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
  26 #include "lock.hh"
  27
  28 /** This is sort of a light-weight RCU idea.
  29     Suitable for when you frequently consult some "readonly" state, which infrequently
  30     gets changed. One way of dealing with this is fully locking access to the state, but
  31     this is rather wasteful.
  32
  33     Instead, in the code below, the frequent users of the state get a "readonly" copy of it,
  34     which they can consult.  On access, we atomically compare if the local copy is still current
  35     with the global one.  If it isn't we do the lock thing, and create a new local copy.
  36
  37     Meanwhile, to upgrade the global state, methods are offered that do appropriate locking
  38     and upgrade the 'generation' counter, signaling to the local copies that they need to be
  39     refreshed on the next access.
  40
  41     Two ways to change the global copy are available:
  42         getCopy(), which delivers a deep copy of the current state, followed by setState()
  43         modify(), which accepts a (lambda)function that modifies the state
  44
  45     NOTE: The actual destruction of the 'old' state happens when the last local state
  46     relinquishes its access to the state.
  47
  48     "read-only"
  49     Sometimes, a 'state' can contain parts that can safely be modified by multiple users, for
  50     example, atomic counters. In such cases, it may be useful to explicitly declare such counters
  51     as mutable.  */
  52
  53 template<typename T> class GlobalStateHolder;
  54
  55 template<typename T>
  56 class LocalStateHolder
  57 {
  58 public:
  59   explicit LocalStateHolder(GlobalStateHolder<T>* source) : d_source(source)
  60   {}
  61
  62   const T* operator->()  // fast const-only access, but see "read-only" above
  63   {
  64     if(d_source->getGeneration() != d_generation) {
  65       d_source->getState(&d_state, & d_generation);
  66     }
  67
  68     return d_state.get();
  69   }
  70   const T& operator*()  // fast const-only access, but see "read-only" above
  71   {
  72     return *operator->();
  73   }
  74
  75   void reset()
  76   {
  77     d_generation=0;
  78     d_state.reset();
  79   }
  80 private:
  81   std::shared_ptr<T> d_state;
  82   unsigned int d_generation{0};
  83   const GlobalStateHolder<T>* d_source;
  84 };
  85
  86 template<typename T>
  87 class GlobalStateHolder
  88 {
  89 public:
  90   GlobalStateHolder() : d_state(std::make_shared<T>())
  91   {}
  92   LocalStateHolder<T> getLocal()
  93   {
  94     return LocalStateHolder<T>(this);
  95   }
  96
  97   void setState(const T& state) //!< Safely & slowly change the global state
  98   {
  99     std::shared_ptr<T> newState = std::make_shared<T>(state);
 100     {
 101       *(d_state.lock()) = std::move(newState);
 102       d_generation++;
 103     }
 104   }
 105
 106   void setState(T&& state) //!< Safely & slowly change the global state
 107   {
 108     std::shared_ptr<T> newState = std::make_shared<T>(std::move(state));
 109     {
 110       *(d_state.lock()) = std::move(newState);
 111       d_generation++;
 112     }
 113   }
 114
 115   T getCopy() const  //!< Safely & slowly get a copy of the global state
 116   {
 117     return *(*(d_state.lock()));
 118   }
 119
 120   //! Safely & slowly modify the global state
 121   template<typename F>
 122   void modify(F act) {
 123     auto state = d_state.lock();
 124     auto newState = *(*state); // and yes, these three steps are necessary, can't ever modify state in place, even when locked!
 125     act(newState);
 126     *state = std::make_shared<T>(std::move(newState));
 127     ++d_generation;
 128   }
 129
 130   typedef T value_type;
 131 private:
 132   unsigned int getGeneration() const
 133   {
 134     return d_generation;
 135   }
 136   void getState(std::shared_ptr<T>* state, unsigned int* generation) const
 137   {
 138     *state = *d_state.lock();
 139     *generation = d_generation;
 140   }
 141   friend class LocalStateHolder<T>;
 142
 143   mutable LockGuarded<std::shared_ptr<T>> d_state;
 144   std::atomic<unsigned int> d_generation{1};
 145 };