Server-side copies with the local backend on macOS failed with `errno -1` when the destination file already existed, for example with `--inplace` or when used under chunker. This was because copyfile(3) with COPYFILE_CLONE implies COPYFILE_EXCL.
This clones to a temporary name in the same directory and renames it over the destination, which keeps APFS cloning for overwrites and leaves the destination untouched if the clone fails.
Ayush Sharma
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