wibox.drawable: Support forced screens

Up to now, a drawable always figured out the screen that it is on by
looking at its position. This causes memleak-like problems with wibars:
A wibar has a screen assigned, but its underlying drawable will end up
referring to another screen. Via this, we were managing to build a long
reference chain of screens and drawable that meant that none of the fake
screens that our test suite added could be garbage collected.

To fix this, add wibox.drawable._force_screen(s). After this function is
called, the normal screen detection based on the position is skipped and
instead the given screen is always used. This breaks the above reference
chain and things become garbage-collectable.

Also, this chains the drawable to the life time of the screen: When the
screen becomes invalid (.valid == false), the drawable will stop
redrawing.

Fixes: https://github.com/awesomeWM/awesome/issues/1237
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/drawable.lua

@@ -30,14 +30,17 @@ local visible_drawables = {}
 local function get_widget_context(self)
     local geom = self.drawable:geometry()
 
-    local sgeos = {}
+    local s = self._forced_screen
+    if not s then
+        local sgeos = {}
 
-    for s in capi.screen do
-        sgeos[s] = s.geometry
+        for s in capi.screen do
+            sgeos[s] = s.geometry
+        end
+
+        s = grect.get_by_coord(sgeos, geom.x, geom.y) or capi.screen.primary
     end
 
-    local s = grect.get_by_coord(sgeos, geom.x, geom.y) or capi.screen.primary
-
     local context = self._widget_context
     local dpi = beautiful.xresources.get_dpi(s)
     if (not context) or context.screen ~= s or context.dpi ~= dpi then
@@ -59,6 +62,7 @@ end
 
 local function do_redraw(self)
     if not self.drawable.valid then return end
+    if self._forced_screen and not self._forced_screen.valid then return end
 
     local surf = surface.load_silently(self.drawable.surface, false)
     -- The surface can be nil if the drawable's parent was already finalized
@@ -274,6 +278,10 @@ function drawable:set_fg(c)
     self._do_complete_repaint()
 end
 
+function drawable:_force_screen(s)
+    self._forced_screen = s
+end
+
 function drawable:_inform_visible(visible)
     self._visible = visible
     if visible then

lib/wibox/init.lua

@@ -86,6 +86,7 @@ function wibox:set_screen(s)
     -- Remember this screen so things work correctly if screens overlap and
     -- (x,y) is not enough to figure out the correct screen.
     self.screen_assigned = s
+    self._drawable:_force_screen(s)
 end
 
 for _, k in pairs{ "buttons", "struts", "geometry", "get_xproperty", "set_xproperty" } do