Fix for CORE-955

[dhsmith1001]

Added liquibase.sqlgenerator.core.ModifyDataTypeGeneratorMySQL to fix CORE-955 "Supress "modifyDataType will lose primary key settings for mysql" warning if changelog says it is ok". ModifyDataTypeGeneratorMySQL uses a DatabaseSnapshot to determine what properties the column has, and add them to the alter statement so the correct properties are maintained, not lost.

[nvoxland]

I think we will run into a problem with your code under updateSQL mode since the column may not exist or may be different than the point it will be at when the generated SQL would be ran.

[dhsmith1001]

Many changes have problems in updateSQL mode -- hence the option to ignore preConditions. This is another time when having an internal model of the database state would solve the problems inherent in updateSQL mode where the database state doesn't change during update processing.

I will add some code to abort processing if the column doesn't exist. Do you have any other suggestions to make it more robust? I don't want to abandon this change, as it's critical for the product I'm working on at the moment.

[dhsmith1001]

I have looked further at this code, and I can return without adding extra clauses to the SQL if the column metadata is not contained in the database snapshot. However I'd like to emit the warning about losing column constraints if and only if the mode is updateSQL rather than update. I can't see a way to determine this from within the warn method of AbstractSQLGenerator interface. Do you know of a way?

[nvoxland]

You could move the warning to requiresUpdatedDatabaseMetadata and still return false, that method is only called when doing an updateSQL. It's not the most obvious code, however. Perhaps we need a new databaseMetadataUpToDate field added to AbstractSQLGenerator