2 * This file is part of PowerDNS or dnsdist.
3 * Copyright -- PowerDNS.COM B.V. and its contributors
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25 /** This is sort of a light-weight RCU idea.
26 Suitable for when you frequently consult some "readonly" state, which infrequently
27 gets changed. One way of dealing with this is fully locking access to the state, but
28 this is rather wasteful.
30 Instead, in the code below, the frequent users of the state get a "readonly" copy of it,
31 which they can consult. On access, we atomically compare if the local copy is still current
32 with the global one. If it isn't we do the lock thing, and create a new local copy.
34 Meanwhile, to upgrade the global state, methods are offered that do appropriate locking
35 and upgrade the 'generation' counter, signaling to the local copies that they need to be
36 refreshed on the next access.
38 Two ways to change the global copy are available:
39 getCopy(), which delivers a deep copy of the current state, followed by setState()
40 modify(), which accepts a (lambda)function that modifies the state
42 NOTE: The actual destruction of the 'old' state happens when the last local state
43 relinquishes its access to the state.
46 Sometimes, a 'state' can contain parts that can safely be modified by multiple users, for
47 example, atomic counters. In such cases, it may be useful to explicitly declare such counters
50 template<typename T> class GlobalStateHolder;
53 class LocalStateHolder
56 explicit LocalStateHolder(GlobalStateHolder<T>* source) : d_source(source)
59 const T* operator->() // fast const-only access, but see "read-only" above
61 if(d_source->getGeneration() != d_generation) {
62 d_source->getState(&d_state, & d_generation);
67 const T& operator*() // fast const-only access, but see "read-only" above
78 std::shared_ptr<T> d_state;
79 unsigned int d_generation{0};
80 const GlobalStateHolder<T>* d_source;
84 class GlobalStateHolder
87 GlobalStateHolder() : d_state(std::make_shared<T>())
89 LocalStateHolder<T> getLocal()
91 return LocalStateHolder<T>(this);
94 void setState(T state) //!< Safely & slowly change the global state
96 std::shared_ptr<T> newState = std::make_shared<T>(state);
98 std::lock_guard<std::mutex> l(d_lock);
104 T getCopy() const //!< Safely & slowly get a copy of the global state
106 std::lock_guard<std::mutex> l(d_lock);
110 //! Safely & slowly modify the global state
113 std::lock_guard<std::mutex> l(d_lock);
114 auto state=*d_state; // and yes, these three steps are necessary, can't ever modify state in place, even when locked!
116 d_state = std::make_shared<T>(state);
121 typedef T value_type;
123 unsigned int getGeneration() const
127 void getState(std::shared_ptr<T>* state, unsigned int* generation) const
129 std::lock_guard<std::mutex> l(d_lock);
131 *generation = d_generation;
133 friend class LocalStateHolder<T>;
134 mutable std::mutex d_lock;
135 std::shared_ptr<T> d_state;
136 std::atomic<unsigned int> d_generation{1};