Add high-level checking for append-only tables

[junjunjd]

Description

Add high-level checking to delete, merge and update operations to return error immediately when the Delta table is append-only.
Use Case
#1747 adds low-level checking for append-only. This issue will add a high-level checking and thus closes #352.

Related Issue(s)
#352

[MrPowers]

@junjunjd - is this done? Can we close this issue?

[junjunjd]

@junjunjd - is this done? Can we close this issue?

@MrPowers The low-level checking is added to commit function. We still need to add some high-level checking so that operations can return an error immediately before calling the commit function.

[roeap]

#1807 adds some checks for protocol compatibility before we start the heavy lifting. To check append only, we theoretically need to see the generated actions on a table. We could try and guess what might get generated - e.g. in a merge we suspect updates - but might end up rejecting operations that may have worked. then there are more obvious ones - like delete - that we could prohibit.

Do you have some specific thoughts on where we could introduce some pre-write checks?

[junjunjd]

@roeap The ProtocolChecker looks great. I added a check_append_only method to be used for high-level checking in delete, update and write with Overwrite save mode.

You are right that for a merge, we won't be able to know if there is data change until the actions are generated. Any high-level checking would need to be added to the execute method when a Remove with data change is generated.

While going over the operations, I also noticed that filesystem check will always be rejected if the table is append-only and files_to_remove is not empty. I think it makes sense to make this operation bypass the low-level checking, otherwise append-only tables will not be able to perform any filesystem check (#1888). What do you think?