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#pragma once
///@file
#include "error.hh"
#include "sync.hh"
#include <queue>
#include <functional>
#include <thread>
#include <atomic>
namespace nix {
MakeError(ThreadPoolShutDown, Error);
/**
* A simple thread pool that executes a queue of work items
* (lambdas).
*/
class ThreadPool
{
public:
ThreadPool(size_t maxThreads = 0);
~ThreadPool();
/**
* An individual work item.
*
* \todo use std::packaged_task?
*/
typedef std::function<void()> work_t;
/**
* Enqueue a function to be executed by the thread pool.
*/
void enqueue(const work_t & t);
/**
* Execute work items until the queue is empty.
*
* \note Note that work items are allowed to add new items to the
* queue; this is handled correctly.
*
* Queue processing stops prematurely if any work item throws an
* exception. This exception is propagated to the calling thread. If
* multiple work items throw an exception concurrently, only one
* item is propagated; the others are printed on stderr and
* otherwise ignored.
*/
void process();
private:
size_t maxThreads;
struct State
{
std::queue<work_t> pending;
size_t active = 0;
std::exception_ptr exception;
std::vector<std::thread> workers;
bool draining = false;
};
std::atomic_bool quit{false};
Sync<State> state_;
std::condition_variable work;
void doWork(bool mainThread);
void shutdown();
};
/**
* Process in parallel a set of items of type T that have a partial
* ordering between them. Thus, any item is only processed after all
* its dependencies have been processed.
*/
template<typename T>
void processGraph(
ThreadPool & pool,
const std::set<T> & nodes,
std::function<std::set<T>(const T &)> getEdges,
std::function<void(const T &)> processNode)
{
struct Graph {
std::set<T> left;
std::map<T, std::set<T>> refs, rrefs;
};
Sync<Graph> graph_(Graph{nodes, {}, {}});
std::function<void(const T &)> worker;
worker = [&](const T & node) {
{
auto graph(graph_.lock());
auto i = graph->refs.find(node);
if (i == graph->refs.end())
goto getRefs;
goto doWork;
}
getRefs:
{
auto refs = getEdges(node);
refs.erase(node);
{
auto graph(graph_.lock());
for (auto & ref : refs)
if (graph->left.count(ref)) {
graph->refs[node].insert(ref);
graph->rrefs[ref].insert(node);
}
if (graph->refs[node].empty())
goto doWork;
}
}
return;
doWork:
processNode(node);
/* Enqueue work for all nodes that were waiting on this one
and have no unprocessed dependencies. */
{
auto graph(graph_.lock());
for (auto & rref : graph->rrefs[node]) {
auto & refs(graph->refs[rref]);
auto i = refs.find(node);
assert(i != refs.end());
refs.erase(i);
if (refs.empty())
pool.enqueue(std::bind(worker, rref));
}
graph->left.erase(node);
graph->refs.erase(node);
graph->rrefs.erase(node);
}
};
for (auto & node : nodes)
pool.enqueue(std::bind(worker, std::ref(node)));
pool.process();
if (!graph_.lock()->left.empty())
throw Error("graph processing incomplete (cyclic reference?)");
}
}
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