openssl: let openssl create FindOpenSSL.cmake script equivalent to official

[madebr]

Specify library name and version: openssl/all

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

This is related to #419
When using penssl packages generated from this recipe,
the generated FindOpenSSL.cmake can be used interchangeably with the offical FindOpenSSL.cmake module..

[conan-center-bot]

All green! 😊

[conan-center-bot]

All green! 😊

[conan-center-bot]

An unexpected error happened and has been reported. Help is on its way! 🏇

[david-sinuela-pix4d]

It is probably a good idea to call find_package_handle_standard_args (https://cmake.org/cmake/help/latest/module/FindPackageHandleStandardArgs.html) to report the error if the files could not be found and set some of the variables that are set in the next few lines.

There is more documentation on regarding writing finders in https://gitlab.kitware.com/cmake/community/wikis/doc/tutorials/How-To-Find-Libraries.

[madebr]

Good idea about find_package_handle_standard_args.

[david-sinuela-pix4d]

Are conan recipes targeting cmake versions older than 3.0 (cmake 3 was released in 2014 https://cmake.org/pipermail/cmake/2014-June/057793.html)? Maybe people using conan are using a reasonably recent version of cmake? Is that a valid assumption?

[madebr]

I've just copied the same logic as the FindXXX.cmake files generated by conan.

But more generally, I believe conan should add a feature for cmake_find_package and cmake_find_package_multi. As this pull request won't co-operate well with the multi-generator.

conan would need a feature that allows this:

def package_info(self):
    self.cpp_info.name = "OpenSSL"
    self.cpp_info.libs = [*self._ssl_crypto_library]
    system_libs = []
    if self.settings.os == "Windows":
        system_libs.extend(["crypt32", "msi", "ws2_32", "advapi32", "user32", "gdi32"])
    elif self.settings.os == "Linux":
        system_libs.extend(["dl", "pthread"])
    self.cpp_info.system_libs.extend(system_libs)

    # By default: the generated cmake targets should inherit the dependencies of this recipe
    ssl_system_libs = ["crypt32", "msi", "ws2_32", "advapi32", "user32", "gdi32"]
    crypto_system_libs = ["crypt32", "msi", "advapi32", "user32", "gdi32"]
    for generator in ("cmake_find_package", "cmake_find_package_multi"):
        ssl_library, crypto_library = self._ssl_crypto_library

        # Set OPENSSL_FOUND unconditionally, is a string by default
        self.cpp_info.generators[generator].add_variable("OPENSSL_FOUND", "ON")
        self.cpp_info.generators[generator].add_variable("OPENSSL_VERSION", self.version, type=CMakeVariableType.String)

        # Add OPENSSL_SSL_LIBRARIES variable: it will be set by find_library calls + is genex ==> will be a generator expression when generator == cmake_find_package_multi
        sslLibraryVariable = self.cpp_info.generators[generator].add_variable("OPENSSL_SSL_LIBRARY", self._ssl_library, type=CMakeVariableType.Library, genex=True)

        sslCMakeTarget = self.cpp_info.generators[generator].createCMakeTarget("OpenSSL::SSL")
        # - override libraries
        sslCMakeTarget.libs = [ssl_library]
        # - override system libraries (SSL requires different system_libs then crypto)
        sslCMakeTarget.system_libs = ssl_system_libs
        # - inherit include directories
        # sslCMakeTarget.includedirs = ["include"]
        # - inherit package dependencies (so in CMake, link to all dependencies (CONAN_PKG::dep1, CONAN_PKG_dep2)
        # sslCMakeTarget.deps = ["dep1", "dep2"]
        # - inherit defines
        # sslCMakeTarget.defines = []
        # - inherit link flags
        # sslCMakeTarget.linkflags = []

        # Add OPENSSL_SSL_LIBRARIES variable: it will be set by find_library calls + is genex ==> will be a generator expression when generator == cmake_find_package_multi
        cryptoLibraryVariable = self.cpp_info.generators[generator].add_variable("OPENSSL_CRYPTO_LIBRARY", self._crypto_library, type=CMakeVariableType.Library, genex=True)

        cryptoCMakeTarget = self.cpp_info.generators[generator].createCMakeTarget("OpenSSL::Crypto")
        cryptoCMakeTarget.libs = [crypto_library]
        cryptoCMakeTarget.system_libs = crypto_system_libs
        # inherit other variables ....

        # Add OPENSSL_LIBRARIES: it will be a list of these variables (which can be cmake variables themselves) + is gen_ex
        self.cpp_info.generators[generator].add_variable("OPENSSL_LIBRARIES", [sslLibraryVariable, cryptoLibraryVariable] + system_libs, type=CMakeVariableType.List, genex=True)

        # Add OPENSSL_INCLUDE_DIR variable: is a path + generator expression
        self.cpp_info.generators[generator].add_variable("OPENSSL_INCLUDE_DIR", os.path.join(self.package_folder, "include"), type=CMakeVariableType.Path, genex=True)

The setting of variables might need some dependency tracking (optionally), but I think this interface can serve a lot of use cases.

[conan-center-bot]

All green! 😊

[SSE4]

@madebr why this is needed? as I understand, our openssl package is fully compatible with FindOpenSSL.cmake (and we test that), which is always available (installed with CMake itself). is there a problem?

[conan-center-bot]

All green in build 5 (165152a136596613a80b8d4e6f92d00d164c55af)! 😊

[SSE4]

@madebr friendly ping, pls answer :) also, conflicts

[madebr]

@SSE4

• CMake's FindOpenSSL.cmake has no support such as cmake_multi_package.
• When packaging projects, they will use OpenSSL::Crypto which cmake_multi_package does not support.
  This is just [question] Mimicking CMake Find Modules #419 and needs support from Conan.
  Otherwise, we will need to endlessly monkey patch CMakeLists.txt scripts.

I will close this pr as support by conan is required.