the manpage for cp says:

     -c    copy files using clonefile(2).  Note that if clonefile(2) is not
           supported for the target filesystem, then cp will fallback to using
           copyfile(2) instead to ensure the copy still succeeds.

is this not the case? it makes it sound like it shouldn't fail, but will fallback gracefully? i think in general we probably need to keep this optimization but figure out a fallback if it doesn't work. we do that in python like this

is this not the case? it makes it sound like it shouldn't fail, but will fallback gracefully? i think in general we probably need to keep this optimization but figure out a fallback if it doesn't work. we do that in python like this

I see the confusion indeed; but looking at the clonefile() manual, it looks like underlying filesystem does not support this call and src and dst are not on the same filesystem are two different error codes. I suspect cp -c is lenient on the first one and not on the second one.

i think in general we probably need to keep this optimization but figure out a fallback if it doesn't work

I'll update my patch

Here is how rules_xcodeproj does it:

if [[ $(stat -f '%d' "$src") == $(stat -f '%d' "$dest") ]]; then
  readonly cp_cmd="cp -c"
else
  readonly cp_cmd="cp"
fi