Failing Local Session Module because of KeyNotFoundException

[harper10]

I keep seeing the following error.

Failing module name: Local Session Module
The given key was not present in the dictionary.

Stack Trace:

System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.Collections.Concurrent.ConcurrentDictionary`2[TKey,TValue].get_Item (System.Collections.Concurrent.TKey key) in /private/tmp/source-mono-mac-4.2.0-branch/bockbuild-mono-4.2.0-pre2-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.0/external/referencesource/mscorlib/system/collections/Concurrent/ConcurrentDictionary.cs:line 955
   at Unosquare.Labs.EmbedIO.Modules.LocalSessionModule.<.ctor>b__4_0 (Unosquare.Labs.EmbedIO.WebServer server, System.Net.HttpListenerContext context) in <filename unknown>:line 0
   at Unosquare.Labs.EmbedIO.WebServer.ProcessRequest (System.Net.HttpListenerContext context) in <filename unknown>:line 0

[geoperez]

Hi @harper10, can you help me with a snippet of your code?

[harper10]

What code are you looking for? The error happens intermittently, and it recently started happening more recently. Most of the requests are served normally. I think the error is in the LocalSessionModules.cs file where the ConcurrentDictionary is being used to get items from the dictionary.

[mariodivece]

Maybe we should consider changing the concurrentdictionary (which is buggy in mono) with a regular dictionary and a synclock object. it would be faster and more lightweight to do. Mario Di Vece President & CIO unosquare.com

…

-------- Original message -------- From: Arthur Harper <notifications@github.com> Date: 12/14/16 1:58 PM (GMT-06:00) To: unosquare/embedio <embedio@noreply.github.com> Subject: Re: [unosquare/embedio] Failing Local Session Module because of KeyNotFoundException (#56) What code are you looking for? The error happens intermittently, and it recently started happening more recently. Most of the requests are served normally. I think the error is in the LocalSessionModules.cs file where the ConcurrentDictionary is being used to get items from the dictionary. - You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub<#56 (comment)>, or mute the thread<https://github.com/notifications/unsubscribe-auth/AAVe6vKHg2p6jWWLTA_ENnWtJXC_wvVMks5rIEpggaJpZM4LNVs4>.

[harper10]

I think the problem here is that when Sessions[key] is called, key is not guaranteed to be in the concurrentDictionary.

var allKeys = Sessions.Keys.ToArray();
foreach (var key in allKeys)
{
    var sessionInfo = Sessions[key];
    if (DateTime.Now.Subtract(sessionInfo.LastActivity) > Expiration)
         Sessions.TryRemove(key, out sessionInfo);
}

The key could have been removed by another thread. Instead

var allKeys = Sessions.Keys.ToArray();
foreach (var key in allKeys)
{
    SessionInfo sessionInfo;
    if (Sessions.TryGetValue(key, out sessionInfo))
    {
        if (DateTime.Now.Subtract(sessionInfo.LastActivity) > Expiration)
            Sessions.TryRemove(key, out sessionInfo);
    }
}

will make sure that the key is still in the dictionary.

[geoperez]

@harper10 I just updated the nuget (1.2.5) with your suggestion. Let me know if it works for you now.

[harper10]

I haven't been able to test it because I am getting the same error referenced in #57

[geoperez]

Can you try again?

[harper10]

The error has changed to
'EmbedIO' already has a dependency defined for 'System.Collections.Specialized'

[geoperez]

Can you paste your project.json?

[mariodivece]

@harper10 sometimes I have found that restarting Visual Studio and reopening the solution helps. Xproj projects seems to still be somewhat buggy when it comes to dependencies. Please paste your project.json file here and try my solution also.

[harper10]

I was able to use the updated nuget client, and I got a different error.

Could not install package 'EmbedIO 1.2.7'. You are trying to install this package into a project that targets '.NETFramework,Version=v4.5', but the package does not contain any assembly references or content files that are compatible with that framework.

Unfortunately I cannot switch from .Net Version 4.5 because my project is using mono. Are there anyway plans for mono to be supported with the new package?

[geoperez]

You can use NET452 with MONO, I think you only need to use the TargetFrameworkVersion=v4.5 parameter. We use it on Travis to build EmbedIO in Mono.

xbuild Unosquare.Labs.EmbedIO.Lib.sln /p:"TargetFrameworkVersion=v4.5;Configuration=Release"

[bufferUnderrun]

@geoperez i've updated to the last v1.2.8 package and still have this issue.

[geoperez]

Let me take a look

[geoperez]

I'm getting another error:

Collection was modified; enumeration operation may not execute.

With the following code:

var webServerUrl = "http://localhost:9090";

            var webServer = new WebServer(webServerUrl);
            webServer.RegisterModule(new LocalSessionModule() { Expiration = TimeSpan.FromSeconds(6) });
            webServer.RegisterModule(new FallbackModule((ws, ctx) =>
            {
                return ctx.JsonResponse(new { Message = "OK" });
            }));

            webServer.RunAsync();

            var rnd = new Random();

            Parallel.ForEach(Enumerable.Range(0, 50), async (s, t, l) =>
            {
                await Task.Delay(TimeSpan.FromSeconds(rnd.Next(1, 10)));

                using (var webClient = new HttpClient())
                {
                    var data = await webClient.GetStringAsync(webServerUrl);
                    data.Info();
                }
            });

            Console.ReadKey();

The error it's because the collection change when you delete a session. So I change to create a new collection only for iteration at LocalSessionModule:

var currentSessions = new Dictionary<string, SessionInfo>(m_Sessions);

                    // expire old sessions
                    foreach (var session in currentSessions)
                    {
                        if (session.Value == null) continue;

                        if (DateTime.Now.Subtract(session.Value.LastActivity) > Expiration)
                            DeleteSession(session.Value);
                    }

I'll publish this change at version 1.4.2.

[bufferUnderrun]

I thought that the lock(SessionsSyncLock) forced threads to sync ? So it does not work this way ?
I will test it

[mariodivece]

It should @bufferUnderrun the lock statement creates a critical section in which no more than 1 thread can enter.

[geoperez]

@bufferUnderrun did you test the new version?

[bufferUnderrun]

@geoperez yes, test 1 day and run in prod for 2 days. You can close the issue. Thanks for you fix.