src/RunGuard.h

/*
	Original Author: Dmitry Sazonov
	StackOverflow: stackoverflow.com/a/28172162/882600
*/

#pragma once

#include <QObject>
#include <QSharedMemory>
#include <QSystemSemaphore>
#include <QCryptographicHash>

class RunGuard
{
	public:
			RunGuard( const QString& key )
				: key( key )
				, memLockKey( generateKeyHash( key, "_memLockKey" ) )
				, sharedmemKey( generateKeyHash( key, "_sharedmemKey" ) )
				, sharedMem( sharedmemKey )
				, memLock( memLockKey, 1 )
			{
				memLock.acquire();
				{
					QSharedMemory fix( sharedmemKey );    // Fix for *nix: http://habrahabr.ru/post/173281/
					fix.attach();
				}
				memLock.release();
			}

			~RunGuard()
			{
				release();
			}

			QString keyString() const { return key; }

			bool isAnotherRunning()
			{
				if ( sharedMem.isAttached() )
					return false;

				memLock.acquire();
				const bool isRunning = sharedMem.attach();
				if ( isRunning )
					sharedMem.detach();
				memLock.release();

				return isRunning;
			}

			bool tryToRun()
			{
				if ( isAnotherRunning() )   // Extra check
					return false;

				memLock.acquire();
				const bool result = sharedMem.create( sizeof( quint64 ) );
				memLock.release();
				if ( !result )
				{
					release();
					return false;
				}

				return true;
			}

			void release()
			{
				memLock.acquire();
				if ( sharedMem.isAttached() )
					sharedMem.detach();
				memLock.release();
			}

			static QString generateKeyHash( const QString& key, const QString& salt )
			{
				QByteArray data;
				data.append( key.toUtf8() );
				data.append( salt.toUtf8() );
				data = QCryptographicHash::hash( data, QCryptographicHash::Sha1 ).toHex();
				return data;
			}

	private:
		const QString key;
		const QString memLockKey;
		const QString sharedmemKey;

		QSharedMemory sharedMem;
		QSystemSemaphore memLock;

		Q_DISABLE_COPY( RunGuard )
};