terminal/src/renderer/base/thread.cpp

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT license.

#include "precomp.h"

#include "thread.hpp"

#pragma hdrstop

using namespace Microsoft::Console::Render;

RenderThread::RenderThread() :
    _pRenderer(nullptr),
    _hThread(nullptr),
    _hEvent(nullptr),
    _hPaintCompletedEvent(nullptr),
    _fKeepRunning(true),
    _hPaintEnabledEvent(nullptr),
    _fNextFrameRequested(false),
    _fWaiting(false)
{
}

RenderThread::~RenderThread()
{
    if (_hThread)
    {
        _fKeepRunning = false; // stop loop after final run
        SignalObjectAndWait(_hEvent, _hThread, INFINITE, FALSE); // signal final paint and wait for thread to finish.

        CloseHandle(_hThread);
        _hThread = nullptr;
    }

    if (_hEvent)
    {
        CloseHandle(_hEvent);
        _hEvent = nullptr;
    }

    if (_hPaintEnabledEvent)
    {
        CloseHandle(_hPaintEnabledEvent);
        _hPaintEnabledEvent = nullptr;
    }

    if (_hPaintCompletedEvent)
    {
        CloseHandle(_hPaintCompletedEvent);
        _hPaintCompletedEvent = nullptr;
    }
}

// Method Description:
// - Create all of the Events we'll need, and the actual thread we'll be doing
//      work on.
// Arguments:
// - pRendererParent: the IRenderer that owns this thread, and which we should
//      trigger frames for.
// Return Value:
// - S_OK if we succeeded, else an HRESULT corresponding to a failure to create
//      an Event or Thread.
[[nodiscard]] HRESULT RenderThread::Initialize(IRenderer* const pRendererParent) noexcept
{
    _pRenderer = pRendererParent;

    HRESULT hr = S_OK;
    // Create event before thread as thread will start immediately.
    if (SUCCEEDED(hr))
    {
        HANDLE hEvent = CreateEventW(nullptr, // non-inheritable security attributes
                                     FALSE, // auto reset event
                                     FALSE, // initially unsignaled
                                     nullptr // no name
        );

        if (hEvent == nullptr)
        {
            hr = HRESULT_FROM_WIN32(GetLastError());
        }
        else
        {
            _hEvent = hEvent;
        }
    }

    if (SUCCEEDED(hr))
    {
        HANDLE hPaintEnabledEvent = CreateEventW(nullptr,
                                                 TRUE, // manual reset event
                                                 FALSE, // initially signaled
                                                 nullptr);

        if (hPaintEnabledEvent == nullptr)
        {
            hr = HRESULT_FROM_WIN32(GetLastError());
        }
        else
        {
            _hPaintEnabledEvent = hPaintEnabledEvent;
        }
    }

    if (SUCCEEDED(hr))
    {
        HANDLE hPaintCompletedEvent = CreateEventW(nullptr,
                                                   TRUE, // manual reset event
                                                   TRUE, // initially signaled
                                                   nullptr);

        if (hPaintCompletedEvent == nullptr)
        {
            hr = HRESULT_FROM_WIN32(GetLastError());
        }
        else
        {
            _hPaintCompletedEvent = hPaintCompletedEvent;
        }
    }

    if (SUCCEEDED(hr))
    {
        HANDLE hThread = CreateThread(nullptr, // non-inheritable security attributes
                                      0, // use default stack size
                                      s_ThreadProc,
                                      this,
                                      0, // create immediately
                                      nullptr // we don't need the thread ID
        );

        if (hThread == nullptr)
        {
            hr = HRESULT_FROM_WIN32(GetLastError());
        }
        else
        {
            _hThread = hThread;
        }
    }

    return hr;
}

DWORD WINAPI RenderThread::s_ThreadProc(_In_ LPVOID lpParameter)
{
    RenderThread* const pContext = static_cast<RenderThread*>(lpParameter);

    if (pContext != nullptr)
    {
        return pContext->_ThreadProc();
    }
    else
    {
        return (DWORD)E_INVALIDARG;
    }
}

DWORD WINAPI RenderThread::_ThreadProc()
{
    while (_fKeepRunning)
    {
        WaitForSingleObject(_hPaintEnabledEvent, INFINITE);

        if (!_fNextFrameRequested.exchange(false))
        {
            // <--
            // If `NotifyPaint` is called at this point, then it will not
            // set the event because `_fWaiting` is not `true` yet so we have
            // to check again below.

