Symlink for README creating problems on Windows

[jonahbeckford]

I am trying to resolve diskuv/dkml-installer-ocaml#12 (get pyml into the Diskuv/Windows OCaml distribution).

One thing I am running into is:

PS Z:\source\test2> opam install stdcompat
...
<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] Failed to get sources of stdcompat.19: "C:\\Users\\beckf\\AppData\\Local\\Programs\\DISKUV~1\\tools\\MSYS2\\usr\\bin\\tar.exe xfz
        /c/Users/beckf/AppData/Local/opam/download-cache/sha512/25/25582731170f2f80eed281c48af4904d1d00ef88a38d1b17853395c19a326d7b11e42c17b69f639402c467ff6f9b83a2e9d4dbab7255ac25d466b85a55674bd0 -C
        /c/Users/beckf/AppData/Local/Temp/opam-32848-5a7b45" exited with code 2

#=== ERROR while fetching sources for stdcompat.19 ============================#
OpamSolution.Fetch_fail("\027[33m#\027[0m \027[33mpath\027[0m        Z:\\source\\test2\n\027[33m#\027[0m \027[33mcommand\027[0m     C:\\Users\\beckf\\AppData\\Local\\Programs\\DISKUV~1\\tools\\MSYS2\\usr\\bin\\tar.exe xfz /c/Users/beckf/AppData/Local/opam/download-cache/sha512/25/25582731170f2f80eed281c48af4904d1d00ef88a38d1b17853395c19a326d7b11e42c17b69f639402c467ff6f9b83a2e9d4dbab7255ac25d466b85a55674bd0 -C /c/Users/beckf/AppData/Local/Temp/opam-32848-5a7b45\n\027[33m#\027[0m \027[33mexit-code\027[0m   2\n\027[33m#\027[0m \027[33menv-file\027[0m    C:\\Users\\beckf\\AppData\\Local\\opam\\log\\log-32848-498a6e.env\n\027[33m#\027[0m \027[33moutput-file\027[0m C:\\Users\\beckf\\AppData\\Local\\opam\\log\\log-32848-498a6e.out\n\027[33m### output ###\n\027[0m\027[33m# \027[0m/c/Users/beckf/AppData/Local/Programs/DISKUV~1/tools/MSYS2/usr/bin/tar: stdcompat-19/README: Cannot create symlink to \226\128\152README.md\226\128\153: No such file or directory\n\027[33m# \027[0m/c/Users/beckf/AppData/Local/Programs/DISKUV~1/tools/MSYS2/usr/bin/tar: Exiting with failure status due to previous errors\n")
...

PS Z:\source\test2> with-dkml bash
$ tar tvfz /c/Users/beckf/AppData/Local/opam/download-cache/sha512/25/25582731170f2f80eed281c48af4904d1d00ef88a38d1b17853395c19a326d7b11e42c17b69f639402c467ff6f9b83a2e9d4dbab7255ac25d466b85a55674bd0 | grep RE
lrwxrwxrwx root/root         0 2022-07-15 09:48 stdcompat-19/README -> README.md
-rw-rw-r-- root/root      4477 2022-07-15 09:48 stdcompat-19/README.md

That log is hard to read, so what is happening here is that:

1. stdcompat-19/README -> README.md entry comes before the stdcompat-19/README.md in the tar archive members, so the tar xfz process launched by opam on Windows will read the stdcompat-19/README -> README.md first.
2. tar xfz will try to create the symlink to README.md ... which doesn't exist yet. Dangling symlinks do not work in MSYS2 (and sometimes don't in Cygwin). Failure!

This needs a better answer in opam itself, or perhaps in the opam publish and dune-release tools.

Ways to mitigate in the short-term:

1. Remove the symlink (it doesn't look necessary at all) and cut a new release stdcompat-20.
2. Upload a patched https://github.com/thierry-martinez/stdcompat/releases/download/v19_patch1/stdcompat-19.tar.gz with either a) the symlink removed or b) the symlink added last. And resubmit stdcompat-19 to the opam repository.

[jonahbeckford]

Link to opam master ticket: ocaml/opam#5406