fiber.cc now uses clock_getttime(), which requires -lrt with glibc prior
2.1. Let's add librt dependency for core library for glibc of those
versions.

We should check for glibc version (using __GLIBC__ and __GLIBC_MINOR__
macros) or perform a check whether we able to use clock_gettime() w/o
librt.

So the patch should look like this:

| diff --git a/CMakeLists.txt b/CMakeLists.txt
| index 7d6c084..ca968eb 100644
| --- a/CMakeLists.txt
| +++ b/CMakeLists.txt
| @@ -116,6 +116,12 @@ else ()
| set(HAVE_CLOCK_GETTIME ${HAVE_CLOCK_GETTIME_DECL})
| endif ()
| set(CMAKE_REQUIRED_LIBRARIES "")
| +# According to `man 2 clockgettime` the glibc wrapper requires
| +# -lrt on glibc versions before 2.17. We need to check whether
| +# the function is available without -lrt to set this linker option
| +# conditionally.
| +check_function_exists(clock_gettime HAVE_CLOCK_GETTIME_WITHOUT_RT)
| +
| check_symbol_exists(__get_cpuid cpuid.h HAVE_CPUID)
|
| # Checks for libev
| diff --git a/src/lib/core/CMakeLists.txt b/src/lib/core/CMakeLists.txt
| index e60b519..72e631b 100644
| --- a/src/lib/core/CMakeLists.txt
| +++ b/src/lib/core/CMakeLists.txt
| @@ -46,3 +46,11 @@ target_link_libraries(core salad small uri decNumber ${LIBEV_LIBRARIES}
| if (ENABLE_BACKTRACE AND NOT TARGET_OS_DARWIN)
| target_link_libraries(core gcc_s ${UNWIND_LIBRARIES})
| endif()
| +
| +# Since fiber.top() introduction, fiber.cc, which is part of core
| +# library, depends on clock_gettime() syscall, so we should set
| +# -lrt when it is appropriate. See a comment for
| +# HAVE_CLOCK_GETTIME_WITHOUT_RT in ${REPO}/CMakeLists.txt.
| +if ("${HAVE_CLOCK_GETTIME}" AND NOT "${HAVE_CLOCK_GETTIME_WITHOUT_RT}")
| + target_link_libraries(core rt)
| +endif()

CCed Serge as author of the patch that introduces the regression. Serge,
it seems we should disable fiber.top() or give explicit error when
HAVE_CLOCK_GETTIME is false.

WBR, Alexander Turenko.

On Fri, Nov 22, 2019 at 04:51:38PM +0300, Alexander V. Tikhonov wrote:
> After the commit 77fa45bd05f8cdd4c0f9bad85185ef5b61528d49

Please, use format commit_hash ('commit message header').

> the unit tests builds failed like:
>
> /opt/rh/devtoolset-6/root/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/ld:
> ../../src/lib/core/libcore.a(fiber.c.o): undefined reference to symbol
> 'clock_gettime@@GLIBC_2.2.5'
> //lib64/librt.so.1: error adding symbols: DSO missing from command line
> collect2: error: ld returned 1 exit status
> test/unit/CMakeFiles/cbus.test.dir/build.make:108: recipe for target
> 'test/unit/cbus.test' failed
> make[2]: *** [test/unit/cbus.test] Error 1
>
> To fix the issue the 'rt' library was added to test unit cmake file only
> for linux and freebsd targets.

It is glibc flaw, no need to change something on FreeBSD.