Wt

Hi Guys,

I got a boost::bad_weak_ptr exception thrown in the following scenario. Wt is trying to destroy a WebSession - the stack trace is shown below:

    tbuWtGui.dll!Tbu::WApplication::finalize()  Line 123    C++
    tbuWtGui.dll!Wt::WebSession::notify(const Wt::WEvent & event={...})  Line 1081  C++
    tbuWtGui.dll!Wt::WApplication::notify(const Wt::WEvent & e={...})  Line 1177    C++
    tbuWtGui.dll!Wt::WebSession::~WebSession()  Line 132    C++
    tbuWtGui.dll!Wt::WebSession::`scalar deleting destructor'()  + 0x31 bytes   C++
    tbuWtGui.dll!boost::checked_delete<Wt::WebSession>(Wt::WebSession * x=0x000000000949b5b0)  Line 34 + 0x2b bytes C++
    tbuWtGui.dll!boost::detail::sp_counted_impl_p<Wt::WebSession>::dispose()  Line 79   C++
    tbuWtGui.dll!boost::detail::sp_counted_base::release()  Line 103    C++
    tbuWtGui.dll!boost::detail::shared_count::~shared_count()  Line 221 C++
    tbuWtGui.dll!boost::shared_ptr<Wt::WebSession>::~shared_ptr<Wt::WebSession>()  + 0x31 bytes C++
    tbuWtGui.dll!boost::shared_ptr<Wt::WebSession>::reset()  Line 387   C++
    tbuWtGui.dll!Wt::WebController::handleAsyncRequest(Wt::WebRequest * request=0x0000000009466b70)  Line 518   C++
    tbuWtGui.dll!Wt::WebController::handleRequest(Wt::WebRequest * request=0x0000000009466b70)  Line 385    C++
    tbuWtGui.dll!Wt::WServer::handleRequest(Wt::WebRequest * request=0x0000000009466b70)  Line 298  C++
    tbuWtGui.dll!http::server::WtReply::consumeRequestBody(const char * begin=0x000000001c4f3919, const char * end=0x000000001c4f3990, bool endOfRequest=true)  Line 79 C++
    tbuWtGui.dll!http::server::RequestParser::parseBody(http::server::Request & req={...}, boost::shared_ptr<http::server::Reply> * reply=0x00000000038df388, const char * & begin=0x000000001c4f3754, const char * end=0x000000001c4f3990)  Line 119   C++
    tbuWtGui.dll!http::server::Connection::handleReadBody()  Line 135 + 0x8a bytes  C++

The above call to Tbu::WApplication::finalize() has blocked while trying to acquire a lock that is shared with my thread that performs "server push" functions. Clearly I need to synchronise these two threads so I don't make changes to applications that are being destroyed. A call to WApplication::finalise() is the first notification I receive that the application and associated session are to be destroyed.

Meanwhile, my "server push" thread is busy making changes to widgets, calling getUpdateLock() and triggerChanged() as required - it does this while holding the lock that the thread noted above is waiting on. The following call stack from the "server push" thread shows where things go wrong:

    tbuWtGui.dll!boost::throw_exception<boost::bad_weak_ptr>(const boost::bad_weak_ptr & e={...})  Line 81 + 0x4d bytes C++
    tbuWtGui.dll!boost::detail::shared_count::shared_count(const boost::detail::weak_count & r={...})  Line 421 + 0x23 bytes    C++
    tbuWtGui.dll!boost::shared_ptr<Wt::WebSession>::shared_ptr<Wt::WebSession><Wt::WebSession>(const boost::weak_ptr<Wt::WebSession> & r={...})  Line 216   C++
    tbuWtGui.dll!boost::enable_shared_from_this<Wt::WebSession>::shared_from_this()  Line 49 + 0xf bytes    C++
    tbuWtGui.dll!Wt::UpdateLockImpl::UpdateLockImpl(Wt::WApplication & app={...})  Line 1050 + 0x49 bytes   C++
    tbuWtGui.dll!Wt::WApplication::UpdateLock::UpdateLock(Wt::WApplication & app={...})  Line 1080 + 0x26 bytes C++
    tbuWtGui.dll!Wt::WApplication::getUpdateLock()  Line 1041 + 0xf bytes   C++

When my "Server push" thread tries to acquire the Update Lock, it creates a shared_ptr to the WebSession. However, the WebSession is already being destructed in the other thread! At the point that the exception was thrown, the use count of the shared_ptr was zero and the weak_count was 2.

It seems to me that either WApplication::finalize() needs to be called before the WebSession destructs or WApplication::getUpdateLock() should not involve the WebSession.

This has occurred on the git version of Wt.

Cheers,

Dan