[gtk/gtk-3-24: 2/3] DnD: fix setting icon in drag-begin

[Benjamin Otte <otte src gnome org>]

commit 41721381548124e72c8a291a8f0d14b33788a8b1
Author: Руслан Ижбулатов <lrn1986 gmail com>
Date:   Tue Oct 16 19:04:54 2018 +0000

    DnD: fix setting icon in drag-begin
    
    Commit 1c96b703 changed the way icon
    information is given to DnD. Previously an icon helper was kept at
    the drag source site. Now an image definition is stored there.
    The difference is that icon helper is an object that changes its
    state in response to an icon being set, thus the object survived
    multiple icon changes. Whereas image definition is destroyed and
    re-created from scratch every time a drag icon is changed.
    This created a problem where gtk_drag_begin_internal() would receive
    the value of site->image_def when a drag just began, then it emits
    "drag-begin" signal, in response to which an application can
    set drag icon, changing the value of site->image_def. However,
    gtk_drag_begin_internal() is unable to know about that change and
    continues to use the old value it received from up the stack.
    
    Not only does it prevent drag icon from being set from "drag-begin",
    it also can induce a crash, since the old image_def value used
    by gtk_drag_begin_internal() points to a freed memory region.
    
    Fix this by only setting a default icon (which is created in-place)
    in gtk_drag_begin_internal() if the caller does not care about icons.
    Otherwise gtk_drag_begin_internal() will return a boolean that indicates
    whether an icon needs to be set. Then the caller can invoke
    gtk_drag_set_icon_definition() to set the icon, if needed.
    
    Fixes #1407.

 gtk/gtkdnd.c        | 40 +++++++++++++++++++---------------------
 gtk/gtkdndprivate.h |  2 +-
 gtk/gtkdragsource.c | 11 ++++++++---
 3 files changed, 28 insertions(+), 25 deletions(-)
---
diff --git a/gtk/gtkdnd.c b/gtk/gtkdnd.c
index dd66413f5d..96edea65f8 100644
--- a/gtk/gtkdnd.c
+++ b/gtk/gtkdnd.c
@@ -1699,18 +1699,20 @@ gtk_drag_is_managed (GtkWidget *source_widget)
     FALSE;
 }
 
-/* Like gtk_drag_begin(), but also takes a GtkIconHelper
- * so that we can set the icon from the source site information
+/* Like gtk_drag_begin(), but also communicates the need to
+ * create an icon for the drag operation back to the caller.
+ * If the caller passes out_needs_icon == NULL, it means that
+ * the caller does not care.
  */
 GdkDragContext *
-gtk_drag_begin_internal (GtkWidget          *widget,
-                         GtkImageDefinition *icon,
-                         GtkTargetList      *target_list,
-                         GdkDragAction       actions,
-                         gint                button,
-                         const GdkEvent     *event,
-                         int                 x,
-                         int                 y)
+gtk_drag_begin_internal (GtkWidget           *widget,
+                         gboolean            *out_needs_icon,
+                         GtkTargetList       *target_list,
+                         GdkDragAction        actions,
+                         gint                 button,
+                         const GdkEvent      *event,
+                         int                  x,
+                         int                  y)
 {
   GtkDragSourceInfo *info;
   GList *targets = NULL;
@@ -1873,20 +1875,16 @@ gtk_drag_begin_internal (GtkWidget          *widget,
    * application may have set one in ::drag_begin, or it may
    * not have set one.
    */
-  if (!info->icon_widget)
+  if (!info->icon_widget && out_needs_icon == NULL)
     {
-      if (icon)
-        {
-          set_icon_helper (info->context, icon, 0, 0);
-        }
-      else
-        {
-          icon = gtk_image_definition_new_icon_name ("text-x-generic");
-          set_icon_helper (info->context, icon, 0, 0);
-          gtk_image_definition_unref (icon);
-        }
+      GtkImageDefinition *icon = gtk_image_definition_new_icon_name ("text-x-generic");
+      set_icon_helper (info->context, icon, 0, 0);
+      gtk_image_definition_unref (icon);
     }
 
+  if (out_needs_icon != NULL)
+    *out_needs_icon = (info->icon_widget == NULL);
+
   if (managed)
     {
       g_signal_connect (context, "drop-performed",
diff --git a/gtk/gtkdndprivate.h b/gtk/gtkdndprivate.h
index f14a456ffb..89f1b0f4db 100644
--- a/gtk/gtkdndprivate.h
+++ b/gtk/gtkdndprivate.h
@@ -43,7 +43,7 @@ struct _GtkDragDestSite
 G_BEGIN_DECLS
 
 GdkDragContext *        gtk_drag_begin_internal         (GtkWidget              *widget,
-                                                         GtkImageDefinition     *icon,
+                                                         gboolean               *out_needs_icon,
                                                          GtkTargetList          *target_list,
                                                          GdkDragAction           actions,
                                                          gint                    button,
diff --git a/gtk/gtkdragsource.c b/gtk/gtkdragsource.c
index 677281aa88..bbe3d2ecad 100644
--- a/gtk/gtkdragsource.c
+++ b/gtk/gtkdragsource.c
@@ -88,6 +88,8 @@ gtk_drag_source_event_cb (GtkWidget *widget,
           GdkEventSequence *sequence;
           GdkEvent *last_event;
           guint button;
+          gboolean needs_icon;
+          GdkDragContext *context;
 
           sequence = gtk_gesture_single_get_current_sequence (GTK_GESTURE_SINGLE (site->drag_gesture));
           last_event = gdk_event_copy (gtk_gesture_get_last_event (site->drag_gesture, sequence));
@@ -95,9 +97,12 @@ gtk_drag_source_event_cb (GtkWidget *widget,
           button = gtk_gesture_single_get_current_button (GTK_GESTURE_SINGLE (site->drag_gesture));
           gtk_event_controller_reset (GTK_EVENT_CONTROLLER (site->drag_gesture));
 
-          gtk_drag_begin_internal (widget, site->image_def, site->target_list,
-                                   site->actions, button, last_event,
-                                   start_x, start_y);
+          context = gtk_drag_begin_internal (widget, &needs_icon, site->target_list,
+                                             site->actions, button, last_event,
+                                             start_x, start_y);
+
+          if (context != NULL && needs_icon)
+            gtk_drag_set_icon_definition (context, site->image_def, 0, 0);
 
           gdk_event_free (last_event);