Alexander,

Thanks a lot for the review and suggested patch, I've used it for the current fix and checked it - it completely passed. I've updated the commit message according the new patch and corrected it as you suggested. 
>Четверг, 28 ноября 2019, 19:41 +03:00 от Alexander Turenko <alexander.turenko@tarantool.org>:
>
>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.
Ok, I've red the man page to know it as you wrote in your patch comments (you misspelled - just not 2.1 but 2.17).
>
>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()
I've used your patch as a fix.
>
>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').
Fixed, also I've checked as you did it and made the same.
>
>> 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.
Ok.
>
>> 
>> Close #4639
>> ---
>> 
>> Github:  https://github.com/tarantool/tarantool/tree/avtikhon/gh-4639-lrt-unit-tests-full-ci
>> Issue:  https://github.com/tarantool/tarantool/issues/4639


-- 
Alexander Tikhonov