[MDEV-29827] Clear error when --event-scheduler=ON is combined with --skip-grant-tables

When the server is started with `--event-scheduler=ON` and with
`--skip-grant-tables` (or built as embedded server which has no grant
tables at all), the event scheduler *appears* to be enabled (`SELECT
@@global.event_scheduler` returns `'ON'`), but attempting to
manipulate it in any way returns a misleading error message:

  "Cannot proceed, because event scheduler is disabled"

Possible solutions:

1. Fast-fail: fail immediately on startup if `EVENT_SCHEDULER` is set to
   any value other than `DISABLED` when starting up without grant
   tables, then prevent `SET GLOBAL event_scheduler` while running.

   Problem: there are existing setup scripts and code which start with
   this combination and assume it will not fail.

2. Warn and change value: if `EVENT_SCHEDULER` is set to any value
   other than `DISABLED` when starting, print a warning and change it
   immediately to `DISABLED`.

   Advantage: The value of the `EVENT_SCHEDULER` system variable after
   startup will be consistent with its functional unavailability.

3. Display a clear error: if `EVENT_SCHEDULER` is enabled, but grant
   tables are not enabled, then ensure error messages clearly explain
   the fact that the combination is not supported.

   Advantage: The error message encountered by the end user when
   attempting to manipulate the event scheduler (such as `CREATE
   EVENT`) is clear and explicit.

This commit implements the combination of solutions (2) and (3): it
will set `EVENT_SCHEDULER=DISABLED` on startup (reflecting the
functional reality) and it will print a startup warning, *and* it will
print a *distinct* error message each time that an end user attempts to
manipulate the event scheduler, so that the end user will clearly understand
the problem even if the startup messages are not visible at that point.

It also adds an MTR test `main.events_skip_grant_tables` to verify the
expected behavior, and the unmodified `main.events_restart` test
continues to demonstrate no change in the error message when the event
scheduler is non-functional for *different* reasons.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the BSD-new
license. I am contributing on behalf of my employer Amazon Web Services

1 files changed, 8 insertions, 0 deletions

diff --git a/mysql-test/main/events_skip_grant_tables.result b/mysql-test/main/events_skip_grant_tables.result
new file mode 100644
index 00000000000..2bd2e383a23
--- /dev/null
+++ b/mysql-test/main/events_skip_grant_tables.result
@@ -0,0 +1,8 @@
+call mtr.add_suppression("Event Scheduler will not function when starting with --skip-grant-tables or --bootstrap.");
+CREATE EVENT test ON SCHEDULE AT CURRENT_TIMESTAMP DO DO NULL;
+ERROR HY000: Event scheduler cannot function with --skip-grant-tables, --bootstrap, or embedded build
+select (@@global.event_scheduler='DISABLED') as expect_1;
+expect_1
+1
+set global event_scheduler=1;
+ERROR HY000: The MariaDB server is running with the --event-scheduler=DISABLED or --skip-grant-tables option so it cannot execute this statement