conf-python-3 + Cygwin(mingw64) + opam 2.2.x = "python3: command not found"

[tahina-pro]

If, after removing any python3.exe in Windows' PATH, I try opam install conf-python-3 with the mingw64 compilers, either from a full-fledged Cygwin environment, or from a Windows opam with its own copy of Cygwin (under C:\Users\...\AppData\Local\opam\.cygwin\root), then Cygwin python3 installs successfully, but then opam fails with:

#=== ERROR while compiling conf-python-3.9.0.0 ================================#
"python3": command not found.

By contrast, if I install Windows (not Cygwin) Python 3.x "by hand", then opam install conf-python-3 succeeds. But that may not be the result I want, because if I then install an opam package such as z3.4.8.5-1, it fails to compile because of directory separators: \ with Windows Python, but I expect Cygwin / because I expect to compile z3.4.8.5-1 with the Cygwin mingw64 compilers.

Is this related to #26130 ? Thank you in advance @UnixJunkie , @dra27 for clarifying!

Output of opam var os: win32
Output of opam var os-distribution: cygwin

Output of opam config report:

# opam config report
# opam-version         2.2.1
# self-upgrade         no
# system               arch=x86_64 os=win32 os-distribution=cygwin os-version=10.0.22621
# solver               builtin-mccs+glpk
# install-criteria     -removed,-count[avoid-version,changed],-count[version-lag,request],-count[version-lag,changed],-count[missing-depexts,changed],-changed
# upgrade-criteria     -removed,-count[avoid-version,changed],-count[version-lag,solution],-count[missing-depexts,changed],-new
# jobs                 13
# repositories         1 (http) (default repo at e89899ac28c3c15f5a5ba4b01d63a5646a1f8a5d)
# pinned               0
# current-switch       default
# invariant            ocaml >= 4.05.0
# compiler-packages    arch-x86_64.1, conf-mingw-w64-gcc-x86_64.1, flexdll.0.43, mingw-w64-shims.0.2.0, ocaml-base-compiler.5.2.0, ocaml-env-mingw64.1, ocaml-options-vanilla.1, system-mingw.1
# mingw-w64-shims:runtime-x86_64 C:\cygwin64\usr\x86_64-w64-mingw32\sys-root\mingw\bin
# mingw-w64-shims:runtime-i686
# ocaml:native         true
# ocaml:native-tools   true
# ocaml:native-dynlink true
# ocaml:stubsdir       C:/Users/taramana/AppData/Local/opam/default/lib/ocaml/stublibs;C:/Users/taramana/AppData/Local/opam/default/lib/ocaml
# ocaml:preinstalled   false
# ocaml:compiler       5.2.0

[tahina-pro]

I suspect one possible reason may be that opam install is not running with a "full Cygwin PATH environment" such as the one that bash --login would provide. More precisely, python3 should be found as /usr/bin/python3 (a symlink to /etc/alternatives/python3, which is itself a symlink to python), but is not.

[UnixJunkie]

I know nothing about windows, and would not be able to test anything related to it. If you have a suggestion to fix this package, it is welcome and others will review it.

[dra27]

This doesn't have an easy solution, I'm afraid. There are two parts to it:

• The package installation failure I think - as you suggest - is related to symlink problems, but possibly also to an issue with .exe searching in PATH in opam at the moment.
• The problem on the z3 side may need some work with Z3 itself - it would be a lot better if the "Windows" build of Z3 is able to cope with both Python-for-Windows (backslashes) and Python-for-Cygwin (forward-slashes), but there's some more nuance and incentive for sorting that. If opam installs Cygwin's python to it's internal installation, then it's only available during package builds - i.e. not from the Command Prompt or PowerShell instance which ran opam.