Add 'None' video device

Fixes #1825

src/video/cameradevice.cpp

@@ -97,6 +97,12 @@ CameraDevice* CameraDevice::open(QString devName, VideoMode mode)
     if (!getDefaultInputFormat())
         return nullptr;
 
+    if (devName == "none")
+    {
+        qDebug() << "Tried to open the null device";
+        return nullptr;
+    }
+
     AVDictionary* options = nullptr;
     if (false);
 #ifdef Q_OS_LINUX
@@ -244,16 +250,18 @@ QVector<QPair<QString, QString>> CameraDevice::getDeviceList()
 {
     QVector<QPair<QString, QString>> devices;
 
+    devices.append({"none", "None"});
+
     if (!getDefaultInputFormat())
             return devices;
 
     if (false);
 #ifdef Q_OS_WIN
     else if (iformat->name == QString("dshow"))
-        devices = DirectShow::getDeviceList();
+        devices += DirectShow::getDeviceList();
 #endif
     else
-        devices = getRawDeviceListGeneric();
+        devices += getRawDeviceListGeneric();
 
     if (idesktopFormat)
     {

src/video/camerasource.cpp

@@ -49,6 +49,8 @@ CameraSource::CameraSource(const QString deviceName, VideoMode mode)
 {
     av_register_all();
     avdevice_register_all();
+
+    isNull = (deviceName == "none");
 }
 
 CameraSource::~CameraSource()
@@ -60,6 +62,12 @@ CameraSource::~CameraSource()
             expected = false;
     }
 
+    if (isNull)
+    {
+        biglock = false;
+        return;
+    }
+
     // Free all remaining VideoFrame
     // Locking must be done precisely this way to avoid races
     for (int i=0; i<freelist.size(); i++)
@@ -94,6 +102,12 @@ bool CameraSource::subscribe()
             expected = false;
     }
 
+    if (isNull)
+    {
+        biglock = false;
+        return true;
+    }
+
     if (device)
     {
         device->open();
@@ -111,6 +125,7 @@ bool CameraSource::subscribe()
     if (!device)
     {
         biglock = false;
+        qWarning() << "Failed to open device!";
         return false;
     }
 
@@ -173,6 +188,12 @@ void CameraSource::unsubscribe()
             expected = false;
     }
 
+    if (isNull)
+    {
+        biglock = false;
+        return;
+    }
+
     if (!device)
     {
         qWarning() << "Unsubscribing with zero subscriber";

src/video/camerasource.h

@@ -73,7 +73,7 @@ private:
     VideoMode mode; ///< What mode we tried to open the device in, all zeros means default mode
     AVCodecContext* cctx, *cctxOrig; ///< Codec context of the camera's selected video stream
     int videoStreamIndex; ///< A camera can have multiple streams, this is the one we're decoding
-    std::atomic_bool biglock, freelistLock; ///< True when locked. Faster than mutexes for video decoding.
+    std::atomic_bool biglock, freelistLock, isNull; ///< True when locked. Faster than mutexes for video decoding.
     std::atomic_int subscriptions; ///< Remember how many times we subscribed for RAII
 };