Add option to fail the gh-ost process on SQL warnings

[grodowski]

My team is investigating using Gh-ost for online schema migrations and we'd like to propose a feature we've been relying on in our current approach with LHM.

A new CLI option (raise_on_warnings?) would query for SHOW WARNINGS after each insert batch and fail the migration if warnings are found. This would optionally prevent data loss when e.g.:

• adding a unique index with duplicate rows
• changing table's collation causing new duplicates
• adding a not null constraint with null values
• resizing columns

An equivalent in LHM is raise_on_warnings: https://github.com/Shopify/lhm/blob/46204927a2933cfe239c77ac84ab58b5c7deb3a0/lib/lhm/chunker.rb#L92-L100

I have a prototype that shows the direction of how this option could be implemented in Applier.ApplyIterationInsertQuery: Shopify#1

Some open questions:

• Is it desired to always process warnings (with the CLI option enabled) or only if a row count discrepancy has been detected. The latter is the current, less strict behaviour of Shopify/lhm

I'd love to hear your suggestions before we propose a PR contribution, thanks!

[meiji163]

We think this is a good feature to have in gh-ost. I believe the lhm approach would be best: check warnings if there is a discrepancy in rows affected, then raise any warnings that aren't "Duplicate entry for <shared unique key>".

We welcome a PR!

[grodowski]

Not much happened here last week due to a team event, but this week I want to evaluate the impact of the count addition in the draft PR. @meiji163 did you have a chance to read through the draft? Curious if there's anything else we'd need to look into to move forward.