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build: enable Seastar to build shared and static libs in a single build
before this change, we respect the CMake variable named `BUILD_SHARED_LIBS`, and build shared libraries if it is set, build static libraries otherwise. but this model cannot fulfill the needs of a parent project which needs to build both static and shared seastar libraries with different configuration in a multi-config generator settings. as `BUILD_SHARED_LIBS` is a CMake variable which cannot be changed at build time, and hence cannot be assigned with different values using generator expression. so, in this change, we add two options - Seastar_BUILD_STATIC_LIBS - Seastar_BUILD_SHARED_LIBS to configure if the build should generated static libraries and shared libraries respectively. but `BUILD_SHARED_LIBS` is still supported, and the behavior is backward compatible. the only user-visible differences are, the libraries of - seastar - seastar_testing - seastar_perf_testing are now aliases of the targets which builds the static or shared libraries. so one cannot build "seastar" as a target anymore, but should specify which library to build: ``` $ cmake --build build --target seastar # does not work anymore $ cmake --build build --target seastar-static $ cmake --build build --target seastar-shared ``` the reason is the limit of an ALIAS library, which cannot be used as a target. but we still need to use a non-interface library to generate .pc file, where, for instance, we use `$<TARGET_FILE_NAME:seastar_testing>` for the library list of a certain Seastar library. but a CMake-based project including seastar can still link against it using "seastar" as a library. please note, in d8a70b3, we pass `--ftls-mode=initial-exec` when building the tree, to address the problem of recursive call of malloc when using seastar allocator. but we cannot use different compiling options when building both shared libraries and static libraries, so we always pass this option as long as `BUILD_SHARED_LIBS` OR `Seastar_BUILD_SHARED_LIBS` is set *and* when we are building dev/release modes. as these two modes enables the seastar allocator. this should not hurt the overall performance, as we are not using global-dynamic mode, but initial-exec. this is verified using ```console $ cmake \ -DCMAKE_CONFIGURATION_TYPES="Debug;RelWithDebInfo" \ -DCMAKE_CROSS_CONFIGS="Debug;RelWithDebInfo" \ -DCMAKE_DEFAULT_CONFIGS="Debug;RelWithDebInfo" \ -DSeastar_BUILD_SHARED_LIBS=ON \ -DSeastar_BUILD_STATIC_LIBS=ON \ -G "Ninja Multi-Config" -B build -S . $ cmake --build build --config RelWithDebInfo --target seastar-static $ readelf -r build/CMakeFiles/seastar-objects.dir/RelWithDebInfo/src/core/reactor.cc.o| \ grep -i tls | tr -s ' ' | cut -d' ' -f 3,5 | uniq R_X86_64_TPOFF32 __tls_guard R_X86_64_PLT32 _ZStlsRSoRKNSt15_[...] ``` and ```console $ cmake -DCMAKE_BUILD_TYPE="RelWithDebInfo" -G "Ninja" -B build -S . $ cmake --build build --target seastar-static $ readelf -r build/CMakeFiles/seastar-objects.dir/src/core/reactor.cc.o| \ grep -i tls | tr -s ' ' | cut -d' ' -f 3,5 | uniq R_X86_64_TPOFF32 __tls_guard R_X86_64_PLT32 _ZStlsRSoRKNSt15_[...] ``` so the TLS variables are using "TPOFF32" relocation type, in general, it implies the initial-exec TLS model. the PLT32 is not specific TLS, it's more related to external symbols linkages. Refs scylladb/scylladb#2717 Signed-off-by: Kefu Chai <[email protected]>
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