Set BUILD_SHARED_LIBS and CMAKE_POSITION_INDEPENDENT_CODE as Cache variables in toolchain

[jcar87]

…riables in CMakeToolchain

Changelog: Bugfix: When recipes have shared and fPIC as options, define BUILD_SHARED_LIBS and CMAKE_POSITION_INDEPENDENT_CODE as CACHE variables in the generated cmake_toolchain.cmake instead of regular variables, so that they are not masked by further calls to options().

Docs: Omit

Close #11840

Notes

The bug in #11840 can happen when CMake is operating on a policy version older than 3.13 and the project's CMake scripts make option() calls for either BUILD_SHARED_LIBS and CMAKE_POSITION_INDEPENDENT_CODE. These calls can shadow the values defined by the toolchain (if the toolchain has been loaded already by project()). Using option() for these variables seems valid as per CMake's own documentation.

These variables are only defined in the generated conan_toolchain.cmake if the recipe has fPIC and shared as options. These may be a tad outside of the scope of CMake toolchains.

By setting them as cache variables, we ensure that option() does not override their values if option() is called after the toolchain is loaded, but we also ensure that if the users manually and explicitly overrides the value via -D flag, that value will be respected too.

One caveat is that for projects that set the values after cmake_minimum_required() but before project() - the behaviour remains undefined, the value will depend on how the variable is set.

[garethsb]

Note that this also resolves the similar issue if the upstream CMakeLists.txt uses:

set(BUILD_SHARED_LIBS ON CACHE BOOL "Build shared libraries")

A workaround in Conan 1.53.0 is:

        tc.cache_variables["CMAKE_POLICY_DEFAULT_CMP0126"] = "NEW"

Whereas CMP077 is for option() variables, CMP0126 is for set(CACHE) ones - see https://cmake.org/cmake/help/latest/policy/CMP0126.html.

[memsharded]

Thanks for the tips @garethsb !

[garethsb]

Unfortunately, it looks like the 1.53.0 workaround wasn't effective... all packages were built, all test packages passed... but actually a shared package had been built and used in some cases. #14227 settled on a different workaround:

-        tc.cache_variables["CMAKE_POLICY_DEFAULT_CMP0126"] = "NEW"
+        tc.variables["BUILD_SHARED_LIBS"] = self.options.shared