[feature] CMake integration: proof of concept of sysroot + CMakeDeps priority

[memsharded]

It is not fully clear, maybe not enough tested:

• A vendor tool, like Android or similar, that could come in a Conan package, and that provides its toolchain.cmake file with the details how to use it.
• That toolchain might contain some libraries
• There are also Conan packages, that via CMakeDeps can be found
• But also some other packages should be found in the sysroot of the vendor toolchain

What would be the best way to integrate? The conan_toolchain.cmake can include the vendor toolchain.cmake, or an intermediate one from the user, but does this guarantee behavior if the vendor toolchain changes priorities of CMake for finding package only in itself?

cc / @maikelvdh

[jcar87]

I think with CMake there isn't currently a solution that covers all cases, but there is potential for one.

When not cross compiling, CMakeToolchain will set CMAKE_PREFIX_PATH to the directory where all the generated CMakeDeps files are. In essence, this results in a search order for find_package:

• Search first in the directory of the generated files ("conan" takes priority)
• Fall back to searching in the system directories (e.g. for system libraries or some utilities provided by the build machine)

When we are cross compiling (CMAKE_CROSSCOMPILING is True) and there is a CMAKE_SYSROOT defined, the behaviour depends on the value of the CMAKE_FIND_ROOT_PATH_* variables - which currently CMakeToolchain sets to BOTH.
That results in the following order:

• Search first in the SYSROOT
• Then search in the directory set by Conan
• Then fall back in the host system's directories

This search order can cause problems if the sysroot contains things that we intend to consume from Conan instead. For example, it is typical that a target system will contain things like zlib or OpenSSL - in which case, CMake might find the ones from the sysroot even if they are listed as Conan dependencies.

There are also going to be libraries that one won't be providing from Conan, but will rely on the SYSROOT to provide. This is anything that we typically consider "system" libraries (e.g. the GCC runtime libraries, X11, OpenGL, etc).

Obviously if one knows which libraries are to be provided by Conan and which libraries are part of the "system" - then one can alter the find_package() calls accordingly, but as we know this is a level of burden that build system maintainers typically don't like doing.

The "ideal" search order would be:

• If it's provided by Conan, pick the one up from Conan.
• Then look in the SYSROOT
• As a last resort, fall back to searching in the build machine - This would typically be for host system executables that the consumer project is not expecting from Conan (to be found with find_program)

If CMake had a fourth option in addition to ONLY, NEVER and BOTH, one that is "both, but look in CMAKE_PREFIX_PATH first, then the sysroot, then the rest of the system) - that would probably achieve the behaviour when we are not cross building.

[puetzk]

If CMake had a fourth option in addition to ONLY, NEVER and BOTH, one that is "both, but look in CMAKE_PREFIX_PATH first, then the sysroot, then the rest of the system

You're in luck, but it's very new - part of the CMake 3.24 dependency provider machinery: see CMAKE_FIND_PACKAGE_REDIRECTS_DIR, introduced for exactly this purpose.

[puetzk]

• Search first in the SYSROOT

• Then search in the directory set by Conan

• Then fall back in the host system's directories

Not quite - it actually never searches "in the SYSROOT" per se. Rather, it takes every path it would have searched, in the order it would have searched them, prefixing each with the sysroot (and, if not found there, with other entries in CMAKE_FIND_ROOT_PATH). CMAKE_PREFIX_PATH isn't first, but it is pretty early (only PackageName_ROOT cache keys or environment variables are earlier).

So one workaround (that I've used) is to bind-mount the CONAN_USER_HOME at that same path within the sysroot, so that it, and thus conan's build folders, are found during this prefixed search.

[SSE4]

that might be confusing, and it needs to be distinguish, for cross-compiling, especially.
there are several things involved:

• sysroot is a directory to look for headers and libraries. it's an input for the cross-compilation process. it might be an SDK path (e.g. something like /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk is typical on Apple platforms)
• prefix is an installation directory, from which programs are actually run on system, something like /usr, /usr/local or /opt are common values on *nix systems (depends if it's software provided by package manager, manually built, or some third party, and so on). it's an output for the cross-compilation process.
• destdir is an installation directory, where built artifacts are actually copied. the final destination is a combo of prefix and destdir. it's also an output for the cross-compilation process.

these are traditional values, now CMake variables are mapped to this:

• DESTDIR just maps to destdir, it's fine
• CMAKE_SYSROOT just maps to sysroot, it's fine, so far simple
• CMAKE_INSTALL_PREFIX just maps to prefix, also okay
• CMAKE_PREFIX_PATH is a list of paths appended to the search order, that affects calls like find_library, find_program and so on. that's where conan should append its directories, as it's a list for libraries installed by user. as conan installs every package to its own directory, we expect that list to be quite long.
• CMAKE_SYSTEM_PREFIX_PATH same as former CMAKE_PREFIX_PATH, but for system libraries. it's initialized with CMAKE_INSTALL_PREFIX.

in general, I believe nothing is needed from conan's side, expect population the CMAKE_PREFIX_PATH with conan's directories. otherwise search order can be controlled via CMake variables listed above, as well as more fine grained CMake variables (like individual find modes for programs/libraries/headers, and variables like CMAKE_INCLUDE_PATH, CMAKE_LIBRARY_PATH, CMAKE_PROGRAM_PATH).
specially, in case of cross-compilation, when on different stages it needs to use host/build/target tools/libraries.