Two dt_dev_process_image_job() timing issues

1. While running dt_dev_process_image_job() it's dt_dev_pixelpipe_process() could return with an
   error flag due to a pipe->shutdown.
   If so, the shutdown flag must be cleared before restarting to safely avoid an endless loop.
2. The preview pipe should raise the signal after updating the final size if that was requested
   by history_postpone_invalidate

Author: jenshannoschwalm

src/develop/develop.c

@@ -444,9 +444,11 @@ void dt_dev_process_image_job(dt_develop_t *dev,
       dt_pthread_mutex_unlock(&pipe->mutex);
       return;
     }
-    // or because the pipeline changed?
+    // or because the pipeline changed or shutdown?
     else
     {
+      if(dt_atomic_exch_int(&pipe->shutdown, FALSE))
+        dt_print_pipe(DT_DEBUG_PIPE, "spurious shutdown", pipe, NULL, pipe->devid, NULL, NULL);
       if(port && port->widget) dt_control_queue_redraw_widget(port->widget);
       goto restart;
     }
@@ -467,16 +469,23 @@ void dt_dev_process_image_job(dt_develop_t *dev,
   dt_control_busy_leave();
   dt_pthread_mutex_unlock(&pipe->mutex);
 
-  if(dev->gui_attached && !dev->gui_leaving && signal != -1)
-    DT_CONTROL_SIGNAL_RAISE(signal);
+  const gboolean signalling = dev->gui_attached && !dev->gui_leaving && signal != -1;
 
-  if(port) return;
+  if(port) // reminder: only the preview pipe is called without a port
+  {
+    if(signalling && signal != DT_SIGNAL_DEVELOP_PREVIEW_PIPE_FINISHED)
+      DT_CONTROL_SIGNAL_RAISE(signal);
+    return;
+  }
 
-  // preview pipe only
+  // rest is for preview pipe only
 
   if(!dev->history_postpone_invalidate)
     dt_image_update_final_size(dev->preview_pipe->output_imgid);
 
+  if(signalling)  // raise this after possibly updating the final size
+    DT_CONTROL_SIGNAL_RAISE(signal);
+
   dev->gui_previous_pipe_time = dt_get_wtime();
 
 #ifdef USE_LUA