Table of Contents

Investigating exception occurrence on Finalizer thread

This section illustrates how to investigate a crash of the SLScripting process in case an exception occurred on the Finalizer thread.

A finalizer can be used to perform cleanup when the class instance is being collected by the garbage collector. Instances of a class that contain a Finalizer get added to the Finalize queue, which then get processed by the Finalizer thread. It is important to make sure that the Finalizer does not throw an exception, as this results in a process crash when the Finalizer thread is executing the Finalizer method of the type that is being freed.

Note
  • Typically, you can avoid having to provide a finalizer by using the SafeHandle or derived classes to wrap any unmanaged handle.
  • We recommend providing a way to explicitly release resources before the garbage collector frees the object by implementing the IDisposable interface to perform the necessary cleanup for the object. This can considerably improve the performance of the application. Note, however, that even if you implement IDisposable, the finalizer acts as a safeguard to clean up the resources if the call to the Dispose method fails.
Tip

For more information about cleaning up resources, refer to:

Investigating the crash dump in Visual Studio

In case a crash occurs because of an exception in the Finalize method of a class instance, a dump of SLManagedScripting is created. The ERRORLOG.TXT file should contain an indication of this exception. For example:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Skyline.Protocol.MyExtension.MyClass.Finalize()

To investigate the crash dump:

  1. Open the SLManagedScripting dump file in Visual Studio, and click the Run Diagnostic Analysis link in the Actions list.

    The Analysis results pane should show that an exception was found on the heap:

    Analysis results
    Visual Studio Analysis results

    In this case, it mentions that a NullReferenceException was found.

  2. Open the Parallel Stacks window (Debug > Windows > Parallel Stacks):

    Visual Studio Parallel Stacks window
    Visual Studio Parallel Stacks window

    This shows that an unhandled exception occurred on the Finalizer thread (OnUnhandledException) while executing the Finalizer of MyClass (~MyClass).

  3. Inspect the source of the class to find out why the exception was thrown.

Investigating the crash dump with WinDbg

Open the dump file with WinDbg (File > Open Dump File).

Tip

Once the dump file is opened, you can try executing the .lastevent and !analyze commands to see if these show some relevant information. Note that these commands might not always provide additional info.

Approach 1: Finding the assembly via the !name2ee command

  1. List all the managed threads by using the !threads command:

    !threads
ThreadCount:      36
UnstartedThread:  0
BackgroundThread: 15
PendingThread:    0
DeadThread:       19
Hosted Runtime:   no
                                                                         Lock  
        ID OSID ThreadOBJ    State GC Mode     GC Alloc Context  Domain   Count Apt Exception
    3    2 12184 053b3510     2b220 Preemptive  145509A8:00000000 053a1b50 0     MTA (Finalizer) System.NullReferenceException 144d100c
    4    9 c39c 0a4da900   102a220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
    6    8 ec30 0abe7dd0   1020220 Preemptive  00000000:00000000 053a1b50 0     Ukn (Threadpool Worker) 
    7   10 dde8 0abe8318     2b220 Preemptive  00000000:00000000 053a1b50 0     MTA 
    8    1 1074 0abe8860     2b220 Preemptive  00000000:00000000 053a1b50 0     MTA 
    9   23 70cc 0abe7888   202b020 Preemptive  00000000:00000000 053a1b50 0     MTA 
  10   24 7fe4 0abea810   202b020 Preemptive  14533158:00000000 053a1b50 0     MTA 
  12  109 ecd0 0abcdae8   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
XXXX   70    0 0abce030     30820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   35    0 0abd6c48     30820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
  13   42 b6ac 0ac22ee0     20220 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   91    0 1555e818     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   68    0 0f31f408     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   27    0 1554fca0     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   96    0 0ac219c0     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX  111    0 0ac213d0     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX  101    0 0ac1f968     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   22    0 155501e8     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   31    0 15550730     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   49    0 0ac203f8     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX    6    0 0ac1e448     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   17    0 0f31f950     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   54    0 0abe7340     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   50    0 0abed798     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
  18   52 157f0 0abe92f0   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
  19   15 4dcc 0f31ff40   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
  20   86 b188 0abedce0   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
  21   56 146d0 0abce578   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
  22   74 12e58 0a46f1c8   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
  23   93 f7c8 0a4716c0   1029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Worker) 
XXXX  114    0 0a471178     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
  24   61 cc58 0ac1df00   8029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Completion Port) 
XXXX  113    0 0a46e1f0     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
XXXX   80    0 15551708     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn 
  25   81 186c 0abecd08   8029220 Preemptive  00000000:00000000 053a1b50 0     MTA (Threadpool Completion Port) 
XXXX   46    0 15550c78     39820 Preemptive  00000000:00000000 053a1b50 0     Ukn

    Here, the first line indicates that a System.NullReferenceException occurred on the Finalizer thread.

