libunwind is reentrant and signal safe, and works faster then then
gcc_eh (plus it has some custom patches for problems that have been
found during it's usage in ClickHouse).
gcc_eh may be missing in the system (if gcc was not installed), and
even if it exists clickhouse uses -nodefaultlibs, so some care should be
made to make it work.
Also this library is tiny and there shouln't be any problem to require
it always (there is already tendency to require some contrib libraries,
i.e. poco).
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
If you will have the same header in both libraries you may have some
tricky failures.
And there is at least one header that exists in both: stdatomic.h
Right now there is a workaround for this header, that allows to use C++
version of this header for C - the workaround is called "set
_LIBCPP_COMPILER_CLANG_BASED not only for CXX" [1] and use include_next
[2].
[1]: 9a457ab3c6/include/__config (L39)
[2]: 9a457ab3c6/include/stdatomic.h (L223-L231)
However #42730 changes this, and now it fails to compile because of this.
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
Before, if you store clickhouse sources in /src, there was a typo and it
produce the following error:
CMake Error in contrib/libcxx-cmake/CMakeLists.txt:
Target "cxx" INTERFACE_INCLUDE_DIRECTORIES property contains path:
"/src"
which is prefixed in the source directory.
Also move "src" into PRIVATE, since it is required only for libcxx
itself.
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
- TSA is a static analyzer build by Google which finds race conditions
and deadlocks at compile time.
- It works by associating a shared member variable with a
synchronization primitive that protects it. The compiler can then
check at each access if proper locking happened before. A good
introduction are [0] and [1].
- TSA requires some help by the programmer via annotations. Luckily,
LLVM's libcxx already has annotations for std::mutex, std::lock_guard,
std::shared_mutex and std::scoped_lock. This commit enables them
(--> contrib/libcxx-cmake/CMakeLists.txt).
- Further, this commit adds convenience macros for the low-level
annotations for use in ClickHouse (--> base/defines.h). For
demonstration, they are leveraged in a few places.
- As we compile with "-Wall -Wextra -Weverything", the required compiler
flag "-Wthread-safety-analysis" was already enabled. Negative checks
are an experimental feature of TSA and disabled
(--> cmake/warnings.cmake). Compile times did not increase noticeably.
- TSA is used in a few places with simple locking. I tried TSA also
where locking is more complex. The problem was usually that it is
unclear which data is protected by which lock :-(. But there was
definitely some weird code where locking looked broken. So there is
some potential to find bugs.
*** Limitations of TSA besides the ones listed in [1]:
- The programmer needs to know which lock protects which piece of shared
data. This is not always easy for large classes.
- Two synchronization primitives used in ClickHouse are not annotated in
libcxx:
(1) std::unique_lock: A releaseable lock handle often together with
std::condition_variable, e.g. in solve producer-consumer problems.
(2) std::recursive_mutex: A re-entrant mutex variant. Its usage can be
considered a design flaw + typically it is slower than a standard
mutex. In this commit, one std::recursive_mutex was converted to
std::mutex and annotated with TSA.
- For free-standing functions (e.g. helper functions) which are passed
shared data members, it can be tricky to specify the associated lock.
This is because the annotations use the normal C++ rules for symbol
resolution.
[0] https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
[1] https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/42958.pdf
* libcxxabi uses exception handling library as public
* Don't set -stdlib for internal libc++ - it poisons the checks.
* Enable capnproto in unbundled build back
* Fix shared build
* Major default libs refactor
* Fix build with gcc_eh
* Link all libraries as a big group.
* Use global interface library as a group
* Build capnproto using our cmake
* Use only internal libunwind
