[ http://issues.apache.org/jira/browse/IBATISNET-134?page=all ]
Gilles Bayon reassigned IBATISNET-134:
--------------------------------------
Assign To: Gilles Bayon
> lock mechanism doesn't prevent duplicate cache keys from being inserted into
> IList persistence
> ----------------------------------------------------------------------------------------------
>
> Key: IBATISNET-134
> URL: http://issues.apache.org/jira/browse/IBATISNET-134
> Project: iBatis for .NET
> Type: Bug
> Components: DataMapper
> Versions: DataMapper 1.1
> Environment: .NET Framework 1.1, Windows XP Professional SP1, SQL Server
> 2000.
> Reporter: Jeff Ziolkowski
> Assignee: Gilles Bayon
>
> Every so often, our application would encounter a "Cannot insert key. Cause:
> Item has already been added." exception.
> Upon further investigation, we found we were able to regularly reproduce this
> exception by having two users concurrently perform an action in our
> application that would call a SQL map to retrieve a list of items to populate
> a drop down list. Previous to these actions, the caches for the SQL map were
> cleared, meaning the map had to go to the SQL Server to retrieve a fresh
> listing.
> It seems the request generated by one user would find that the key for the
> list wasn't present in the IList collection and would proceed to go to the
> SQL Server to retrieve the list, but before the thread has the opportunity to
> retrieve the data from the SQL Server and place it in the IList, the request
> generated by the second user comes along and finds the key isn't present in
> the IList collection and ALSO proceeds to retrieve the data from the SQL
> Server. In the end, the requests from both users ends up retrieving data
> from the SQL Server and attempts to place both entries in the IList which
> causes the collection to throw the duplicate key not allowed exeception.
> Here's the property in question. It comes from the LRUCacheController class.
> /// <summary>
> /// Adds an item with the specified key and value into cached
> data.
> /// Gets a cached object with the specified key.
> /// </summary>
> /// <value>The cached object or <c>null</c></value>
> public object this [object key]
> {
> get
> {
> lock (this)
> {
> _keyList.Remove(key);
> _keyList.Add(key);
> return _cache[key];
> }
> }
> set
> {
> lock (this)
> {
> _cache.Add(key, value);
> _keyList.Add(key);
> if (_keyList.Count >
> _cacheSize)
> {
> object oldestKey =
> _keyList[0];
> _keyList.Remove(0);
>
> _cache.Remove(oldestKey);
> }
> }
> }
> }
> Microsoft has noted that the lock(this) statement shouldn't be used on
> non-static members, as multiple instances of the object can exist and
> therefore cause synchronization issues. In a multi-threaded scenario, with
> multiple requests for the same map coming in on different threads, this seems
> like a problem. In addition, within the set section of this code... there
> should be a check to see if the key already exists in the collection just to
> be safe.
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