ClickHouse/dbms/Common/RWLock.h

#pragma once

#include <Core/Types.h>

#include <list>
#include <vector>
#include <mutex>
#include <condition_variable>
#include <map>
#include <string>
#include <unordered_map>


namespace DB
{

class RWLockImpl;
using RWLock = std::shared_ptr<RWLockImpl>;


/// Implements shared lock with FIFO service
/// Can be acquired recursively (several calls for the same query) in Read mode
///
/// NOTE: it is important to allow acquiring the same lock in Read mode without waiting if it is already
/// acquired by another thread of the same query. Otherwise the following deadlock is possible:
/// - SELECT thread 1 locks in the Read mode
/// - ALTER tries to lock in the Write mode (waits for SELECT thread 1)
/// - SELECT thread 2 tries to lock in the Read mode (waits for ALTER)
class RWLockImpl : public std::enable_shared_from_this<RWLockImpl>
{
public:
    enum Type
    {
        Read,
        Write,
    };

    static RWLock create() { return RWLock(new RWLockImpl); }

    /// Just use LockHolder::reset() to release the lock
    class LockHolderImpl;
    friend class LockHolderImpl;
    using LockHolder = std::shared_ptr<LockHolderImpl>;

    /// Waits in the queue and returns appropriate lock
    /// Empty query_id means the lock is acquired out of the query context (e.g. in a background thread).
    LockHolder getLock(Type type, const String & query_id);

    /// Use as query_id to acquire a lock outside the query context.
    inline static const String NO_QUERY = String();

private:
    RWLockImpl() = default;

    struct Group;
    using GroupsContainer = std::list<Group>;
    using OwnerQueryIds = std::unordered_map<String, size_t>;

    /// Group of locking requests that should be granted concurrently
    /// i.e. a group can contain several readers, but only one writer
    struct Group
    {
        const Type type;
        size_t refererrs;

        std::condition_variable cv; /// all locking requests of the group wait on this condvar

        explicit Group(Type type_) : type{type_}, refererrs{0} {}
    };

    GroupsContainer queue;
    OwnerQueryIds owner_queries;

    mutable std::mutex mutex;
};


}