MSYS2 Environment configuration in opam-repository

MSYS2 installs a friendly fork of Cygwin, by default in C:\msys64. This is based on Cygwin, but uses a different packaging system for the native Windows compiler toolchains from Cygwin's. MSYS2 has a concept of an Environment, and each environment has one compiler toolchain. These toolchains are installed in entirely separate root directories (i.e. they have strictly separated bin, include and lib directories, rather than using multilib approaches) and have their own package namespace.

An MSYS2 Environment is activated in a not dissimilar way to a Visual Studio Tools Command Prompt. The base Environment (MSYS) provides the POSIX utilities needed to support configuration and build of software (and packages installed in this are based on Cygwin's newlib and POSIX emulation layer, rather than native Windows libraries and the Win32 API) and then one additional Environment can then be selected on top of this, causing its bin directory, etc. to be selected in the environment.

This project provides an msys2 package and a series of virtual packages to select which additional MSYS2 Environment should be activated for a given switch.

Mechanism

MSYS2's Environment is fundamentally derived from the MSYSTEM environment variable (which is what the various Terminal shortcuts set before then starting the login shell). The value MSYS refers to the base "default" Environment (that is a POSIX gcc based on Cygwin's newlib) and is assumed to have been set-up by the user (or opam). The remaining 6 values seelect actual native compilation toolchains:

MSYSTEM	Toolchain	Architecture	C Library	C++ Library
UCRT64	gcc	x86_64	ucrt	libstdc++
CLANG64	llvm	x86_64	ucrt	libc++
CLANGARM64	llvm	aarch64	ucrt	libc++
CLANG32	llvm	i686	ucrt	libc++
MINGW64	gcc	x86_64	msvcrt	libstdc++
MINGW32	gcc	i686	msvcrt	libstdc++

Each of these six toolchains is represented by an msys2-toolchain package, msys2-ucrt64, msys2-clang64, etc. Only one of these six packages may be installed in a given opam switch.

gen_config.sh sets MSYSTEM to one of these six values and then sources /etc/msystem (which is what MSYS2's /etc/profile also does). /etc/msystem is responsible for setting 6 environment variables:

• MSYSTEM_CARCH / MINGW_CARCH are identical and are the Architecture from the table above
• MSYSTEM_CHOST / MINGW_CHOST are identical and the target triplet of the compiler (effectively "$MSYSTEM_CARCH-w64-mingw32")
• MSYSTEM_PREFIX is the Unix-style path to the Environment's files (e.g. /mingw64 for the MINGW64 environment)
• MINGW_PACKAGE_PREFIX is the prefix used for packages in the MSYS2 package repository. For example, to install libzstd for the current environment, one installs "$MINGW_PACKAGE_PREFIX-zstd" (e.g. mingw-w64-clang-x86_64-zstd)

The msys2 package:

• Verifies that MSYSTEM_CARCH equals MINGW_CARCH and MSYSTEM_CHOST equals MINGW_CHOST
• Exports opam package variables with:

      _:carch = "$MSYSTEM_CARCH"
      _:chost = "$MSYSTEM_CHOST"
      _:root = "$MSYSTEM_PREFIX"
      _:native-root = "$(cygpath -w $MSYSTEM_PREFIX)"
      _:package-prefix = "$MINGW_PACKAGE_PREFIX"

• Adds MSYSTEM and the six variables set by /etc/msystem to the user's environment (via opam's setenv mechanism)
• Unconditionally sets (again, via setenv):

      export CONFIG_SITE='/etc/config.site'
      export PKG_CONFIG_PATH="$MSYSTEM_PREFIX/lib/pkgconfig:$MSYSTEM_PREFIX/share/pkgconfig"
      export PKG_CONFIG_SYSTEM_INCLUDE_PATH="$MSYSTEM_PREFIX/include"
      export PKG_CONFIG_SYSTEM_LIBRARY_PATH="$MSYSTEM_PREFIX/lib"
      export ACLOCAL_PATH="$MSYSTEM_PREFIX/share/aclocal:$MSYSTEM_PREFIX/usr/share/aclocal"

• PATH, MANPATH and INFOPATH have the appropriate directories under MINGW_PREFIX added using setenv's += operator

This process mirrors that done by /etc/profile in MSYS2.

opam package

gen_config.sh is intended for use in opam-repository and generates an opam .config file. The script itself must be run under MSYS2, not Cygwin (it will gracefully fail to find /etc/msystem if run on Cygwin).