Add a test case capturing problems with weak_connect_signal (#520)

This adds a cyclic dependency between the function used for the weak signal and
an object with a __gc metamethod. The problem occurs since the GC will first
finalize the object (call its __gc metamethod) and only in the next generation
will it actually collect the garbage and remove the functions from weak-tables
using it as a key.

And yes this needs special code for Lua 5.1 because there __gc doesn't work on
tables. :-(

Signed-off-by: Uli Schlachter <psychon@znc.in>

spec/gears/object_spec.lua

@@ -131,6 +131,35 @@ describe("gears.object", function()
             obj:weak_connect_signal("signal", cb)
         end)
     end)
+
+    it("weak signal is disconnected before __gc runs", function()
+        local finalized = false
+        do
+            local userdata
+            local function callback()
+                error("Signal should already be disconnected")
+
+                -- This should reference the object...
+                print("userdata:", userdata)
+            end
+            obj:weak_connect_signal("signal", callback)
+
+            -- Now create an object and tie its lifetime to the callback
+            local function gc()
+                finalized = true
+            end
+            if _VERSION <= "Lua 5.1" then
+                local userdata = newproxy(true)
+                getmetatable(userdata).__gc = gc
+                getmetatable(userdata).callback = callback
+            else
+                userdata = setmetatable({callback}, { __gc = gc })
+            end
+        end
+        collectgarbage("collect")
+        assert.is_true(finalized)
+        obj:emit_signal("signal")
+    end)
 end)
 
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