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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <string>
#include "mongo/db/client.h"
#include "mongo/db/service_context.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/stdx/mutex.h"
#include "mongo/util/concurrency/thread_name.h"
#include "mongo/util/intrusive_counter.h"
#include "mongo/util/out_of_line_executor.h"
namespace mongo {
/**
* ClientStrand is a reference counted type for loaning Clients to threads.
*
* ClientStrand maintains the lifetime of its wrapped Client object and provides functionality to
* "bind" that Client to one and only one thread at a time. Its functions are synchronized.
*/
class ClientStrand final : public RefCountable {
static constexpr auto kDiagnosticLogLevel = 3;
public:
static constexpr auto kUnableToRecoverClient = "Unable to recover Client for ClientStrand";
/**
* A simple RAII guard to set and release Clients.
*/
class Guard {
public:
Guard() = default;
Guard(Guard&&) = default;
Guard& operator=(Guard&&) = default;
Guard(const Guard&) = delete;
Guard& operator=(const Guard&) = delete;
Guard(ClientStrand* strand) : _strand(strand) {
// Hold the lock for as long as the Guard is around. This forces other consumers to
// queue behind the Guard.
_strand->_mutex.lock();
_strand->_isBound.store(true);
_strand->_setCurrent();
}
~Guard() {
dismiss();
}
void dismiss() noexcept {
auto strand = std::exchange(_strand, {});
if (!strand) {
return;
}
strand->_releaseCurrent();
strand->_isBound.store(false);
strand->_mutex.unlock();
}
Client* get() noexcept {
return _strand->getClientPointer();
}
Client* operator->() noexcept {
return get();
}
Client& operator*() noexcept {
return *get();
}
private:
boost::intrusive_ptr<ClientStrand> _strand;
};
/**
* A simple wrapping executor to run tasks while a Client is bound.
*/
class Executor final : public OutOfLineExecutor {
public:
Executor(ClientStrand* strand, ExecutorPtr exec)
: _strand(strand), _exec(std::move(exec)) {}
void schedule(Task task) override;
private:
boost::intrusive_ptr<ClientStrand> _strand;
ExecutorPtr _exec;
};
/**
* Make a new ClientStrand from a UniqueClient.
*/
static boost::intrusive_ptr<ClientStrand> make(ServiceContext::UniqueClient client);
/**
* Acquire an owning ClientStrand given a client.
*
* This will return nullptr if the Client does not belong to a ClientStrand.
*/
static boost::intrusive_ptr<ClientStrand> get(Client* client);
ClientStrand(ServiceContext::UniqueClient client)
: _clientPtr(client.get()),
_client(std::move(client)),
_threadName(ThreadNameRef{_client->desc()}) {}
/**
* Get a pointer to the underlying Client.
*/
Client* getClientPointer() noexcept {
return _clientPtr;
}
/**
* Set the current Client for this thread and return a RAII guard to release it eventually.
*
* If the Client is currently bound, this function will block until the Client is available.
*/
auto bind() {
return Guard(this);
}
/**
* Run a Task with the Client bound to the current thread.
*
* This function runs the task inline and assumes that the Client is not already bound to the
* current thread. If the Client is currently bound, this function will block until it is
* released.
*/
template <typename Task, typename... Args>
void run(Task task, Args&&... args) {
auto guard = bind();
return task(std::forward<Args>(args)...);
}
/**
* Make a wrapped executor around another.
*/
ExecutorPtr makeExecutor(ExecutorPtr exec) {
return std::make_shared<Executor>(this, std::move(exec));
}
/**
* Return if the strand is currently bound to a Client.
*/
bool isBound() const noexcept {
return _isBound.load();
}
private:
/**
* Bind the Client to the current thread.
*
* This is only valid to call if no other thread has the Client bound.
*/
void _setCurrent() noexcept;
/**
* Release the Client from the current thread.
*
* This is valid to call multiple times on the same thread. It is not valid to mix this with
* Client::releaseCurrent().
*/
void _releaseCurrent() noexcept;
Client* const _clientPtr;
stdx::mutex _mutex; // NOLINT
// Once we have stdx::atomic::wait(), we can get rid of the mutex in favor of this variable.
AtomicWord<bool> _isBound{false};
ServiceContext::UniqueClient _client;
ThreadNameRef _threadName;
ThreadNameRef _oldThreadName;
};
inline void ClientStrand::Executor::schedule(Task task) {
_exec->schedule([task = std::forward<Task>(task), strand = _strand](Status status) mutable {
strand->run(std::move(task), std::move(status));
});
}
using ClientStrandPtr = boost::intrusive_ptr<ClientStrand>;
} // namespace mongo
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