            _fWaiting.store(true);

            // check again now (see comment above)
            if (!_fNextFrameRequested.exchange(false))
            {
                // Wait until a next frame is requested.
                WaitForSingleObject(_hEvent, INFINITE);
            }

            // <--
            // If `NotifyPaint` is called at this point, then it _will_ set
            // the event because `_fWaiting` is `true`, but we're not waiting
            // anymore!
            // This can probably happen quite often: imagine a scenario where
            // we are waiting, and the terminal calls `NotifyPaint` twice
            // very quickly.
            // In that case, both calls might end up calling `SetEvent`. The
            // first one will resume this thread and the second one will
            // `SetEvent` the event. So the next time we wait, the event will
            // already be set and we won't actually wait.
            // Because it can happen often, and because rendering is an
            // expensive operation, we should reset the event to not render
            // again if nothing changed.

            _fWaiting.store(false);

            // see comment above
            ResetEvent(_hEvent);
        }

        ResetEvent(_hPaintCompletedEvent);

        LOG_IF_FAILED(_pRenderer->PaintFrame());

        SetEvent(_hPaintCompletedEvent);

        // extra check before we sleep since it's a "long" activity, relatively speaking.
        if (_fKeepRunning)
        {
            Sleep(s_FrameLimitMilliseconds);
        }
    }

    return S_OK;
}

void RenderThread::NotifyPaint()
{
    if (_fWaiting.load())
    {
        SetEvent(_hEvent);
    }
    else
    {
        _fNextFrameRequested.store(true);
    }
}

void RenderThread::EnablePainting()
{
    SetEvent(_hPaintEnabledEvent);
}

void RenderThread::WaitForPaintCompletionAndDisable(const DWORD dwTimeoutMs)
{
    // When rendering takes place via DirectX, and a console application
    // currently owns the screen, and a new console application is launched (or
    // the user switches to another console application), the new application
    // cannot take over the screen until the active one relinquishes it. This
    // blocking mechanism goes as follows:
    //
    // 1. The console input thread of the new console application connects to
    // ConIoSrv;
    // 2. While servicing the new connection request, ConIoSrv sends an event to
    // the active application letting it know that it has lost focus;
    // 3.1 ConIoSrv waits for a reply from the client application;
    // 3.2 Meanwhile, the active application receives the focus event and calls
    // this method, waiting for the current paint operation to
    // finish.
    //
    // This means that the new application is waiting on the connection request
    // reply from ConIoSrv, ConIoSrv is waiting on the active application to
    // acknowledge the lost focus event to reply to the new application, and the
    // console input thread in the active application is waiting on the renderer
    // thread to finish its current paint operation.
    //
    // Question: what should happen if the wait on the paint operation times
    // out?
    //
    // There are three options:
    //
    // 1. On timeout, the active console application could reply with an error
    // message and terminate itself, effectively relinquishing control of the
    // display;
    //
    // 2. ConIoSrv itself could time out on waiting for a reply, and forcibly
    // terminate the active console application;
    //
    // 3. Let the wait time out and let the user deal with it. Because the wait
    // occurs on a single iteration of the renderer thread, it seemed to me that
    // the likelihood of failure is extremely small, especially since the client
    // console application that the active conhost instance is servicing has no
    // say over what happens in the renderer thread, only by proxy. Thus, the
    // chance of failure (timeout) is minimal and since the OneCoreUAP console
    // is not a massively used piece of software, it didn’t seem that it would
    // be a good use of time to build the requisite infrastructure to deal with
    // a timeout here, at least not for now. In case of a timeout DirectX will
    // catch the mistake of a new application attempting to acquire the display
    // while another one still owns it and will flag it as a DWM bug. Right now,
    // the active application will wait one second for the paint operation to
    // finish.
    //
    // TODO: MSFT: 11833883 - Determine action when wait on paint operation via
    //       DirectX on OneCoreUAP times out while switching console
    //       applications.

    ResetEvent(_hPaintEnabledEvent);
    WaitForSingleObject(_hPaintCompletedEvent, dwTimeoutMs);
}