File uploading code spawns all parts uploading into background. If this "spawning" fails (not the uploading code itself), any fiber that was spawned before is orphaned. It will eventually stop on its own, by while it's alive it may use(-after-free) the do_upload_file object.
Another issue with not handling spawn exception, is that multipart upload object is not aborted in this case. So it's leaked until garbage collector picks it up, which is not critical, but unpleasant.
Closesscylladb/scylladb#21139
* github.com:scylladb/scylladb:
s3/client: Restore indentation after previous patch
s3/client: Catch do_upload_file::upload_part() exceptions
d6a79fefda