mirror of
https://github.com/ClickHouse/ClickHouse.git
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104 lines
4.3 KiB
C++
104 lines
4.3 KiB
C++
#pragma once
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#include <base/types.h>
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#include <chrono>
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#include <list>
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#include <vector>
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#include <mutex>
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#include <condition_variable>
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#include <map>
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#include <string>
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#include <unordered_map>
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namespace DB
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{
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class RWLockImpl;
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using RWLock = std::shared_ptr<RWLockImpl>;
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/// Implements Readers-Writers locking algorithm that serves requests in "Phase Fair" order.
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/// (Phase Fair RWLock as suggested in https://www.cs.unc.edu/~anderson/papers/rtsj10-for-web.pdf)
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/// It is used for synchronizing access to various objects on query level (i.e. Storages).
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///
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/// In general, ClickHouse processes queries by multiple threads of execution in parallel.
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/// As opposed to the standard OS synchronization primitives (mutexes), this implementation allows
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/// unlock() to be called by a thread other than the one, that called lock().
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/// It is also possible to acquire RWLock in Read mode without waiting (FastPath) by multiple threads,
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/// that execute the same query (share the same query_id).
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///
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/// NOTE: it is important to allow acquiring the same lock in Read mode without waiting if it is already
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/// acquired by another thread of the same query. Otherwise the following deadlock is possible:
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/// - SELECT thread 1 locks in the Read mode
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/// - ALTER tries to lock in the Write mode (waits for SELECT thread 1)
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/// - SELECT thread 2 tries to lock in the Read mode (waits for ALTER)
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///
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/// NOTE: it is dangerous to acquire lock with NO_QUERY, because FastPath doesn't
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/// exist for this case and deadlock, described in previous note,
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/// may occur in case of recursive locking.
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class RWLockImpl : public std::enable_shared_from_this<RWLockImpl>
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{
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public:
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enum Type
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{
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Read,
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Write,
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};
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static RWLock create() { return RWLock(new RWLockImpl); }
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/// Just use LockHolder::reset() to release the lock
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class LockHolderImpl;
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friend class LockHolderImpl;
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using LockHolder = std::shared_ptr<LockHolderImpl>;
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/// Empty query_id means the lock is acquired from outside of query context (e.g. in a background thread).
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LockHolder getLock(Type type, const String & query_id,
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const std::chrono::milliseconds & lock_timeout_ms = std::chrono::milliseconds(0), bool throw_in_fast_path = true);
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/// Use as query_id to acquire a lock outside the query context.
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inline static const String NO_QUERY = String();
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inline static const auto default_locking_timeout_ms = std::chrono::milliseconds(120000);
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/// Returns all query_id owning locks (both read and write) right now.
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/// !! This function are for debugging and logging purposes only, DO NOT use them for synchronization!
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std::unordered_map<String, size_t> getOwnerQueryIds() const;
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String getOwnerQueryIdsDescription() const;
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private:
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/// Group of locking requests that should be granted simultaneously
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/// i.e. one or several readers or a single writer
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struct Group
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{
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const Type type;
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size_t requests = 0;
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bool ownership = false; /// whether this group got ownership? (that means `cv` is notified and the locking requests should stop waiting)
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std::condition_variable cv; /// all locking requests of the group wait on this condvar
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explicit Group(Type type_) : type{type_} {}
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};
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using GroupsContainer = std::list<Group>;
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using OwnerQueryIds = std::unordered_map<String /* query_id */, size_t /* num_owners */>;
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mutable std::mutex internal_state_mtx;
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GroupsContainer readers_queue;
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GroupsContainer writers_queue;
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GroupsContainer::iterator rdlock_owner{readers_queue.end()}; /// last group with ownership in readers_queue in read phase
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/// or readers_queue.end() in writer phase
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GroupsContainer::iterator wrlock_owner{writers_queue.end()}; /// equals to writers_queue.begin() in write phase
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/// or writers_queue.end() in read phase
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OwnerQueryIds owner_queries;
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RWLockImpl() = default;
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void unlock(GroupsContainer::iterator group_it, const String & query_id) noexcept;
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void dropOwnerGroupAndPassOwnership(GroupsContainer::iterator group_it) noexcept;
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void grantOwnership(GroupsContainer::iterator group_it) noexcept;
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void grantOwnershipToAllReaders() noexcept;
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};
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}
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