152 lines
3.4 KiB
C++
152 lines
3.4 KiB
C++
#pragma once
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#include <functional>
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#include <limits>
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#include <list>
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#include <memory>
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#include <cassert>
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#include "sync.hh"
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#include "ref.hh"
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namespace nix {
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/* This template class implements a simple pool manager of resources
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of some type R, such as database connections. It is used as
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follows:
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class Connection { ... };
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Pool<Connection> pool;
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{
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auto conn(pool.get());
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conn->exec("select ...");
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}
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Here, the Connection object referenced by ‘conn’ is automatically
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returned to the pool when ‘conn’ goes out of scope.
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*/
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template <class R>
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class Pool
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{
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public:
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/* A function that produces new instances of R on demand. */
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typedef std::function<ref<R>()> Factory;
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/* A function that checks whether an instance of R is still
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usable. Unusable instances are removed from the pool. */
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typedef std::function<bool(const ref<R> &)> Validator;
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private:
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Factory factory;
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Validator validator;
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struct State
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{
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size_t inUse = 0;
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size_t max;
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std::vector<ref<R>> idle;
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};
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Sync<State> state;
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std::condition_variable wakeup;
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public:
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Pool(size_t max = std::numeric_limits<size_t>::max(),
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const Factory & factory = []() { return make_ref<R>(); },
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const Validator & validator = [](ref<R> r) { return true; })
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: factory(factory)
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, validator(validator)
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{
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auto state_(state.lock());
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state_->max = max;
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}
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~Pool()
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{
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auto state_(state.lock());
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assert(!state_->inUse);
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state_->max = 0;
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state_->idle.clear();
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}
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class Handle
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{
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private:
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Pool & pool;
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std::shared_ptr<R> r;
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friend Pool;
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Handle(Pool & pool, std::shared_ptr<R> r) : pool(pool), r(r) { }
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public:
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Handle(Handle && h) : pool(h.pool), r(h.r) { h.r.reset(); }
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Handle(const Handle & l) = delete;
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~Handle()
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{
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if (!r) return;
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{
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auto state_(pool.state.lock());
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state_->idle.push_back(ref<R>(r));
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assert(state_->inUse);
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state_->inUse--;
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}
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pool.wakeup.notify_one();
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}
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R * operator -> () { return &*r; }
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R & operator * () { return *r; }
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};
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Handle get()
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{
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{
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auto state_(state.lock());
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/* If we're over the maximum number of instance, we need
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to wait until a slot becomes available. */
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while (state_->idle.empty() && state_->inUse >= state_->max)
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state_.wait(wakeup);
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while (!state_->idle.empty()) {
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auto p = state_->idle.back();
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state_->idle.pop_back();
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if (validator(p)) {
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state_->inUse++;
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return Handle(*this, p);
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}
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}
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state_->inUse++;
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}
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/* We need to create a new instance. Because that might take a
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while, we don't hold the lock in the meantime. */
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try {
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Handle h(*this, factory());
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return h;
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} catch (...) {
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auto state_(state.lock());
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state_->inUse--;
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throw;
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}
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}
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unsigned int count()
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{
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auto state_(state.lock());
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return state_->idle.size() + state_->inUse;
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}
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};
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}
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