David Pritchard reported May 29 at 08:33 PM

I have an x64 UWP C++/CX app that makes heavy use of co_await. I find that Visual Studio 15.7 cannot be used for our app; coroutines consistently crash in release builds. From the release notes, it sounds like there was a substantial revamp of the coroutine code in 15.7 - that change appears to be triggering the issues I see.

Here's an example code block to search for Bluetooth pens that crashes every time for me:

struct PenInfo { bool m_bAtLeastOneSurfacePen = false; bool m_bAtLeastOneNonSurfacePen = false; };

concurrency::task<PenInfo> TestCoroutine() { using namespace Windows::Devices::Bluetooth;

PenInfo result; auto devices = co_await Windows::Devices::Enumeration::DeviceInformation::FindAllAsync( Windows::Devices::Bluetooth::BluetoothLEDevice::GetDeviceSelector() );

for ( auto device : devices ) { if ( device->Name == "Surface Pen" ) result.m_bAtLeastOneSurfacePen = true; else { BluetoothLEDevice ^bleDevice = co_await BluetoothLEDevice::FromIdAsync( device->Id ); if ( bleDevice && bleDevice->Appearance->Category == BluetoothLEAppearanceCategories::HumanInterfaceDevice && bleDevice->Appearance->SubCategory == BluetoothLEAppearanceSubcategories::DigitalPen ) { result.m_bAtLeastOneNonSurfacePen = true; } } } co_return result; }

An x64 release build will crash with an access violation reading an 0xCCDDCCDD pointer (more details below). It will crash after resuming the coroutine after the first co_await call - i.e., before actually iterating through the results.

An x64 debug build is fine, and an x86 debug or release build is fine.

Any build on Visual Studio 15.6.x appears to be fine.

In the attached zipfile - if the DISABLE_SECOND_AWAIT flag is set to 1, the crash goes away.

Further details on error messages

0xC0000005: Access violation reading location 0x00000000CCDDCCDD. occurred

• [Inline Frame] App2.exe!__abi_winrt_cast_to(bool) Line 499
• App2.exe!Windows::Foundation::await_resume <Windows::Devices:: Enumeration:: DeviceInformationCollection ^>(Windows::Foundation::IAsyncOperation <Windows::Devices:: Enumeration:: DeviceInformationCollection ^> ^ _Task) Line 382
• App2.exe!TestCoroutine$_ResumeCoro$2() Line 26

A disassembly window shows an error inside _VerifyStateForResultsCall(_Task->Status). (In the process of stripping down code to put this repro together, some slightly more complex variations of this code also saw crashes in VerifyStateForResultsCall instead of _abi_winrt_cast_to.)

Further details on computer system:

• Visual Studio Community 15.7.2
• Windows 10 Fall Creators Update
• Surface Pro 3
• Bluetooth devices connected: Surface Pen and Surface Type Cover