pdns/sholder.hh

   1 /*
   2  * This file is part of PowerDNS or dnsdist.
   3  * Copyright -- PowerDNS.COM B.V. and its contributors
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of version 2 of the GNU General Public License as
   7  * published by the Free Software Foundation.
   8  *
   9  * In addition, for the avoidance of any doubt, permission is granted to
  10  * link this program with OpenSSL and to (re)distribute the binaries
  11  * produced as the result of such linking.
  12  *
  13  * This program is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16  * GNU General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU General Public License
  19  * along with this program; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  21  */
  22 #include <memory>
  23 #include <atomic>
  24 #include <mutex>
  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.
  29
  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.
  33
  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.
  37
  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
  41
  42     NOTE: The actual destruction of the 'old' state happens when the last local state
  43     relinquishes its access to the state.
  44
  45     "read-only"
  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
  48     as mutable.  */
  49
  50 template<typename T> class GlobalStateHolder;
  51
  52 template<typename T>
  53 class LocalStateHolder
  54 {
  55 public:
  56   explicit LocalStateHolder(GlobalStateHolder<T>* source) : d_source(source)
  57   {}
  58
  59   const T* operator->()  // fast const-only access, but see "read-only" above
  60   {
  61     if(d_source->getGeneration() != d_generation) {
  62       d_source->getState(&d_state, & d_generation);
  63     }
  64
  65     return d_state.get();
  66   }
  67   const T& operator*()  // fast const-only access, but see "read-only" above
  68   {
  69     return *operator->();
  70   }
  71
  72   void reset()
  73   {
  74     d_generation=0;
  75     d_state.reset();
  76   }
  77 private:
  78   std::shared_ptr<T> d_state;
  79   unsigned int d_generation{0};
  80   const GlobalStateHolder<T>* d_source;
  81 };
  82
  83 template<typename T>
  84 class GlobalStateHolder
  85 {
  86 public:
  87   GlobalStateHolder() : d_state(std::make_shared<T>())
  88   {}
  89   LocalStateHolder<T> getLocal()
  90   {
  91     return LocalStateHolder<T>(this);
  92   }
  93
  94   void setState(T state) //!< Safely & slowly change the global state
  95   {
  96     std::shared_ptr<T> newState = std::make_shared<T>(state);
  97     {
  98       std::lock_guard<std::mutex> l(d_lock);
  99       d_state = newState;
 100       d_generation++;
 101     }
 102   }
 103
 104   T getCopy() const  //!< Safely & slowly get a copy of the global state
 105   {
 106     std::lock_guard<std::mutex> l(d_lock);
 107     return *d_state;
 108   }
 109
 110   //! Safely & slowly modify the global state
 111   template<typename F>
 112   void modify(F act) {
 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!
 115     act(state);
 116     d_state = std::make_shared<T>(state);
 117     ++d_generation;
 118   }
 119
 120
 121   typedef T value_type;
 122 private:
 123   unsigned int getGeneration() const
 124   {
 125     return d_generation;
 126   }
 127   void getState(std::shared_ptr<T>* state, unsigned int* generation) const
 128   {
 129     std::lock_guard<std::mutex> l(d_lock);
 130     *state=d_state;
 131     *generation = d_generation;
 132   }
 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};
 137 };