fix: use Long_val for sendfile() parameters to fix file copying in docker

[tonyfettes]

I was using dune in docker, and every it copies some large file in a data_only_dirs it reports that sendfile: No such file or directory. I tried digging into the source and I think converting the ocaml integer to 32-bit version will make it impossible to copy any file that is larger than 4 GB. I have tested in a docker, but I cannot really confirm whether it really works on real linux machine (I'm using macOS).

[emillon]

Thanks, I'll try to test this.

[emillon]

I checked that it fixes the issue with the test in #10334

[emillon]

@dra27 does this look ok or are there any considerations for using Long_val here?

[dra27]

This will fix files over 4GB on 64-bit systems, but it will still overflow on 32-bit systems. It's a slightly confusingly-named macro, but Long_val converts an OCaml integer value to a value with the same size as a pointer (so a 64-bit integer on 64-bit systems) whereas Int_val converts to a C int (which is always 32bits, as already noted).

The only way to fix this for 32-bit systems is to use an int64 or, with slightly heavier-weight C code, an int63.

I'm not sure how much that level of 32-bit support really matters for Dune, though?

[rgrinberg]

Yes, there's no need to bother 32 bit support for dune itself.