MDEV-7394 : Making slave_skip_errors writable at runtime when slave is stopped

[Mahmoud-kh1]

Now we can update slave_skip_errors at runtime when slaves stopped

• This feature makes the slave_skip_errors system variable dynamic allowing it
  to be changed at runtime when the replication slave is stopped.
• Previously slave_skip_errors was read only at runtime and required a
  server restart to be changed.
• Runtime updates are now validated and safely rejected when the slave is
  running preventing inconsistent replication state.

Key Changes

• Added ON_CHECK handler to verify that updates are only allowed while the
  slave is stopped.
• Added ON_UPDATE handler to reinitialize the internal skip error state
  when the variable is changed.
• Added an rpl mtr test that verifies slave_skip_errors can be changed
  dynamically when the slave is stopped and verified that updates are rejected while the slave is running.

behavior now is like that :

Feature :
MDEV-7394

[CLAassistant]

All committers have signed the CLA.

[Mahmoud-kh1]

The following test cases fail because they assume that slave_skip_errors is read only which is no longer true which making their checks fail also
sys_vars.sysvars_server_notembedded
main.variables-notembedded
sys_vars.slave_skip_errors_basic

[gkodinov]

This is a preliminary review. I'd like to request changes mostly because there are tests that need to be re-recorded (and possibly fixed too).

The rest of the comments are just my own limited take on the change. Feel free to ignore and leave for the final review.

[gkodinov]

remove all these lines please.

[Mahmoud-kh1]

thanks for your review I will do this

[gkodinov]

here you can do what you removed from slave.cc: assign the printable names to the variable so it's more readable.

[Mahmoud-kh1]

ok I will do this , thanks

[gkodinov]

please also add actual functionality tests for the newly set variable values.

[Mahmoud-kh1]

ok I will do this

[Mahmoud-kh1]

thanks for the great feedback.
I have added a new test I'd be grateful if you take a look on it , and should I fix the tests that fail because it expect the variable to be read-only or leave it for now.

[gkodinov]

This is bad pattern. I'd initialize the bitmap to empty at the start of the server as it's done now.
And I'd update its contents as needed without de-allocating and re-allocating it.

[grooverdan]

Thank you so much for implementing my 11 year old bug report. I'd be very grateful if you stick through the review process on this. There's a lot to keep correct in the server to implement this change.