DLNA: Network Shares Cause Blocking Threads in Windows Explorer During Copying Events

TL;DR – DLNA shares have a bug which can cause Explorer to stop copying data across the share, nigh indefinitely. The easiest work-around is to restart Windows Explorer, delete the target file in the previous copy operation, and start anew.

I’ve discovered a bug that I can’t seem to get addressed because the assembly isn’t publicly documented, anywhere, but I figured that I would write about what happens to explain it to those of you who run into it.

First, we need to cover what DLNA (Digital Living Network Alliance) is. (Wiki article is here.) DLNA is a standard by which multiple libraries can be accessed for sharing/streaming, without having had a proprietary library to communicate between them.

Plex Media Server is one such media streaming service built using the DLNA libraries for sharing resources. One of the DLNA features, in Windows, is that it appears as a Network Share/Location in Windows Explorer.

So, we have a Plex Media Server and it’s serving DLNA. The media on the server is browsable, as it if were a dedicated network share. If we treat it as such and copy from one location to another, this is when this particular bug surfaces.

Microsoft has implement DLNA in x86 and x64 processes via assemblies included in Windows. In particular, for this bug, we care about the mfnetcore.dll assembly, which can be found in either the System32 or the SysWow64 folders.

Here’s a dump of the stack during repro:

81 TID:0e60 kb kbn kbnL kn knL kpn kPn
# Child-SP RetAddr Call Site
00 00000000`2fc8eb18 00007ffb`272683d3 ntdll!NtWaitForSingleObject+0x14
01 00000000`2fc8eb20 00007ffa`e0845d59 KERNELBASE!WaitForSingleObjectEx+0x93
02 00000000`2fc8ebc0 00007ffa`f135812f mfnetcore!MFGetSupportedDLNAProfileInfo+0xaa69
03 00000000`2fc8ec60 00007ffb`27ba03ed mfplat!CStreamOnMFByteStream::Read+0xef
04 00000000`2fc8ecb0 00007ffb`27ad01c5 windows_storage!SHCopyStreamWithProgress2+0x1ad
05 00000000`2fc8ed90 00007ffb`27ad03ce windows_storage!CCopyOperation::_CopyResourceStreams+0x89
06 00000000`2fc8ee00 00007ffb`278e50df windows_storage!CCopyOperation::_CopyResources+0x17e
07 00000000`2fc8eea0 00007ffb`276f784f windows_storage!CCopyOperation::Do+0x1b5cbf
08 00000000`2fc8efa0 00007ffb`276f5d4f windows_storage!CCopyWorkItem::_DoOperation+0x9b
09 00000000`2fc8f080 00007ffb`276f657a windows_storage!CCopyWorkItem::_SetupAndPerformOp+0x2a3
0a 00000000`2fc8f370 00007ffb`276f2f1e windows_storage!CCopyWorkItem::ProcessWorkItem+0x152
0b 00000000`2fc8f620 00007ffb`276f3907 windows_storage!CRecursiveFolderOperation::Do+0x1be
0c 00000000`2fc8f6c0 00007ffb`276f33d6 windows_storage!CFileOperation::_EnumRootDo+0x277
0d 00000000`2fc8f760 00007ffb`276fd25c windows_storage!CFileOperation::PrepareAndDoOperations+0x1c6
0e 00000000`2fc8f830 00007ffb`2874c525 windows_storage!CFileOperation::PerformOperations+0x10c
0f 00000000`2fc8f890 00007ffb`2874acf0 shell32!CFSDropTargetHelper::_MoveCopyHIDA+0x269
10 00000000`2fc8f940 00007ffb`2874d517 shell32!CFSDropTargetHelper::_Drop+0x220
11 00000000`2fc8fe20 00007ffb`29b6c315 shell32!CFSDropTargetHelper::s_DoDropThreadProc+0x37
12 00000000`2fc8fe50 00007ffb`2ab17974 SHCore!_WrapperThreadProc+0xf5
13 00000000`2fc8ff30 00007ffb`2aeba271 kernel32!BaseThreadInitThunk+0x14
14 00000000`2fc8ff60 00000000`00000000 ntdll!RtlUserThreadStart+0x21

Note that frames that we, generally, care about are in orange and red at the top of the stack and those are the last instructions executed in the thread. In this case, we’re waiting on a response from the request to get the supported DLNA profile information and this is demonstrated by the fact that we’re waiting on an object at the top of the stack. Essentially, we have an open/blocking request that has never completed and the thread will have to die to unblock the request.

We can see the block happen on other native threads. Specifically, in the dump that I created, there were three threads with the same stacks, shown as below.

97 TID:3cdc kb kbn kbnL kn knL kpn kPn
# Child-SP RetAddr Call Site
00 00000000`3448f9e8 00007ffb`299bf5cd win32u!NtUserMsgWaitForMultipleObjectsEx+0x14
01 00000000`3448f9f0 00007ffa`f88e2cfd user32!RealMsgWaitForMultipleObjectsEx+0x1d
02 00000000`3448fa30 00007ffa`f88e2c24 duser!CoreSC::Wait+0x75
03 00000000`3448fa80 00007ffb`299d05d1 duser!MphWaitMessageEx+0x104
04 00000000`3448fae0 00007ffb`2aef33c4 user32!_ClientWaitMessageExMPH+0x21
05 00000000`3448fb30 00007ffb`26f51224 ntdll!KiUserCallbackDispatcherContinue
06 00000000`3448fb98 00007ffa`fad54913 win32u!NtUserWaitMessage+0x14
07 00000000`3448fba0 00007ffa`fad547a9 explorerframe!CExplorerFrame::FrameMessagePump+0x153
08 00000000`3448fc20 00007ffa`fad546f6 explorerframe!BrowserThreadProc+0x85
09 00000000`3448fca0 00007ffa`fad55a12 explorerframe!BrowserNewThreadProc+0x3a
0a 00000000`3448fcd0 00007ffa`fad670c2 explorerframe!CExplorerTask::InternalResumeRT+0x12
0b 00000000`3448fd00 00007ffb`2785b58c explorerframe!CRunnableTask::Run+0xb2
0c 00000000`3448fd40 00007ffb`2785b245 windows_storage!CShellTask::TT_Run+0x3c
0d 00000000`3448fd70 00007ffb`2785b125 windows_storage!CShellTaskThread::ThreadProc+0xdd
0e 00000000`3448fe20 00007ffb`29b6c315 windows_storage!CShellTaskThread::s_ThreadProc+0x35
0f 00000000`3448fe50 00007ffb`2ab17974 SHCore!_WrapperThreadProc+0xf5
10 00000000`3448ff30 00007ffb`2aeba271 kernel32!BaseThreadInitThunk+0x14
11 00000000`3448ff60 00000000`00000000 ntdll!RtlUserThreadStart+0x21

Work-arounds: This bug is pretty ugly and there aren’t a whole lot of work-arounds for it. One could wait for the lifetime of the thread to cause an abort, which could be a considerable amount of time. The work-around that I typically opt for is to restart Windows Explorer process, via Task Manager, delete the file and try to copy it again. Sure, it takes a lot of time but it’s considerably a far lower cost, time-wise, than waiting for a thread to become unblocked due to a timeout.