naughty.notification: Do not hold a strong reference to the screen.

The screen will leak if there is active notifications when the screen
is removed. Note that as of this commit, it still does, but "less".

lib/naughty/notification.lua

@@ -15,6 +15,7 @@
 -- @copyright 2017 Emmanuel Lepage Vallee
 -- @coreclassmod naughty.notification
 ---------------------------------------------------------------------------
+local capi     = { screen = screen }
 local gobject  = require("gears.object")
 local gtable   = require("gears.table")
 local gsurface = require("gears.surface")
@@ -579,14 +580,14 @@ end
 
 local properties = {
     "message" , "title"   , "timeout" , "hover_timeout" ,
-    "screen"  , "position", "ontop"   , "border_width"  ,
+    "app_name", "position", "ontop"   , "border_width"  ,
     "width"   , "font"    , "icon"    , "icon_size"     ,
     "fg"      , "bg"      , "height"  , "border_color"  ,
     "shape"   , "opacity" , "margin"  , "ignore_suspend",
     "destroy" , "preset"  , "callback", "actions"       ,
     "run"     , "id"      , "ignore"  , "auto_reset_timeout",
     "urgency" , "image"   , "images"  , "widget_template",
-    "max_width", "app_name",
+    "max_width",
 }
 
 for _, prop in ipairs(properties) do
@@ -718,6 +719,23 @@ function notification:append_actions(new_actions)
 
 end
 
+function notification:set_screen(s)
+    assert(not self._private.screen)
+
+    s = s and capi.screen[s] or nil
+
+    -- Avoid an infinite loop in the management code.
+    if s == self._private.weak_screen[1] then return end
+
+    self._private.weak_screen = setmetatable({s}, {__mode="v"})
+
+    self:emit_signal("property::screen", s)
+end
+
+function notification:get_screen()
+    return self._private.weak_screen[1]
+end
+
 --TODO v6: remove this
 local function convert_actions(actions)
     gdebug.deprecate(
@@ -869,7 +887,7 @@ local function create(args)
     n:_connect_everything(naughty.emit_signal)
 
     -- Avoid modifying the original table
-    local private = {}
+    local private = {weak_screen = setmetatable({}, {__mode="v"})}
     rawset(n, "_private", private)
 
     -- Allow extensions to create override the preset with custom data
@@ -882,7 +900,11 @@ local function create(args)
     end
 
     for k, v in pairs(args) do
-        private[k] = v
+        -- Don't keep a strong reference to the screen, Lua 5.1 GC wont be
+        -- smart enough to unwind the mess of circular weak references.
+        if k ~= "screen" then
+            private[k] = v
+        end
     end
 
     -- It's an automatic property