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- 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
40 lines
1.1 KiB
C++
40 lines
1.1 KiB
C++
#pragma once
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#include "SharedLibraryHandler.h"
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#include <base/defines.h>
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#include <unordered_map>
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#include <mutex>
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namespace DB
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{
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/// Each library dictionary source has unique UUID. When clone() method is called, a new UUID is generated.
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/// There is a unique mapping from diciotnary UUID to sharedLibraryHandler.
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class SharedLibraryHandlerFactory final : private boost::noncopyable
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{
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public:
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static SharedLibraryHandlerFactory & instance();
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SharedLibraryHandlerPtr get(const std::string & dictionary_id);
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void create(
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const std::string & dictionary_id,
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const std::string & library_path,
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const std::vector<std::string> & library_settings,
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const Block & sample_block,
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const std::vector<std::string> & attributes_names);
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bool clone(const std::string & from_dictionary_id, const std::string & to_dictionary_id);
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bool remove(const std::string & dictionary_id);
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private:
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/// map: dict_id -> sharedLibraryHandler
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std::unordered_map<std::string, SharedLibraryHandlerPtr> library_handlers TSA_GUARDED_BY(mutex);
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std::mutex mutex;
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};
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}
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