Intermittent IIS App Pool Crash using EntityFramework 6.1.3 about ef6

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Upgrading the .Net version from 4.5.2 to 4.6.1 in our production environment solved our intermittent IIS app pool crashing issues.

[divega]

@sglencross can you provide the LINQ query?

Also, can you try setting context.Configuration.UseDatabaseNullSemantics = true before that particular query? We have had previous reports that the default (UseDatabaseNullSemantics = false) can cause larger trees that in a very complicated query can cause stack overflow.

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Setting this before our query is problematic as we have abstracted our context from the linq operations on the DBSet. The query related to the stack overflow above is spread out across several classes and would be difficult to summarize without changing its behaviour.

We do have a simpler query not related to the stack dump above but has also intermittently crashed our IIS app pool. We suspect it is also having the same kind of issue.

//_dbTerms is a IDbSet<Term> exposed on the context
public IEnumerable<TermModel> Execute(int languageId, Guid clientId)
{
    var clientSpecificTerms = _dbTerms.Where(t => t.ClientId == clientId && t.LanguageId == languageId);

    var languageFallbackTerms = _dbTerms.Where(t => t.ClientId == null && t.LanguageId == languageId &&
                                                                    clientSpecificTerms.All(clientTerm => clientTerm.Key != t.Key));

    var clientDefaultTerms = _dbTerms.Where(t => t.ClientId == clientId && t.LanguageId == Languages.English.Id &&
                                                                clientSpecificTerms.All(clientTerm => clientTerm.Key != t.Key) &&
                                                                languageFallbackTerms.All(languageFallback => languageFallback.Key != t.Key));

    var defaultTerms = _dbTerms.Where(t => t.LanguageId == Languages.English.Id && t.ClientId == null &&
                                                            clientSpecificTerms.All(clientTerm => clientTerm.Key != t.Key) &&
                                                            languageFallbackTerms.All(languageFallback => languageFallback.Key != t.Key) &&
                                                            clientDefaultTerms.All(clientDefaultTerm => clientDefaultTerm.Key != t.Key));

    return clientSpecificTerms.Union(languageFallbackTerms)
                              .Union(clientDefaultTerms)
                              .Union(defaultTerms)
                              .OrderBy(t => t.Key)
                              .Select(t => new TermModel { Key = t.Key, Value = t.Value })
                              .ToList();
}

This is the generated SQL

SELECT 1 AS [C1], [Project2].[Key] AS[Key], [Limit1].[Value] AS[Value] FROM 
    (SELECT[Distinct1].[Key] AS[Key]FROM ( SELECT DISTINCT[Extent1].[Key] AS[Key] FROM[dbo].[Terms] AS[Extent1] 
            WHERE([Extent1].[LanguageId] IN(@languageId,1)) 
            AND([Extent1].[ClientId] = @clientId OR[Extent1].[ClientId] IS NULL) 
        ) AS[Distinct1] 
    ) AS [Project2] 
    OUTER APPLY (SELECT TOP (1) [Project3].[Value] AS [Value] FROM 
        (SELECT CASE WHEN([Extent2].[LanguageId] = @languageId) THEN cast(1 as bit) WHEN([Extent2].[LanguageId] <> @languageId) THEN cast(0 as bit) END AS[C1], 
                CASE WHEN([Extent2].[ClientId] = @clientId) THEN cast(1 as bit) WHEN(NOT(([Extent2].[ClientId] = @clientId) AND((CASE WHEN([Extent2].[ClientId] IS NULL) THEN cast(1 as bit) ELSE cast(0 as bit) END) = 0))) THEN cast(0 as bit) END AS[C2], [Extent2].[Value] AS[Value] 
            FROM[dbo].[Terms] AS[Extent2] 
            WHERE ([Extent2].[LanguageId] IN (@languageId,1)) 
            AND ([Extent2].[ClientId] = @clientId OR[Extent2].[ClientId] IS NULL) 
            AND (([Project2].[Key] = [Extent2].[Key]) OR(([Project2].[Key] IS NULL) 
            AND ([Extent2].[Key] IS NULL))) 
    ) AS [Project3] 
    ORDER BY [Project3].[C1] DESC, [Project3].[C2] DESC) AS[Limit1]

Would changing the UseDatabaseNullSemantics setting on this query prevent the intermittent crashes happening with this query?

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We are currently on .Net 4.5.2 and have been informed that there is a known issue that may be contributing to our crashes. Specifically the CLR issue addressed in this hotfix:
https://support.microsoft.com/en-us/kb/3139544#bookmark-issuesresolve
We are going to move to .Net 4.6.1 in the next week or two which will hopefully solve our issue.