Client session tracking

[insom]

This adds support for Oracle/Percona MySQL 5.7+ session tracking to the client.

Docs on the function: https://dev.mysql.com/doc/refman/5.7/en/mysql-session-track-get-first.html

At Shopify, we wanted to be able to tell which GTID was generated by a given COMMIT so that when we are reading from delayed replicas we can be sure that a given replica has the data that we want on it. This is pretty much the exact flow that's described by the Oracle worklog entry describing the implementation of the feature

Example usage:

require 'mysql2'
c = Mysql2::Client.new(:host => "127.0.0.1", :username => "root", :password => "test1", :flags => "SESSION_TRACK", :database => "test");
c.query("SET SESSION session_track_gtids=OWN_GTID")
c.query("INSERT INTO test VALUES (1)");
gtid = c.session_track(Mysql2::Client::SESSION_TRACK_GTIDS)[0]

c2 = Mysql2::Client.new(:host => "replica.host")
c2.query("SELECT WAIT_FOR_EXECUTED_GTID_SET('#{gtid}')")
# waits forever, by default, probably not what people want
c2.query("SELECT * FROM test")

I mirrored the implementation of client.last_id -- I noticed that wasn't documented in the README.md so I hadn't added this, but if you have any suggestions about how or where to document this feature I'd appreciate that.

I believe that all the ifdefing is right to not compile this feature for MariaDB (which doesn't support it).

[sodabrew]

Thank you!

Few things needed:

• Unit tests (which should skip if MySQL < 5.7)
• README updates to add a section about how to use this feature
• Example code should use the init_command feature because it is works with reconnect to ensure the initial SQL is executed even if the connection drops and is transparently reconnected.

require 'mysql2'

c = Mysql2::Client.new(
    host: "127.0.0.1",
    username: "root",
    password: "test1",
    database: "test",
    flags: "SESSION_TRACK",
    init_command: "SET SESSION session_track_gtids=OWN_GTID")
c.query("INSERT INTO test VALUES (1)")
gtid = c.session_track(SESSION_TRACK_GTIDS).first

[sodabrew]

The pattern I'd like to follow is to always define this method on the object, and within the method, if the feature is unavailable, to return Qnil.

[bibstha]

@sodabrew I've made the following changes

1. Always defined session_track method and conditionally returns nil.
2. Updated readme with information on how to call session_track method.

[bibstha]

Also seems like the failing test is due to network failure. I'm unable to rerun the failing build, hum.

[bibstha]

Hi @sodabrew, do you have some time to review this PR?

[kirs]

I think this example might be wrong? Should it be flags: Mysql2::Client::SESSION_TRACK?

[insom]

You can pass all sorts of things to flags 😁 -- https://github.com/brianmario/mysql2/blob/master/lib/mysql2/client.rb#L56L57 -- so this will work. It might be better to encourage the use of the constant for style reasons though? The other examples do.

[jeremy]

This is a great feature. Thank you for contributing it.

[simi]

Is there any chance to disable/extend this rubocop rule in specs only? rspec is usually bunch of huge blocks by design.

[sodabrew]

Thanks again for this the support for this feature!