[ncruces]

You can do it “in place” as the target can be the same as the source. And you can definitely resize it, both truncate it and extend it. The only restriction, really, is that all parts except for the last one need to be at least 5MiB.

GCS compose can also have target be one of the source objects, so you can append (and/or prepend) “in place.”

For GCS compose the suffix/prefix need to be separate visible objects (though you can put a lifecycle on them). For multipart, the parts are not really objects ever.

The performance isn't great because because updating the “index” is slow and rate limited, not because the APIs aren't there.