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 #pragma once
  23 #include <memory>
  24 #include <atomic>
  25 #include <mutex>
  26 /** This is sort of a light-weight RCU idea.
  27     Suitable for when you frequently consult some "readonly" state, which infrequently
  28     gets changed. One way of dealing with this is fully locking access to the state, but
  29     this is rather wasteful.
  30
  31     Instead, in the code below, the frequent users of the state get a "readonly" copy of it,
  32     which they can consult.  On access, we atomically compare if the local copy is still current
  33     with the global one.  If it isn't we do the lock thing, and create a new local copy.
  34
  35     Meanwhile, to upgrade the global state, methods are offered that do appropriate locking
  36     and upgrade the 'generation' counter, signaling to the local copies that they need to be
  37     refreshed on the next access.
  38
  39     Two ways to change the global copy are available:
  40         getCopy(), which delivers a deep copy of the current state, followed by setState()
  41         modify(), which accepts a (lambda)function that modifies the state
  42
  43     NOTE: The actual destruction of the 'old' state happens when the last local state
  44     relinquishes its access to the state.
  45
  46     "read-only"
  47     Sometimes, a 'state' can contain parts that can safely be modified by multiple users, for
  48     example, atomic counters. In such cases, it may be useful to explicitly declare such counters
  49     as mutable.  */
  50
  51 template<typename T> class GlobalStateHolder;
  52
  53 template<typename T>
  54 class LocalStateHolder
  55 {
  56 public:
  57   explicit LocalStateHolder(GlobalStateHolder<T>* source) : d_source(source)
  58   {}
  59
  60   const T* operator->()  // fast const-only access, but see "read-only" above
  61   {
  62     if(d_source->getGeneration() != d_generation) {
  63       d_source->getState(&d_state, & d_generation);
  64     }
  65
  66     return d_state.get();
  67   }
  68   const T& operator*()  // fast const-only access, but see "read-only" above
  69   {
  70     return *operator->();
  71   }
  72
  73   void reset()
  74   {
  75     d_generation=0;
  76     d_state.reset();
  77   }
  78 private:
  79   std::shared_ptr<T> d_state;
  80   unsigned int d_generation{0};
  81   const GlobalStateHolder<T>* d_source;
  82 };
  83
  84 template<typename T>
  85 class GlobalStateHolder
  86 {
  87 public:
  88   GlobalStateHolder() : d_state(std::make_shared<T>())
  89   {}
  90   LocalStateHolder<T> getLocal()
  91   {
  92     return LocalStateHolder<T>(this);
  93   }
  94
  95   void setState(T state) //!< Safely & slowly change the global state
  96   {
  97     std::shared_ptr<T> newState = std::make_shared<T>(state);
  98     {
  99       std::lock_guard<std::mutex> l(d_lock);
 100       d_state = newState;
 101       d_generation++;
 102     }
 103   }
 104
 105   T getCopy() const  //!< Safely & slowly get a copy of the global state
 106   {
 107     std::lock_guard<std::mutex> l(d_lock);
 108     return *d_state;
 109   }
 110
 111   //! Safely & slowly modify the global state
 112   template<typename F>
 113   void modify(F act) {
 114     std::lock_guard<std::mutex> l(d_lock);
 115     auto state=*d_state; // and yes, these three steps are necessary, can't ever modify state in place, even when locked!
 116     act(state);
 117     d_state = std::make_shared<T>(state);
 118     ++d_generation;
 119   }
 120
 121
 122   typedef T value_type;
 123 private:
 124   unsigned int getGeneration() const
 125   {
 126     return d_generation;
 127   }
 128   void getState(std::shared_ptr<T>* state, unsigned int* generation) const
 129   {
 130     std::lock_guard<std::mutex> l(d_lock);
 131     *state=d_state;
 132     *generation = d_generation;
 133   }
 134   friend class LocalStateHolder<T>;
 135   mutable std::mutex d_lock;
 136   std::shared_ptr<T> d_state;
 137   std::atomic<unsigned int> d_generation{1};
 138 };