Improve tracking of DrmBuffers in DrmBackend

We need to prevent double deletions from the gbm callback. Therefore
we track all buffers in DrmBackend and pass the DrmBackend as user
data.

drm_backend.cpp

@@ -481,16 +481,25 @@ OpenGLBackend *DrmBackend::createOpenGLBackend()
 
 DrmBuffer *DrmBackend::createBuffer(const QSize &size)
 {
-    return new DrmBuffer(this, size);
+    DrmBuffer *b = new DrmBuffer(this, size);
+    m_buffers << b;
+    return b;
 }
 
 DrmBuffer *DrmBackend::createBuffer(gbm_surface *surface)
 {
 #if HAVE_GBM
-    return new DrmBuffer(this, surface);
+    DrmBuffer *b = new DrmBuffer(this, surface);
+    m_buffers << b;
+    return b;
 #endif
 }
 
+void DrmBackend::bufferDestroyed(DrmBuffer *b)
+{
+    m_buffers.removeAll(b);
+}
+
 DrmOutput::DrmOutput(DrmBackend *backend)
     : m_backend(backend)
 {
@@ -633,8 +642,14 @@ DrmBuffer::DrmBuffer(DrmBackend *backend, const QSize &size)
 #if HAVE_GBM
 static void gbmCallback(gbm_bo *bo, void *data)
 {
-    Q_UNUSED(bo);
-    delete reinterpret_cast<DrmBuffer*>(data);
+    DrmBackend *backend = reinterpret_cast<DrmBackend*>(data);
+    const auto &buffers = backend->buffers();
+    for (auto buffer: buffers) {
+        if (buffer->gbm() == bo) {
+            delete buffer;
+            return;
+        }
+    }
 }
 #endif
 
@@ -653,12 +668,13 @@ DrmBuffer::DrmBuffer(DrmBackend *backend, gbm_surface *surface)
     if (drmModeAddFB(m_backend->fd(), m_size.width(), m_size.height(), 24, 32, m_stride, gbm_bo_get_handle(m_bo).u32, &m_bufferId) != 0) {
         qWarning(KWIN_CORE) << "drmModeAddFB failed";
     }
-    gbm_bo_set_user_data(m_bo, this, gbmCallback);
+    gbm_bo_set_user_data(m_bo, m_backend, gbmCallback);
 #endif
 }
 
 DrmBuffer::~DrmBuffer()
 {
+    m_backend->bufferDestroyed(this);
     delete m_image;
     if (m_memory) {
         munmap(m_memory, m_bufferSize);

drm_backend.h

@@ -62,6 +62,10 @@ public:
     QVector<DrmOutput*> outputs() const {
         return m_outputs;
     }
+    QVector<DrmBuffer*> buffers() const {
+        return m_buffers;
+    }
+    void bufferDestroyed(DrmBuffer *b);
 
 Q_SIGNALS:
     void screensQueried();
@@ -92,6 +96,7 @@ private:
     int m_cursorIndex = 0;
     int m_pageFlipsPending = 0;
     bool m_active = false;
+    QVector<DrmBuffer*> m_buffers;
 };
 
 class DrmOutput
@@ -153,6 +158,9 @@ public:
     quint32 stride() const {
         return m_stride;
     }
+    gbm_bo *gbm() const {
+        return m_bo;
+    }
     void releaseGbm();
 
 private: