MySQL Bugs: #80962: Replication does not work when @@GLOBAL.SERVER

Bug #80962	Replication does not work when @@GLOBAL.SERVER_UUID is missing on the master
Submitted:	5 Apr 2016 15:57	Modified:	1 Jul 2016 10:38
Reporter:	Kenny Gryp	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Replication	Severity:	S3 (Non-critical)
Version:	5.7	OS:	Any
Assigned to:		CPU Architecture:	Any
Tags:	Contribution, replication

Description:
In commit https://github.com/mysql/mysql-server/commit/14d664f0ddca7c69d44599693ffcd3455557e7e6#diff..., the code that fetches the SERVER_UUID changed from SHOW GLOBAL VARIABLES to SELECT @@GLOBAL, which now returns an error if the variable is not there instead of an empty row.

However, as you can see in https://github.com/grypyrg/percona-server/blob/5.7/sql/rpl_slave.cc#L2429-L2437 the code actually catches the non presence of a row which is using the SHOW GLOBAL VARIABLES way.

This patch just made it handle SELECT @@GLOBAL properly.

This caused any replication from MySQL < 5.6 to MySQL 5.7 to break.

How to repeat:
setup replication between 55 and 57

Suggested fix:
See attached patch

replication_55_57.patch

Attachment: replication_55_57.patch (application/octet-stream, text), 1.32 KiB.

(Thanks to Ceri Williams for finding the broken code)

Please check for related bug: http://bugs.mysql.com/bug.php?id=79539. Thanks.

Yes, the related bug #79539 shows that the condition will never be used.

The patch I added does not remove the condition, but moves the condition to where it now belongs, to adapt along with the change from SHOW GLOBAL VARIABLES to SELECT @@GLOBAL.

if not, you can't replicate from a master that has no server_uuid at all, which is reported in bug #79272

55 to 57 replication is broken ONLY BECAUSE fetching the server_uuid from the master changed from SHOW GLOBAL VARIABLES to SELECT @@GLOBAL. 
There is no other known reason why replication from 55 to 57 would not work.

I know that replication from 55 to 57 is not officially supported according to the mysql documentation, 
but I recommend to consider fixing this bug anyway. It doesn't automatically mean you have to start officially support 55 to 57.

Thank you for the feedback!
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[20 May 2016 8:42] David Moss

This has been fixed in upcoming versions and the following was added to the
5.7.13 changelog:

Slaves running MySQL 5.7 could not connect to a MySQL 5.5 master due to an
error retrieving the server_uuid, which is not part of MySQL 5.5. This was
caused by changes in the method of retrieving the server_uuid.

(Fixed without using the contribution.)