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5a4f21c50f
- 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
50 lines
1.8 KiB
C++
50 lines
1.8 KiB
C++
#pragma once
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#include <Access/SettingsProfileElement.h>
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#include <base/defines.h>
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#include <Core/UUID.h>
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#include <boost/container/flat_set.hpp>
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#include <mutex>
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namespace DB
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{
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struct SettingsProfilesInfo;
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/// Watches settings profiles for a specific user and roles.
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class EnabledSettings
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{
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public:
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struct Params
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{
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UUID user_id;
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boost::container::flat_set<UUID> enabled_roles;
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SettingsProfileElements settings_from_enabled_roles;
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SettingsProfileElements settings_from_user;
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auto toTuple() const { return std::tie(user_id, enabled_roles, settings_from_enabled_roles, settings_from_user); }
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friend bool operator ==(const Params & lhs, const Params & rhs) { return lhs.toTuple() == rhs.toTuple(); }
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friend bool operator !=(const Params & lhs, const Params & rhs) { return !(lhs == rhs); }
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friend bool operator <(const Params & lhs, const Params & rhs) { return lhs.toTuple() < rhs.toTuple(); }
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friend bool operator >(const Params & lhs, const Params & rhs) { return rhs < lhs; }
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friend bool operator <=(const Params & lhs, const Params & rhs) { return !(rhs < lhs); }
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friend bool operator >=(const Params & lhs, const Params & rhs) { return !(lhs < rhs); }
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};
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/// Returns the default settings come from settings profiles defined for the user
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/// and the roles passed in the constructor.
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std::shared_ptr<const SettingsProfilesInfo> getInfo() const;
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~EnabledSettings();
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private:
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friend class SettingsProfilesCache;
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explicit EnabledSettings(const Params & params_);
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void setInfo(const std::shared_ptr<const SettingsProfilesInfo> & info_);
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const Params params;
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std::shared_ptr<const SettingsProfilesInfo> info TSA_GUARDED_BY(mutex);
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mutable std::mutex mutex;
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};
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}
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