accepting --assume-rbr, remove SUPER requirement

[shlomi-noach]

Related issue: #132

Avoid restarting replication, avoid need for SUPER

gh-ost cannot verify the replication thread truly uses RBR, and wishes to restart replication (stop slave; start slave;). However that requires SUPER privilege.
This PR supports --assume-rbr where the user clearly indicates "yes, this is truly and really RBR, and the replica is using RBR". If binlog_format=ROW then gh-ost is happy to proceed without restarting replication, hence without requiring SUPER privilege.

This PR [briefly explain what is does]

• contributed code is using same conventions as original code
• code is formatted via gofmt (please avoid goimports)
• code is built via ./build.sh

[SuperQ]

Normally I would call this --assume-rbr, but "trust" is also fine.

👍

[pbitty]

Hi @shlomi-noach, have you considered the approach of detecting whether ROW format is actually being used? By sending some test updates to a table, we could see the format of the events arriving in the binlog.

Something like this:

1. On master: insert into _rbrtest_gho (a) values (1), (2), (3);
2. Wait for the corresponding Write_rows events in the binlog

If the event arrives as a Query event then we know we're using STATEMENT format and can return the appropriate error message.

This may be out of scope of the actual PR, and I can open an issue if you prefer to have the discussion there.

PS. Kudos on coming up with such a novel approach to live schema changes.

[shlomi-noach]

@pbitty this is a good suggestion, thank you! It may complicate stuff a bit, but seems worthwhile. I think I might first iterate with --trust-rbr and then iterate again with your idea.

[shlomi-noach]

@SuperQ --assume-rbr does sound better. Will change.

[SuperQ]

Maybe we can file a feature request upstream to extend the SHOW SLAVE STATUS to include the binlog mode for the running thread.

[SuperQ]

👍 Nice

[shlomi-noach]

Noting down that in the event gh-ost thinks replication uses RBR where in fact it does not, the current behavior for migration is to just not work, because gh-ost will not be able to intercept its very own changelog table events.

I'm actually good to merge this, and iterate later with double-safety checks, such as:

• oh, I'm waiting for too long for my own changelog event, and replication seems fine, let's inject a noop DDL statement (drop view if exists no_such_view_1234567890randomnumber) and see if we're able to intercept that, hence SBR.

[shlomi-noach]

Need to add documentation:

• mentioning this flag
• discussing SUPER
• and anywhere SUPER or RDS is discussed