  2. Switch to the Finalizer thread.

    Switch to this thread by using the ~3s command (3 is the value for the thread in the first column of the !threads command output).

  3. Execute the !clrstack command.

    0:003> !clrstack
OS Thread Id: 0x12184 (3)
Child SP       IP Call Site
0a14e91c 773d3f3c [InlinedCallFrame: 0a14e91c] 
0a14e918 0ce2221e DomainBoundILStubClass.IL_STUB_PInvoke(IntPtr, UInt32, System.Runtime.InteropServices.SafeHandle, MINIDUMP_TYPE, IntPtr, IntPtr, IntPtr)
0a14e91c 0ce22146 [InlinedCallFrame: 0a14e91c] Skyline.DataMiner.Net.Tools+NativeMethods.MiniDumpWriteDump(IntPtr, UInt32, System.Runtime.InteropServices.SafeHandle, MINIDUMP_TYPE, IntPtr, IntPtr, IntPtr)
0a14e980 0ce22146 Skyline.DataMiner.Net.Tools+NativeMethods.DumpProcess(System.String, Boolean)
0a14e9b4 0ce22081 Skyline.DataMiner.Net.Tools.DumpProcess(System.String)
0a14e9d8 0ce2159d Skyline.DataMiner.Net.ToolsSpace.CrashHandler.LogUnhandledException(System.String, System.Exception, Boolean)
0a14ea30 0ce2095b .OnUnhandledException(System.Object, System.UnhandledExceptionEventArgs)
0a14eb9c 74922536 [GCFrame: 0a14eb9c] 
0a14ec48 74922536 [GCFrame: 0a14ec48] 
0a14ecf4 74922536 [GCFrame: 0a14ecf4] 
0a14ee14 74922536 [GCFrame: 0a14ee14] 
0a14f7d0 74922536 [DebuggerU2MCatchHandlerFrame: 0a14f7d0] 
0a14f5c4 0ce20856 Skyline.Protocol.MyExtension.MyClass.Finalize()
0a14f7d0 7496c971 [DebuggerU2MCatchHandlerFrame: 0a14f7d0]

    The stack shows that an unhandled exception occurred while executing the Finalizer of the Skyline.Protocol.MyExtension.MyClass type.

  4. Determine the assembly this type belongs to (and therefore the connector), by executing the following command: !name2ee * Skyline.Protocol.MyExtension.MyClass.

    --------------------------------------
Module:      0ce00a14
Assembly:    SLC-C-SLProtocolNew.1.0.0.1.QAction.5, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
--------------------------------------
Module:      0ce01a04
Assembly:    ExampleProtocol.1.0.0.1.QAction.1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Token:       02000002
MethodTable: 0ce02f2c
EEClass:     0da35d64
Name:        Skyline.Protocol.MyExtension.MyClass
--------------------------------------
Module:      0ce01dbc
Assembly:    ExampleProtocol.1.0.0.1.QAction.900012, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
--------------------------------------
Module:      0ce021e8
Assembly:    ExampleProtocol.1.0.0.1.QAction.2, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
--------------------------------------

    The output lists all the assemblies, and for each assembly that defines this type, the type is mentioned. In this case, you can see that the assembly ExampleProtocol.1.0.0.1.QAction.1 defines a type Skyline.Protocol.MyExtension.MyClass, so you should inspect the Finalizer of the MyClass type defined in QAction 1 of the ExampleProtocol connector version 1.0.0.1.

  5. Inspect the source of the class to find out why the exception was thrown:

    In WinDbg, you can use the !dumpclass command to retrieve more information about the type using its EEClass value as argument: !dumpclass 0da35d64.

