using System;

using ServiceStack.Common;

using ServiceStack.Text;

namespace ServiceStack.Redis

{

public class RedisLock

: IDisposable

{

private readonly RedisClient redisClient;

private readonly string key;

public RedisLock(RedisClient redisClient, string key, TimeSpan? timeOut)

{

this.redisClient = redisClient;

this.key = key;

ExecExtensions.RetryUntilTrue(

() =>

{

//This pattern is taken from the redis command for SETNX http://redis.io/commands/setnx

//Calculate a unix time for when the lock should expire

TimeSpan realSpan = timeOut ?? new TimeSpan(365, 0, 0, 0); //if nothing is passed in the timeout hold for a year

DateTime expireTime = DateTime.UtcNow.Add(realSpan);

string lockString = (expireTime.ToUnixTimeMs() + 1).ToString();

//Try to set the lock, if it does not exist this will succeed and the lock is obtained

var nx = redisClient.SetEntryIfNotExists(key, lockString);

if (nx)

return true;

//If we've gotten here then a key for the lock is present. This could be because the lock is

//correctly acquired or it could be because a client that had acquired the lock crashed (or didn't release it properly).

//Therefore we need to get the value of the lock to see when it should expire

string lockExpireString = redisClient.Get<string>(key);

long lockExpireTime;

if (!long.TryParse(lockExpireString, out lockExpireTime))

return false;

//If the expire time is greater than the current time then we can't let the lock go yet

if (lockExpireTime > DateTime.UtcNow.ToUnixTimeMs())

return false;

//If the expire time is less than the current time then it wasn't released properly and we can attempt to

//acquire the lock. This is done by setting the lock to our timeout string AND checking to make sure

//that what is returned is the old timeout string in order to account for a possible race condition.

return redisClient.GetAndSetEntry(key, lockString) == lockExpireString;

},

timeOut

);

}

public void Dispose()

{

redisClient.Remove(key);

}

}

}