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