    0:003> !dumpclass 0da35d64
Class Name:      Skyline.Protocol.MyExtension.MyClass
mdToken:         02000002
File:            ExampleProtocol.1.0.0.1.QAction.1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Parent Class:    72dc6118
Module:          0ce01a04
Method Table:    0ce02f2c
Vtable Slots:    4
Total Method Slots:  5
Class Attributes:    100001  
Transparency:        Critical
NumInstanceFields:   0
NumStaticFields:     0

  6. Use the !dumpmt -md 0ce02f2c command (where 0ce02f2c is the Method Table address of the type as seen from the output of the !dumpclass command) to see the methods this type defines:

    0:003> !dumpmt -md 0ce02f2c
EEClass:         0da35d64
Module:          0ce01a04
Name:            Skyline.Protocol.MyExtension.MyClass
mdToken:         02000002
File:            ExampleProtocol.1.0.0.1.QAction.1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
BaseSize:        0xc
ComponentSize:   0x0
Slots in VTable: 6
Number of IFaces in IFaceMap: 0
--------------------------------------
MethodDesc Table
    Entry MethodDe    JIT Name
731a9a58 72dc9e9c PreJIT System.Object.ToString()
7326a370 72f261d8 PreJIT System.Object.Equals(System.Object)
73198e60 72f261f8 PreJIT System.Object.GetHashCode()
0ce20840 0ce02f08    JIT Skyline.Protocol.MyExtension.MyClass.Finalize()
0d37de08 0ce02f18   NONE Skyline.Protocol.MyExtension.MyClass..ctor()
0ce20820 0ce02efc    JIT Skyline.Protocol.MyExtension.MyClass.PerformAction()

  7. To see the IL code of the Finalize method, use the !u 0ce02f08 command (where 0ce02f08 is the address denoted for the Finalize method in the output of the !dumpmt command).

Approach 2: Finding the assembly via the !eestack command

Another approach to find the type and file that defines the method where the exception occurred is to use the !eestack command. The following steps illustrate how to do this with another example where an exception occurs in the Finalizer thread.

  1. Like in the previous section, use the !threads command to list all the managed threads.

  2. Show stacks using the !eestack command.

    If you see that an exception occurred on the Finalizer thread, you can use the !eestack command to list the call stacks.

    The output for the Finalizer thread might show an exception, and just below the exception you might find more info on what Finalize method was being executed (in the output, look at the output for Thread X, where X denotes the number for the Finalizer thread as seen in the output of the !threads command, which is 3 in this case.):

    ...
---------------------------------------------
Thread  3
....
00000000 00004015 00004015 ====> Exception cxr@0a05f680
0a05fb18 0fbdc482 (MethodDesc 0b51b8bc +0x12 Skyline.DataMiner.Scripting.ConcreteSLProtocol.Log(System.String, Skyline.DataMiner.Scripting.LogType, Skyline.DataMiner.Scripting.LogLevel))
0a05fb28 0f25ea3e (MethodDesc 15633010 +0x4e QAction.Dispose()), calling 0ae63e7e
0a05fb30 0f25ea55 (MethodDesc 15633010 +0x65 QAction.Dispose()), calling clr!JIT_WriteBarrierESI
0a05fb44 0f25f3b1 (MethodDesc 15632fbc +0x19 QAction.Finalize()), calling (MethodDesc 15633010 +0 QAction.Dispose())
...

  3. Use the !dumpmd command to obtain more information about the Finalize method.

    You can use the method descriptor address that is denoted for the QAction.Finalize: !dumpmd 15632fbc.

    0:003> !dumpmd 15632fbc
Method Name:  QAction.Finalize()
Class:        0da35d64
MethodTable:  15633044
mdToken:      06000001
Module:       15632a40
IsJitted:     yes
CodeAddr:     0f25f398
Transparency: Safe critical

  4. Use the !dumpclass command with the address provided on the Class: line to obtain more information about the class: !dumpclass 0da35d64.

    0:003> !dumpclass 156196fc
Class Name:      QAction
mdToken:         02000003
File:            Example Protocol.1.0.0.1.QAction.1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Parent Class:    6bbf15c8
Module:          15632a40
Method Table:    15633044
Vtable Slots:    6
Total Method Slots:  6
Class Attributes:    100001  
Transparency:        Critical
NumInstanceFields:   6
NumStaticFields:     0
      MT    Field   Offset                 Type VT     Attr    Value Name
1563334c  400000a        4 ...ConnectionFactory  0 instance           connectionFactory
115707e0  400000c        c ...ing.SLProtocolExt  0 instance           protocolExt
6bc08788  400000d       18       System.Boolean  1 instance           _disposed
6bc08788  400000e       19       System.Boolean  1 instance           _startDispose
0505a66c  400000f       10 ...er.Net.Connection  0 instance           _slnetConnection
6bc024d4  4000010       14        System.String  0 instance           _subscriptionSetName

    Now the line that starts with File: shows the assembly where this method is defined, which includes the protocol name and QAction ID.

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