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// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <mutex>
namespace search::attribute {
class InterlockGuard;
/**
* Class used to serialize getting enum change exclusive lock. This
* eliminates the need for defining a locking order when getting enum
* change shared locks. Scenario avoided is:
*
* Threads T1, T2: Grouping queries
* Threads T3, T4: Attribute writer threads
*
* Thread T1 gets shared lock on A1
* Thread T2 gets shared lock on A2
* Theead T3 tries to get exclusive lock on A1
* Theead T4 tries to get exclusive lock on A2
* Thread T1 tries to get shared lock on A2
* Thread T2 tries to get shared lock on A1
*
* With the interlock properly used, thread T3 will hold the
* interlock, preventing thread T4 from registering intent to get
* write lock on A2, thus thread T1 can get a shared lock on A2 and complete.
*/
class Interlock {
std::mutex _mutex;
friend class InterlockGuard;
public:
Interlock() noexcept
: _mutex()
{
}
virtual ~Interlock() { }
};
/**
* Class used to serialize getting enum change exclusive lock. The guard
* is passed to EnumModifier constructor to signal that interlock is held.
*/
class InterlockGuard
{
std::lock_guard<std::mutex> _guard;
public:
InterlockGuard(Interlock &interlock)
: _guard(interlock._mutex)
{
}
~InterlockGuard() { }
};
}
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