Lock.cs

Go to the documentation of this file.

// 
//  Copyright 2010-2015 Deveel
// 
//    Licensed under the Apache License, Version 2.0 (the "License");
//    you may not use this file except in compliance with the License.
//    You may obtain a copy of the License at
// 
//        http://www.apache.org/licenses/LICENSE-2.0
// 
//    Unless required by applicable law or agreed to in writing, software
//    distributed under the License is distributed on an "AS IS" BASIS,
//    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//    See the License for the specific language governing permissions and
//    limitations under the License.
//

using System;
using System.Threading;

namespace Deveel.Data.Transactions {
    public sealed class Lock {
        private bool exclusiveMode;
        private int sharedAccess;

        internal Lock(LockingQueue queue, LockingMode mode, AccessType accessType) {
            Queue = queue;
            AccessType = accessType;
            Mode = mode;
            Queue.Acquire(this);
        }

        private LockingQueue Queue { get; set; }

        public AccessType AccessType { get; private set; }

        public LockingMode Mode { get; private set; }

        private bool WasChecked { get; set; }

        internal ILockable Lockable {
            get { return Queue.Lockable; }
        }

        private void StartMode() {
            lock (this) {
                // If currently in exclusive mode, block until not.

                while (exclusiveMode) {
                    Monitor.Wait(this);
                }

                if (Mode == LockingMode.Exclusive) {
                    // Set this thread to exclusive mode, and wait until all shared modes
                    // have completed.

                    exclusiveMode = true;
                    while (sharedAccess > 0) {
                        Monitor.Wait(this);
                    }
                } else if (Mode == LockingMode.Shared) {
                    // Increase the threads counter that are in shared mode.

                    ++sharedAccess;
                } else {
                    throw new InvalidOperationException("Invalid mode");
                }
            }
        }

        private void EndMode() {
            lock (this) {
                if (Mode == LockingMode.Exclusive) {
                    exclusiveMode = false;
                    Monitor.PulseAll(this);
                } else if (Mode == LockingMode.Shared) {
                    --sharedAccess;
                    if (sharedAccess == 0 && exclusiveMode) {
                        Monitor.PulseAll(this);
                    } else if (sharedAccess < 0) {
                        sharedAccess = 0;
                        Monitor.PulseAll(this);
                        throw new Exception("Too many 'Sahred Locks Release' calls");
                    }
                } else {
                    throw new InvalidOperationException("Invalid mode");
                }
            }
        }

        internal void Acquire() {
            StartMode();
        }

        internal void Release() {
            EndMode();
            Queue.Release(this);

            // TODO: if the lock was not check, silently report the error to the system
        }

        internal void CheckAccess(AccessType accessType) {
            if (AccessType == AccessType.Write && 
                accessType != AccessType.Write)
                throw new InvalidOperationException("Access error on Lock: Tried to Write to a non Write Lock.");

            if (!WasChecked) {
                Queue.CheckAccess(this);
                WasChecked = true;
            }
        }
    }
}