilip Kumar <[hidden email]>
Discussion: https://postgr.es/m/20210124061758.GA11756@nol

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8deb6b38dc4c7a7fd4719ee45e4b00d62b27dffe

Modified Files
--------------
contrib/amcheck/t/001_verify_heapam.pl | 14 +++++++++++++-
contrib/amcheck/verify_heapam.c        |  7 -------
src/backend/access/heap/README.tuplock |  7 ++++---
src/backend/access/heap/hio.c          |  8 +++-----
4 files changed, 20 insertions(+), 16 deletions(-)