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