lix/src/libutil/pool.hh
Eelco Dolstra 89dc62c174
RemoteStore: Add option to drop old connections from the pool
This is a hack to make hydra-queue-runner free its temproots
periodically, thereby ensuring that garbage collection of the
corresponding paths is not blocked until the queue runner is
restarted.

It would be better if temproots could be released earlier than at
process exit. I started working on a RAII object returned by functions
like addToStore() that releases temproots. However, this would be a
pretty massive change so I gave up on it for now.
2017-09-14 18:10:38 +02:00

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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;
}
void incCapacity()
{
auto state_(state.lock());
state_->max++;
/* we could wakeup here, but this is only used when we're
* about to nest Pool usages, and we want to save the slot for
* the nested use if we can
*/
}
void decCapacity()
{
auto state_(state.lock());
state_->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--;
wakeup.notify_one();
throw;
}
}
size_t count()
{
auto state_(state.lock());
return state_->idle.size() + state_->inUse;
}
size_t capacity()
{
return state.lock()->max;
}
void flushBad()
{
auto state_(state.lock());
std::vector<ref<R>> left;
for (auto & p : state_->idle)
if (validator(p))
left.push_back(p);
std::swap(state_->idle, left);
}
};
}