Spark 3.4: Create non-existing Tag/Branch when using CREATE OR REPLACE

[nastra]

Currently, executing ALTER TABLE x CREATE OR REPLACE TAG xyz will fail with Tag does not exist: xyz.

As a user I'd expect this to create the tag due to the CREATE OR REPLACE usage. The same issue happens with branches.

[nastra]

there was a test gap for branches where a pure replace and a replace on a non-existing branch wasn't exercised, therefore I added replaceBranch() / replaceBranchDoesNotExist() / createOrReplace() (similar tests already exist in TestTagDDL)

[danielcweeks]

This is a little hard to reason through, what about simplifying to something more like:

val exists = null == iceberg.table().refs().get(tag)

if (create && !exists) {
  if (exists && ifNotExists) {
    return Nil
  }
  manageSnapshot.createTag(tag, snapshotId)
} else if (replace && exists) {
  manageSnapshot.replaceTag(tag, snapshotId)
}

[nastra]

that will change the expected behavior unfortunaly. Tests are expecting ALTER TABLE x REPLACE TAG non-existing to throw an exception, therefore we need to distinguish whether CREATE OR REPLACE is used here vs REPLACE

[danielcweeks]

Would removing the exists variable from the second if statement achieve that? Then it would always call replaceTag if it's not a create and result in the expected throw?

[nastra]

yes, this would restore the behavior I mentioned above, but it would change a different behavior where ALTER TABLE x CREATE TAG t is expected to fail when t already exists. The conditions are quite tricky unfortunately and from what I've seen is that we need to explicitly handle create && replace && !refExists