TableLocker.h

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// # TableLocker.h: Class to hold a (user) lock on a table
// # Copyright (C) 1998,2000
// # Associated Universities, Inc. Washington DC, USA.
// #
// # This library is free software; you can redistribute it and/or modify it
// # under the terms of the GNU Library General Public License as published by
// # the Free Software Foundation; either version 2 of the License, or (at your
// # option) any later version.
// #
// # This library 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 GNU Library General Public
// # License for more details.
// #
// # You should have received a copy of the GNU Library General Public License
// # along with this library; if not, write to the Free Software Foundation,
// # Inc., 675 Massachusetts Ave, Cambridge, MA 02139, USA.
// #
// # Correspondence concerning AIPS++ should be addressed as follows:
// #        Internet email: casa-feedback@nrao.edu.
// #        Postal address: AIPS++ Project Office
// #                        National Radio Astronomy Observatory
// #                        520 Edgemont Road
// #                        Charlottesville, VA 22903-2475 USA
 
#ifndef TABLES_TABLELOCKER_H
#define TABLES_TABLELOCKER_H
 
// # Includes
#include <casacore/casa/aips.h>
#include <casacore/tables/Tables/Table.h>
#include <casacore/tables/Tables/TableLock.h>
 
namespace casacore {  // # NAMESPACE CASACORE - BEGIN
 
// <summary>
// Class to hold a (user) lock on a table.
// </summary>
 
// <use visibility=export>
 
// <reviewed reviewer="UNKNOWN" date="before2004/08/25" tests="tTableLockSync.cc">
// </reviewed>
 
// <prerequisite>
// # Classes you should understand before using this one.
//   <li> <linkto class=Table>Table</linkto>
//   <li> <linkto class=TableLock>TableLock</linkto>
// </prerequisite>
 
// <synopsis>
// Class TableLocker can be used to acquire a (user) lock on a table.
// The lock can be a read or write lock.
// The destructor only releases the lock if the lock was acquired by the
// constructor.
// <p>
// TableLocker simply uses the <src>lock</src> and <src>unlock</src>
// function of class Table.
// The advantage of TableLocker over these functions is that the
// destructor of TableLocker is called automatically by the system,
// so unlocking the table does not need to be done explicitly and
// cannot be forgotten. Especially in case of exception handling this
// can be quite an adavantage.
// <p>
// This class is meant to be used with the UserLocking option.
// It can, however, also be used with the other locking options.
// In case of PermanentLocking(Wait) it won't do anything at all.
// In case of AutoLocking it will acquire and release the lock when
// needed. However, it is possible that the system releases an
// auto lock before the TableLocker destructor is called.
// </synopsis>
 
// <example>
// <srcblock>
// // Open a table to be updated.
// Table myTable ("theTable", TableLock::UserLocking, Table::Update);
// // Start of some critical section requiring a lock.
// {
//     TableLocker lock1 (myTable);
//     ... write the data
// }
// // The TableLocker destructor invoked by } unlocked the table.
// </srcblock>
// </example>
 
// <motivation>
// TableLocker makes it easier to unlock a table.
// </motivation>
 
// # <todo asof="$DATE:$">
// # A List of bugs, limitations, extensions or planned refinements.
// # </todo>
 
class TableLocker {
 public:
  // The constructor acquires a read or write lock on a table
  // which is released by the destructor.
  // If the table was already locked, the destructor will
  // not unlock the table.
  // <br>
  // The number of attempts (default = forever) can be specified when
  // acquiring the lock does not succeed immediately. When nattempts>1,
  // the system waits 1 second between each attempt, so nattempts
  // is more or less equal to a wait period in seconds.
  // An exception is thrown when the lock cannot be acquired.
  explicit TableLocker(Table& table, FileLocker::LockType = FileLocker::Write, uInt nattempts = 0);
 
  // If locked, the destructor releases the lock and flushes the data.
  ~TableLocker();
 
  // The copy constructor and assignment are not possible.
  // Note that only one lock can be held on a table, so copying a
  // TableLocker object imposes great difficulties which objects should
  // release the lock.
  // It can be solved by turning TableLocker into a handle class
  // with a reference counted body class.
  // However, that will only be done when the need arises.
  // <group>
  TableLocker(const TableLocker&) = delete;
  TableLocker& operator=(const TableLocker&) = delete;
  // </group>
 
  // Has this process the read or write lock, thus can the table
  // be read or written safely?
  Bool hasLock(FileLocker::LockType = FileLocker::Write) const;
 
 private:
  // # Variables.
  Table itsTable;
  bool itsHadLock;
};
 
inline Bool TableLocker::hasLock(FileLocker::LockType type) const { return itsTable.hasLock(type); }
 
}  // namespace casacore
 
#endif