[gimp/gimp-2-10] app: in GimpHistogramEditor, don't block when creating bg histogram

[Ell <ell src gnome org>]

commit a299270f8a45b896a47af92ee8ccc723609fc783
Author: Ell <ell_se yahoo com>
Date:   Fri Jan 4 15:13:43 2019 -0500

    app: in GimpHistogramEditor, don't block when creating bg histogram
    
    In GimpHistogramEditor, when the drawable preview is frozen, don't
    duplicate the main histogram as the bg histogram if calculation is
    still ongoing, since this will block until histogram calculation is
    complete.  In particular, this creates a noticeable stall when
    beginning a paint stroke while the histogram is being calculated.
    
    Instead, defer the creation of the bg histogram to the completion
    of the calculation of the main histogram.
    
    (cherry picked from commit 471efee769555a04782f7f9fe544b256b229753a)

 app/widgets/gimphistogrameditor.c | 69 ++++++++++++++++++++++++++++++++++-----
 app/widgets/gimphistogrameditor.h |  5 ++-
 2 files changed, 65 insertions(+), 9 deletions(-)
---
diff --git a/app/widgets/gimphistogrameditor.c b/app/widgets/gimphistogrameditor.c
index 2e485c8404..4a249f9ff1 100644
--- a/app/widgets/gimphistogrameditor.c
+++ b/app/widgets/gimphistogrameditor.c
@@ -337,6 +337,8 @@ gimp_histogram_editor_set_image (GimpImageEditor *image_editor,
           editor->idle_id = 0;
         }
 
+      editor->update_pending = FALSE;
+
       g_signal_handlers_disconnect_by_func (image_editor->image,
                                             gimp_histogram_editor_update,
                                             editor);
@@ -463,8 +465,24 @@ gimp_histogram_editor_calculate_async_callback (GimpAsync           *async,
 {
   editor->calculate_async = NULL;
 
-  if (gimp_async_is_finished (async))
-    gimp_histogram_editor_info_update (editor);
+  if (gimp_async_is_finished (async) && editor->histogram)
+    {
+      if (editor->bg_pending)
+        {
+          GimpHistogramView *view = GIMP_HISTOGRAM_BOX (editor->box)->view;
+
+          editor->bg_histogram = gimp_histogram_duplicate (editor->histogram);
+
+          gimp_histogram_view_set_background (view, editor->bg_histogram);
+        }
+
+      gimp_histogram_editor_info_update (editor);
+    }
+
+  editor->bg_pending = FALSE;
+
+  if (update_pending)
+    gimp_histogram_editor_update (editor);
 }
 
 static gboolean
@@ -544,16 +562,42 @@ gimp_histogram_editor_frozen_update (GimpHistogramEditor *editor,
       if (! editor->bg_histogram &&
           gtk_widget_is_drawable (GTK_WIDGET (editor)))
         {
-          if (gimp_histogram_editor_validate (editor))
-            editor->bg_histogram = gimp_histogram_duplicate (editor->histogram);
+          if (editor->idle_id)
+            {
+              g_source_remove (editor->idle_id);
 
-          gimp_histogram_view_set_background (view, editor->bg_histogram);
+              gimp_histogram_editor_idle_update (editor);
+            }
+
+          if (gimp_histogram_editor_validate (editor))
+            {
+              if (editor->calculate_async)
+                {
+                  editor->bg_pending = TRUE;
+                }
+              else
+                {
+                  editor->bg_histogram = gimp_histogram_duplicate (
+                    editor->histogram);
+
+                  gimp_histogram_view_set_background (view,
+                                                      editor->bg_histogram);
+                }
+            }
         }
     }
-  else if (editor->bg_histogram)
+  else
     {
-      g_clear_object (&editor->bg_histogram);
-      gimp_histogram_view_set_background (view, NULL);
+      if (editor->bg_histogram)
+        {
+          g_clear_object (&editor->bg_histogram);
+          gimp_histogram_view_set_background (view, NULL);
+        }
+
+      editor->bg_pending = FALSE;
+
+      if (editor->update_pending)
+        gimp_async_cancel_and_wait (editor->calculate_async);
     }
 }
 
@@ -569,6 +613,15 @@ gimp_histogram_editor_buffer_update (GimpHistogramEditor *editor,
 static void
 gimp_histogram_editor_update (GimpHistogramEditor *editor)
 {
+  if (editor->bg_pending)
+    {
+      editor->update_pending = TRUE;
+
+      return;
+    }
+
+  editor->update_pending = FALSE;
+
   if (editor->calculate_async)
     gimp_async_cancel_and_wait (editor->calculate_async);
 
diff --git a/app/widgets/gimphistogrameditor.h b/app/widgets/gimphistogrameditor.h
index a3e663e2fa..55c094d768 100644
--- a/app/widgets/gimphistogrameditor.h
+++ b/app/widgets/gimphistogrameditor.h
@@ -41,11 +41,14 @@ struct _GimpHistogramEditor
   GimpDrawable         *drawable;
   GimpHistogram        *histogram;
   GimpHistogram        *bg_histogram;
-  GimpAsync            *calculate_async;
 
   guint                 idle_id;
   gboolean              recompute;
 
+  GimpAsync            *calculate_async;
+  gboolean              bg_pending;
+  gboolean              update_pending;
+
   GtkWidget            *menu;
   GtkWidget            *box;
   GtkWidget            *labels[6];