awful.titlebar: Fix GC for titlebar widgets (#2830)

Some titlebar widgets (`awful.titlebar.widget.titlewidget`,
`awful.titlebar.widget.button` and other specific button widgets) could
not be garbage collected until the associated client was unmanaged,
because the signal connection used to update the widget was never
destroyed, and the signal handling function was keeping a reference to
the widget in its environment.  This resulted in high memory usage when
the titlebar widgets were recreated multiple times for the same client
(this does not happen with the default Awesome configuration, but may be
needed for dynamic titlebar reconfiguration in a custom config).

Modify the code to use weak tables instead of direct signal connections
to avoid keeping strong references to widgets.  The widget update
functions still keep strong references to the widget itself (creating a
reference loop, but the Lua GC should handle it correctly) and the
client object, but this should not be a problem.

One publicly visible change is that `awful.titlebar.widget.titlewidget`
now has an `update` function, like the button widgets.

Signed-off-by: Sergey Vlasov <sigprof@gmail.com>

lib/awful/titlebar.lua

@@ -13,6 +13,8 @@
 ---------------------------------------------------------------------------
 
 local error = error
+local pairs = pairs
+local table = table
 local type = type
 local gmath = require("gears.math")
 local abutton = require("awful.button")
@@ -601,6 +603,33 @@ function titlebar.toggle(c, position)
     end
 end
 
+local instances = {}
+
+-- Do the equivalent of
+--     c:connect_signal(signal, widget.update)
+-- without keeping a strong reference to the widget.
+local function update_on_signal(c, signal, widget)
+    local sig_instances = instances[signal]
+    if sig_instances == nil then
+        sig_instances = setmetatable({}, { __mode = "k" })
+        instances[signal] = sig_instances
+        capi.client.connect_signal(signal, function(cl)
+            local widgets = sig_instances[cl]
+            if widgets then
+                for _, w in pairs(widgets) do
+                    w.update()
+                end
+            end
+        end)
+    end
+    local widgets = sig_instances[c]
+    if widgets == nil then
+        widgets = setmetatable({}, { __mode = "v" })
+        sig_instances[c] = widgets
+    end
+    table.insert(widgets, widget)
+end
+
 --- Create a new titlewidget. A title widget displays the name of a client.
 -- Please note that this returns a textbox and all of textbox' API is available.
 -- This way, you can e.g. modify the font that is used.
@@ -612,7 +641,8 @@ function titlebar.widget.titlewidget(c)
     local function update()
         ret:set_text(c.name or titlebar.fallback_name)
     end
-    c:connect_signal("property::name", update)
+    ret.update = update
+    update_on_signal(c, "property::name", ret)
     update()
 
     return ret
@@ -717,8 +747,8 @@ function titlebar.widget.button(c, name, selector, action)
 
     -- We do magic based on whether a client is focused above, so we need to
     -- connect to the corresponding signal here.
-    c:connect_signal("focus", update)
+    update_on_signal(c, "focus", ret)
-    c:connect_signal("unfocus", update)
+    update_on_signal(c, "unfocus", ret)
 
     return ret
 end
@@ -728,7 +758,7 @@ end
 -- @staticfct awful.titlebar.widget.floatingbutton
 function titlebar.widget.floatingbutton(c)
     local widget = titlebar.widget.button(c, "floating", aclient.object.get_floating, aclient.floating.toggle)
-    c:connect_signal("property::floating", widget.update)
+    update_on_signal(c, "property::floating", widget)
     return widget
 end
 
@@ -741,7 +771,7 @@ function titlebar.widget.maximizedbutton(c)
     end, function(cl, state)
         cl.maximized = not state
     end)
-    c:connect_signal("property::maximized", widget.update)
+    update_on_signal(c, "property::maximized", widget)
     return widget
 end
 
@@ -752,7 +782,7 @@ function titlebar.widget.minimizebutton(c)
     local widget = titlebar.widget.button(c, "minimize",
                                           function() return "" end,
                                           function(cl) cl.minimized = not cl.minimized end)
-    c:connect_signal("property::minimized", widget.update)
+    update_on_signal(c, "property::minimized", widget)
     return widget
 end
 
@@ -770,7 +800,7 @@ function titlebar.widget.ontopbutton(c)
     local widget = titlebar.widget.button(c, "ontop",
                                           function(cl) return cl.ontop end,
                                           function(cl, state) cl.ontop = not state end)
-    c:connect_signal("property::ontop", widget.update)
+    update_on_signal(c, "property::ontop", widget)
     return widget
 end
 
@@ -781,7 +811,7 @@ function titlebar.widget.stickybutton(c)
     local widget = titlebar.widget.button(c, "sticky",
                                           function(cl) return cl.sticky end,
                                           function(cl, state) cl.sticky = not state end)
-    c:connect_signal("property::sticky", widget.update)
+    update_on_signal(c, "property::sticky", widget)
     return widget
 end