should we finalize in this case?

hmm maybe? TBH, if I had to write that code from zero I would just panic(), as that's a clear developer problem.
Up to you, I can add it.

If we make this also finalise then we could eventually deprecate Finalize() so that Close() is always what you call at the end of use of a readwrite, you just have the option of also making it readonly before you do that. So I think my vote would be to add a finalisation phase in here if it's not already done, then we could update docs to make it clear you can do either.

This is a bit odd to me. When working on a file, there is two outcomes: Finalize() and Discard(). Here you want, in the odd case that the caller didn't choose one, arbitrary choose Finalize()?

With the current PR, you get three clear paths:

• Discard()
• Finalize()
• FinalizeReadOnly() + Close()

If Close() also finalize, we get:

• Discard()
• Finalize()
• Close()
• FinalizeReadOnly() + Close()

I get that you might want this (although the current PR version look more obvious to me):

• Discard()
• Close()
• FinalizeReadOnly() + Close()

In that case, I'd suggest to go there in one go, and skip the confusing in-between.