Session leak in the Query Plan Cache

It looks like whenever a session is used for the first, uncached execution of a query, the Query Plan Cache will hold a reference to that NhQueryable (and hence, that session) via the Expression Tree generated by the Linq extension methods.

Here is an example of a SOS.dll !gcroot dump:

Thread 2264:

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    • WARNING: Unable to verify checksum for System.ni.dll
      0000000018e6ecb0 000007fef6638de6 System.Net.TimerThread.ThreadProc()
      r12: (interior)
      -> 00000006fff977e8 System.Object[]
      -> 00000003fff90658 System.Web.Hosting.ObjectCacheHost
      -> 00000006800b5cd0 System.Collections.Generic.Dictionary`2[[System.Runtime.Caching.MemoryCache, System.Runtime.Caching],[System.Web.Hosting.ObjectCacheHost+MemoryCacheInfo, System.Web]]
      -> 00000006800b5df0 System.Collections.Generic.Dictionary`2+Entry[[System.Runtime.Caching.MemoryCache, System.Runtime.Caching],[System.Web.Hosting.ObjectCacheHost+MemoryCacheInfo, System.Web]][]
      -> 00000004fffc3248 System.Runtime.Caching.MemoryCache
      -> 00000004fffc3298 System.Object[]
      -> 00000004fffc4410 System.Runtime.Caching.MemoryCacheStore
      -> 00000004fffc44e0 System.Runtime.Caching.CacheExpires
      [... removed repetitive cache timer instances ...]
      -> 00000003fff8da38 System.Web.RequestTimeoutManager
      -> 00000003fff8da70 System.Object[]
      -> 00000003fff8db20 System.Web.Util.DoubleLinkList
      -> 0000000403438450 System.Web.RequestTimeoutManager+RequestTimeoutEntry
      -> 0000000403436ee8 System.Web.HttpContext
      -> 0000000502f505b0 System.Collections.Hashtable
      -> 0000000502f856d8 System.Collections.Hashtable+bucket[]
      -> 0000000502f50660 Autofac.Core.Lifetime.LifetimeScope
      -> 0000000502f506e0 System.Collections.Generic.Dictionary`2[[System.Guid, mscorlib],[System.Object, mscorlib]]
      -> 0000000502f91678 System.Collections.Generic.Dictionary`2+Entry[[System.Guid, mscorlib],[System.Object, mscorlib]][]
      -> 0000000502f51e58 NHibernate.Impl.SessionImpl
      -> 00000001fff942e8 NHibernate.Impl.SessionFactoryImpl
      -> 00000001fff95588 NHibernate.Engine.Query.QueryPlanCache
      -> 00000001fff96918 NHibernate.Util.SoftLimitMRUCache
      -> 00000001fff969b0 NHibernate.Util.LRUMap
      -> 00000001fff969e0 NHibernate.Util.SequencedHashMap+Entry
      -> 00000001002beb80 NHibernate.Util.SequencedHashMap+Entry
      -> 00000001002b9930 NHibernate.Engine.Query.HQLExpressionQueryPlan
      -> 00000001002b9290 NHibernate.Linq.NhLinqExpression
      -> 00000001002b9410 System.Linq.Expressions.MethodCallExpressionN
      -> 00000001002b93f0 System.Runtime.CompilerServices.TrueReadOnlyCollection`1[[System.Linq.Expressions.Expression, System.Core]]
      -> 00000001002b93c0 System.Object[]
      -> 00000001002b91c0 System.Linq.Expressions.ConstantExpression
      -> 00000001002b9188 NHibernate.Linq.NhQueryable`1[[REDACTED]]
      -> 00000001002b91a8 NHibernate.Linq.DefaultQueryProvider
      -> 00000001002b3f08 NHibernate.Impl.SessionImpl

Further notes are available in the Google Groups postings:
https://groups.google.com/d/topic/nhusers/v_6WCod79XE/discussion
https://groups.google.com/d/topic/nhibernate-development/agsCK1cZpkg/discussion

I've got a test case and a candidate fix – I'll make a pull request soon.