Fix crash when struts change during grab operation

Since meta_workspace_invalidate_work_area() frees the edges
workspace->screen_edges and workspace->monitor_edges, we must clean up
our cached edge resistance data when the invalidate_work_area() is
called on the active workspace, or when the workspace changes.

Make the computation of the edge resistance data lazy so that it
will be recomputed the next time we try to access it.
meta_display_compute_resistance_and_snapping_edges() is made
private to edge-resistance.c

Invaliding the data when active workspace changes also will improve
correctness for edge resistance when the current workspace changes
during a grab operation. (Even with this fix we still don't try to
handle window positions changing during a grab operation; that can't
cause a crash since, unlike screen and monitor edges, the window edges
are freshly allocated, it will just cause slight oddness in that
corner case.)

Root cause tracked down due to much effort by Jon Nettleton.
https://bugzilla.gnome.org/show_bug.cgi?id=608800

4 files changed, 24 insertions, 18 deletions

diff --git a/src/core/display-private.h b/src/core/display-private.h
index dc270a96..f204af0f 100644
--- a/src/core/display-private.h
+++ b/src/core/display-private.h
@@ -420,8 +420,7 @@ void meta_display_ungrab_focus_window_button (MetaDisplay *display,
                                               MetaWindow  *window);
 
 /* Next two functions are defined in edge-resistance.c */
-void meta_display_compute_resistance_and_snapping_edges (MetaDisplay *display);
-void meta_display_cleanup_edges                         (MetaDisplay *display);
+void meta_display_cleanup_edges              (MetaDisplay *display);
 
 /* make a request to ensure the event serial has changed */
 void     meta_display_increment_event_serial (MetaDisplay *display);
diff --git a/src/core/display.c b/src/core/display.c
index 6a4831fb..bcb262af 100644
--- a/src/core/display.c
+++ b/src/core/display.c
@@ -3662,18 +3662,6 @@ meta_display_begin_grab_op (MetaDisplay *display,
   g_assert (display->grab_window != NULL || display->grab_screen != NULL);
   g_assert (display->grab_op != META_GRAB_OP_NONE);
 
-  /* If this is a move or resize, cache the window edges for
-   * resistance/snapping
-   */
-  if (meta_grab_op_is_resizing (display->grab_op) ||
-      meta_grab_op_is_moving (display->grab_op))
-    {
-      meta_topic (META_DEBUG_WINDOW_OPS,
-                  "Computing edges to resist-movement or snap-to for %s.\n",
-                  window->desc);
-      meta_display_compute_resistance_and_snapping_edges (display);
-    }
-
   /* Save the old stacking */
   if (GRAB_OP_IS_WINDOW_SWITCH (display->grab_op))
     {
diff --git a/src/core/edge-resistance.c b/src/core/edge-resistance.c
index 51e77d54..aa4fcb6a 100644
--- a/src/core/edge-resistance.c
+++ b/src/core/edge-resistance.c
@@ -63,6 +63,8 @@ struct MetaEdgeResistanceData
   ResistanceDataForAnEdge bottom_data;
 };
 
+static void compute_resistance_and_snapping_edges (MetaDisplay *display);
+
 /* !WARNING!: this function can return invalid indices (namely, either -1 or
  * edges->len); this is by design, but you need to remember this.
  */
@@ -550,7 +552,9 @@ apply_edge_resistance_to_each_side (MetaDisplay         *display,
   gboolean                modified;
   int new_left, new_right, new_top, new_bottom;
 
-  g_assert (display->grab_edge_resistance_data != NULL);
+  if (display->grab_edge_resistance_data == NULL)
+    compute_resistance_and_snapping_edges (display);
+
   edge_data = display->grab_edge_resistance_data;
 
   if (auto_snap)
@@ -671,7 +675,8 @@ meta_display_cleanup_edges (MetaDisplay *display)
   MetaEdgeResistanceData *edge_data = display->grab_edge_resistance_data;
   GHashTable *edges_to_be_freed;
 
-  g_assert (edge_data != NULL);
+  if (edge_data == NULL) /* Not currently cached */
+    return;
 
   /* We first need to clean out any window edges */
   edges_to_be_freed = g_hash_table_new_full (g_direct_hash, g_direct_equal,
@@ -938,8 +943,8 @@ initialize_grab_edge_resistance_data (MetaDisplay *display)
   edge_data->bottom_data.keyboard_buildup = 0;
 }
 
-void
-meta_display_compute_resistance_and_snapping_edges (MetaDisplay *display)
+static void
+compute_resistance_and_snapping_edges (MetaDisplay *display)
 {
   GList *stacked_windows;
   GList *cur_window_iter;
@@ -952,6 +957,11 @@ meta_display_compute_resistance_and_snapping_edges (MetaDisplay *display)
    */
   GSList *rem_windows, *rem_win_stacking;
 
+  g_assert (display->grab_window != NULL);
+  meta_topic (META_DEBUG_WINDOW_OPS,
+              "Computing edges to resist-movement or snap-to for %s.\n",
+              display->grab_window->desc);
+
   /*
    * 1st: Get the list of relevant windows, from bottom to top
    */
diff --git a/src/core/workspace.c b/src/core/workspace.c
index b85ab53c..928f3de6 100644
--- a/src/core/workspace.c
+++ b/src/core/workspace.c
@@ -382,6 +382,10 @@ meta_workspace_activate_with_focus (MetaWorkspace *workspace,
   if (workspace->screen->active_workspace)
     workspace_switch_sound(workspace->screen->active_workspace, workspace);
 
+  /* Free any cached pointers to the workspaces's edges from
+   * a current resize or move operation */
+  meta_display_cleanup_edges (workspace->screen->display);
+
   /* Note that old can be NULL; e.g. when starting up */
   old = workspace->screen->active_workspace;
   
@@ -542,6 +546,11 @@ meta_workspace_invalidate_work_area (MetaWorkspace *workspace)
               "Invalidating work area for workspace %d\n",
               meta_workspace_index (workspace));
 
+  /* If we are in the middle of a resize or move operation, we
+   * might have cached pointers to the workspace's edges */
+  if (workspace == workspace->screen->active_workspace)
+    meta_display_cleanup_edges (workspace->screen->display);
+
   g_free (workspace->work_area_xinerama);
   workspace->work_area_xinerama = NULL;