qt(6)/material: ensure that we always update the context in setBuffer()

Scenario is that there are two (or more) GstGLContext's wrapping Qt's GL
context from either multiple qml(6)glsink or qml(6)glsrc elements.  Call flow is this:

1. material 1 setBuffer()
2. material 1 bind()
3. material 2 setBuffer()
4. material 2 bind()

If the call to setBuffer() reuses the same buffer as previous call, then the
qt context is not updated in the material.  If however the previously used qt
context by the material had been deactivated or freed, then bind() would fail
and could result in a critical like so:

gst_gl_context_thread_add: assertion 'context->priv->active_thread == g_thread_self ()' failed

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7970>

subprojects/gst-plugins-good/ext/qt/gstqsgmaterial.cc

@@ -451,16 +451,23 @@ GstQSGMaterial::setCaps (GstCaps * caps)
 gboolean
 GstQSGMaterial::setBuffer (GstBuffer * buffer)
 {
-  GST_LOG ("%p setBuffer %" GST_PTR_FORMAT, this, buffer);
+  GstGLContext *qt_context;
+  gboolean ret = FALSE;
+
   /* FIXME: update more state here */
-  if (!gst_buffer_replace (&this->buffer_, buffer))
+  if (gst_buffer_replace (&this->buffer_, buffer)) {
-    return FALSE;
+    GST_LOG ("%p setBuffer new buffer %" GST_PTR_FORMAT, this, buffer);
 
-  this->buffer_was_bound = FALSE;
+    this->buffer_was_bound = FALSE;
+    ret = TRUE;
+  }
 
-  g_weak_ref_set (&this->qt_context_ref_, gst_gl_context_get_current ());
+  qt_context = gst_gl_context_get_current ();
+  GST_DEBUG ("%p setBuffer with qt context %" GST_PTR_FORMAT, this, qt_context);
 
-  return TRUE;
+  g_weak_ref_set (&this->qt_context_ref_, qt_context);
+
+  return ret;
 }
 
 /* only called from the streaming thread with scene graph thread blocked */
@@ -481,7 +488,7 @@ void
 GstQSGMaterial::bind(GstQSGMaterialShader *shader, GstVideoFormat v_format)
 {
   const GstGLFuncs *gl;
-  GstGLContext *context, *qt_context;
+  GstGLContext *context, *qt_context = NULL;
   GstGLSyncMeta *sync_meta;
   GstMemory *mem;
   GstGLMemory *gl_mem;
@@ -492,10 +499,6 @@ GstQSGMaterial::bind(GstQSGMaterialShader *shader, GstVideoFormat v_format)
     memset (&this->v_frame, 0, sizeof (this->v_frame));
   }
 
-  qt_context = GST_GL_CONTEXT (g_weak_ref_get (&this->qt_context_ref_));
-  if (!qt_context)
-    goto out;
-
   if (!this->buffer_)
     goto out;
   if (GST_VIDEO_INFO_FORMAT (&this->v_info) == GST_VIDEO_FORMAT_UNKNOWN)
@@ -505,6 +508,10 @@ GstQSGMaterial::bind(GstQSGMaterialShader *shader, GstVideoFormat v_format)
   if (!this->mem_)
     goto out;
 
+  qt_context = GST_GL_CONTEXT (g_weak_ref_get (&this->qt_context_ref_));
+  if (!qt_context)
+    goto out;
+
   gl = qt_context->gl_vtable;
 
   /* FIXME: should really lock the memory to prevent write access */
@@ -514,6 +521,8 @@ GstQSGMaterial::bind(GstQSGMaterialShader *shader, GstVideoFormat v_format)
     goto out;
   }
 
+  GST_DEBUG ("%p attempting to bind with context %" GST_PTR_FORMAT, this, qt_context);
+
   mem = gst_buffer_peek_memory (this->buffer_, 0);
   g_assert (gst_is_gl_memory (mem));
   gl_mem = (GstGLMemory *)mem;

subprojects/gst-plugins-good/ext/qt6/gstqsg6material.cc

@@ -473,15 +473,19 @@ GstQSG6Material::setCaps (GstCaps * caps)
 gboolean
 GstQSG6Material::setBuffer (GstBuffer * buffer)
 {
-  GST_LOG ("%p setBuffer %" GST_PTR_FORMAT, this, buffer);
+  GstGLContext *qt_context = gst_gl_context_get_current ();
+
+  GST_LOG ("%p setBuffer %" GST_PTR_FORMAT " with qt context %" GST_PTR_FORMAT,
+      this, buffer, qt_context);
   /* FIXME: update more state here */
+
+  g_weak_ref_set (&this->qt_context_ref_, qt_context);
+
   if (!gst_buffer_replace (&this->buffer_, buffer))
     return FALSE;
 
   this->buffer_was_bound = false;
 
-  g_weak_ref_set (&this->qt_context_ref_, gst_gl_context_get_current ());
-
   if (this->v_frame.buffer) {
     gst_video_frame_unmap (&this->v_frame);
     memset (&this->v_frame, 0, sizeof (this->v_frame));
@@ -567,7 +571,7 @@ video_format_to_texel_size (GstVideoFormat format, guint plane)
 QSGTexture *
 GstQSG6Material::bind(GstQSG6MaterialShader *shader, QRhi * rhi, QRhiResourceUpdateBatch *res_updates, guint plane, GstVideoFormat v_format)
 {
-  GstGLContext *qt_context, *context;
+  GstGLContext *qt_context = NULL, *context;
   GstMemory *mem;
   GstGLMemory *gl_mem;
   GstGLSyncMeta *sync_meta;
@@ -578,15 +582,17 @@ GstQSG6Material::bind(GstQSG6MaterialShader *shader, QRhi * rhi, QRhiResourceUpd
   QSize tex_size;
   QRhiTexture::Flags flags = {};
 
-  qt_context = GST_GL_CONTEXT (g_weak_ref_get (&this->qt_context_ref_));
-  if (!qt_context)
-    goto out;
-
   if (!this->buffer_)
     goto out;
   if (GST_VIDEO_INFO_FORMAT (&this->v_info) == GST_VIDEO_FORMAT_UNKNOWN)
     goto out;
 
+  qt_context = GST_GL_CONTEXT (g_weak_ref_get (&this->qt_context_ref_));
+  if (!qt_context)
+    goto out;
+
+  GST_DEBUG ("%p attempting to bind with context %" GST_PTR_FORMAT, this, qt_context);
+
   mem = gst_buffer_peek_memory (this->buffer_, plane);
   g_assert (gst_is_gl_memory (mem));
   gl_mem = (GstGLMemory *) mem;