WL#12892: Deprecate slave-rows-search-algorithms

EXECUTIVE SUMMARY
=================

This worklog implements a deprecation warning when user engages the
command line option (or the corresponding system variable)
--slave_rows_search_algorithms.

BACKGROUND
==========

The option slave-rows-search-algorithms controls how the replication
applier looks up rows in tables, when applying UPDATE or DELETE row
events. The default for this option is HASH_SCAN,INDEX_SCAN as of 8.0,
and this is always best, both for performance and (in a corner case)
for correctness. So it is better to not give the possibility to use
any other algorithm.

User Stories
------------

U1. As a user, I don't want the server to have options that should
    never be changed, so that there is less to learn in order to use
    the server.

U2. As a user, I want replication to always be as fast as possible,
    and not have options to make it slower.

U3. As a user, I want replication to always work, and not have options
    that might break it.

Notes:

 1. For indexed tables, it is always faster to use INDEX_SCAN.

 2. For unindexed tables, it is always faster to use HASH_SCAN.

 3. For indexed tables, using TABLE_SCAN,HASH_SCAN can make the slave
    stop with an error (duplicate key) in cases like the following:

    CREATE TABLE t (a INT PRIMARY KEY, b INT UNIQUE NOT NULL);
    INSERT INTO t VALUES (1, 2), (2, 1);
    UPDATE t SET b = b - 1 WHERE b > 0;

    (I.e., rows are updated on master in the order of a secondary index, and
    the row updates are order-dependent in such a way that doing the same row
    updates in primary key order will create collisions in some index.)

 4. For indexed tables, using TABLE_SCAN,HASH_SCAN can make the slave
    stop with an error (key not found) in cases like the following:

    --delimiter |
    CREATE FUNCTION f1 () RETURNS INT BEGIN
      UPDATE t1 SET a = 2 WHERE a = 1;
      UPDATE t1 SET a = 3 WHERE a = 2;
      RETURN 0;
    END|
    --delimiter ;
    CREATE TABLE t1 (a INT);
    INSERT INTO t1 VALUES (1);
    SELECT f1();

Functional Requirements
=======================

FR1: Server SHALL emit a warning, at server start time, when
     configuring slave_rows_search_algorithms in the server
     configuration file.

FR2: Server SHALL emit a warning, at server start time, when passing
     slave_rows_search_algorithms as a server start up option on the
     command line.

FR3: Warning shall be emitted in the user session when the user reads
     the variable @@global.slave_rows_search_algorithms.

FR4: Warning shall be emitted in the user session when the user SETS
     the variable @@global.slave_rows_search_algorithms.

FR5: Warning shall be emitted in the user session when the user issues
     SET PERSIST slave_rows_search_algorithms or SET PERSIST_ONLY
     slave_rows_search_algorithms.

User Visible Changes
====================

- Server emits a deprecation warning in the session when the user
  selects @@global.slave_rows_search_algorithms.

  "Warning 1287 '@@slave_rows_search_algorithms' is deprecated and will
   be removed in a future release."

- Server emits a deprecation warning in the session when the user sets
  @@global.slave_rows_search_algorithms.

  "Warning 1287 '@@slave_rows_search_algorithms' is deprecated and
  will be removed in a future release."

- Server emits a deprecation warning at start up if the
  slave_rows_search_algorithms config option is set, either through
  the config file or through a command line option.

   "2019-07-09T22:45:35.948175Z 0 [Warning] [MY-011069] [Server] The
    syntax '--slave-rows-search-algorithms' is deprecated and will be
    removed in a future release."

Upgrades
========

After upgrading to the version that contains this change then the user
will get the aforementioned deprecation warnings. This, however, shall
not break backwards compatibility.

Security
========

There are no security considerations.

Observability
=============

There are no observability considerations. There is no new
functionality added only a new warning is emitted.

LOW LEVEL CHANGES
=================

1. Deploy DEPRECATED_VAR("") in the slave_rows_search_algorithms
   system variable declaration.

2. Deploy conditional calls to push_deprecated_warn in
   mysql_get_one_options, for slave_rows_search_algorithms so that the
   warning is pushed at server start time, when the options is set.

3. Add deprecation test case validating session and server start
   warnings.