db-lab1/bserv/database.cpp

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#include "pch.h"
#include "bserv/database.hpp"
namespace bserv {
std::shared_ptr<db_connection> db_connection_manager::get_or_block() {
// `counter_lock_` must be acquired first.
// exchanging this statement with the next will cause dead-lock,
// because if the request is blocked by `counter_lock_`,
// the destructor of `db_connection` will not be able to put
// itself back due to the `queue_lock_` has already been acquired
// by this request!
counter_lock_.lock();
// `queue_lock_` is acquired so that only one thread will
// modify the `queue_`
std::lock_guard<std::mutex> lg{ queue_lock_ };
std::shared_ptr<raw_db_connection_type> conn = queue_.front();
queue_.pop();
// if there are no connections in the `queue_`,
// `counter_lock_` remains to be locked
// so that the following requests will be blocked
if (queue_.size() != 0) counter_lock_.unlock();
return std::make_shared<db_connection>(*this, conn);
}
db_connection::~db_connection() {
std::lock_guard<std::mutex> lg{ mgr_.queue_lock_ };
mgr_.queue_.emplace(conn_);
// if this is the first available connection back to the queue,
// `counter_lock_` is unlocked so that the blocked requests will
// be notified
if (mgr_.queue_.size() == 1)
mgr_.counter_lock_.unlock();
}
} // bserv