public void Test()
{
// WARNING: This test case makes use of reflection resulting in failures if internals change, but reflection was needed to allow for simulation.
// This test simulates a heavy load on the QueryPlanCache by making it use a SoftLimitMRUCache instance with a size of 1 instead of the default of 128.
// Since this cache moves the most recently used query plans to the top, pushing down the less used query plans until they are removed from the cache,
// the smaller the size of the cache the sooner the query plans will be dropped, making it easier to simulate the problem.
//
// What is the exact problem:
// -> When executing a LINQ query that has a contains with only one element in the collection the same queryExpression string is generated by 2 different types
// of IQueryExpression, the 'NHibernate.Impl.ExpandedQueryExpression' and the 'NHibernate.Linq.NhLinqExpression' and that key is used to store a query plan
// in the QueryPlanCache if a query plan is requested and not found in the cache.
// -> The 'NHibernate.Linq.NhLinqExpression' is typically added during the DefaultQueryProvider.PrepareQuery and the 'NHibernate.Impl.ExpandedQueryExpression'
// less likely during the execution of the LINQ query
// -> Unfortunately the PrepareQuery is casting the returned query plan's QueryExpression to a NhLinqExpression, which it assumes will always be the case, but this
// is not true in a heavy loaded environment where the cache entries are constantly moving when other queries are being executed at the same time.
// -> If you look at the following method inside the DefaultQueryProvider class, then you'll see that by drilling down in the PrepareQuery and in the ExecuteQuery, that
// both operations are actually requesting the query plan from the QueryPlanCache at some point
// public virtual object Execute(Expression expression)
// {
// IQuery query;
// NhLinqExpression nhQuery;
// NhLinqExpression nhLinqExpression = PrepareQuery(expression, out query, out nhQuery);
// return ExecuteQuery(nhLinqExpression, query, nhQuery);
// }
//
// When they are requesting the corresponding query plan according to the QueryExpression's key the PrepareQuery assumes it will get back a NhLinqExpression, while it
// is perfectly possible that the corresponding query plan has a QueryExpression of type ExpandedQueryExpression that has been added during the ExecuteQuery because
// when a request was made for the query plan during the execution, the load on the cache has put the query plan with a QueryExpression of type NhLinqExpression and with
// the same key somewhere at the bottom of the MRU cache and it might even have been removed from the cache, resulting in adding a query plan with a QueryExpression value
// of type ExpandedQueryExpression. When the same LINQ query is executed afterwards, it will go through the PrepareQuery again, assuming that what is returned is a
// NhLinqExpression, while in reality it is an ExpandedQueryExpression, resulting in a cast exception. This problem might even go away due to the same load, pushing out
// the cached query plan with a QueryExpression of ExpandedQueryExpression and have a NhLinqExpression added back again during the next Prepare.
//
// So this test will simulate the pushing out by clearing the cache as long as the QueryExpression of the query plan is NhLinqExpression, once it is an ExpandedQueryExpression
// it will stop clearing the cache, and the exception will occur, resulting in a failure of the test.
// The test will pass once all LINQ expression are executed (1000 max) and no exception occured
var cache = new SoftLimitMRUCache(1);
var queryPlanCacheType = typeof(QueryPlanCache);
// get the planCache field on the QueryPlanCache and overwrite it with the restricted cache
queryPlanCacheType
.GetField("planCache", BindingFlags.Instance | BindingFlags.NonPublic)
.SetValue(Sfi.QueryPlanCache, cache);
// Initiate a LINQ query with a contains with one item in it, of which we know that the underlying IQueryExpression implementations
// aka NhLinqExpression and the ExpandedQueryExpression generate the same key.
IEnumerable <int> personIds = new List <int>
{
1
};
ISession session = null;
try
{
session = OpenSession();
var allLinqQueriesSucceeded = false;
// Setup an action delegate that will be executed on a separate thread and that will execute the LINQ query above multiple times.
// This will constantly interact with the cache (Once in the PrepareQuery method of the DefaultQueryProvider and once in the Execute)
System.Action queryExecutor = () =>
{
var sessionToUse = Sfi.OpenSession();
try
{
for (var i = 0; i < 1000; i++)
{
(from person in session.Query <Person>()
where personIds.Contains(person.Id)
select person).ToList();
}
allLinqQueriesSucceeded = true;
}
finally
{
if (sessionToUse != null && sessionToUse.IsOpen)
{
sessionToUse.Close();
}
}
};
(from person in session.Query <Person>()
where personIds.Contains(person.Id)
select person).ToList();
// the planCache now contains one item with a key of type HQLQueryPlanKey,
// so we are going to retrieve the generated key so that we can use it afterwards to interact with the cache.
// The softReferenceCache field value from the SoftLimitMRUCache cache instance contains this key
var field = cache.GetType().GetField("softReferenceCache", BindingFlags.NonPublic | BindingFlags.Instance);
var softReferenceCache = (IEnumerable)field.GetValue(cache);
// Since the cache only contains one item, the first one will be our key
var queryPlanCacheKey = ((DictionaryEntry)softReferenceCache.First()).Key;
// Setup an action that will be run on another thread and that will do nothing more than clearing the cache as long
// as the value stored behind the cachekey is not of type ExpandedQueryExpression, which triggers the error.
// By running this constantly in concurrency with the thread executing the query, the odds of having the wrong
// QueryExpression in the cache (wrong as in the PrepareQuery is not expecting it) augments, simulating the workings
// of the MRU algorithm under load.
System.Action cacheCleaner = () =>
{
while (!allLinqQueriesSucceeded)
{
var hqlExpressionQueryPlan = (QueryExpressionPlan)cache[queryPlanCacheKey];
if (hqlExpressionQueryPlan != null)
{
if (hqlExpressionQueryPlan.QueryExpression.GetType().FullName.Contains("NHibernate.Impl.ExpandedQueryExpression"))
{
// we'll stop clearing the cache, since the cache now has a different query expression type than expected by the code
break;
}
}
cache.Clear();
// we sleep a little, just to make sure the cache is not constantly empty ;-)
Thread.Sleep(50);
}
};
var queryExecutorTask = Task.Run(queryExecutor);
var cacheCleanerTask = Task.Run(cacheCleaner);
queryExecutorTask.Wait();
cacheCleanerTask.Wait();
Assert.IsTrue(allLinqQueriesSucceeded);
}
finally
{
if (session != null)
{
session.Close();
}
}
}
