Unbreak ReaderFooter test

Tests: Update the ffi.metatype wrapper

(Better idea: move to busted master).

spec/unit/commonrequire.lua

@@ -1,12 +1,37 @@
--- don't try to overwrite metatables so we can use --auto-insulate-tests
--- shamelessly copied from https://github.com/Olivine-Labs/busted/commit/db6d8b4be8fd099ab387efeb8232cfd905912abb
-local ffi = require "ffi"
-local old_metatype = ffi.metatype
-local exists = {}
-ffi.metatype = function(def, mttable)
-    if exists[def] then return exists[def] end
-    exists[def] = old_metatype(def, mttable)
-    return exists[def]
+-- Check if we're running a busted version recent enough that we don't need to deal with the LuaJIT hacks...
+-- That currently means > 2.0.0 (i.e., scm-2, which isn't on LuaRocks...).
+local busted_ok = false
+for name, _ in pairs(package.loaded) do
+    if name == "busted.luajit" then
+        busted_ok = true
+        break
+    end
+end
+
+-- Don't try to overwrite metatables so we can use --auto-insulate-tests
+-- Shamelessly copied from https://github.com/Olivine-Labs/busted/commit/2dfff99bda01fd3da56fd23415aba5a2a4cc0ffd
+if not busted_ok then
+    local ffi = require "ffi"
+
+    local original_metatype = ffi.metatype
+    local original_store = {}
+    ffi.metatype = function (primary, ...)
+        if original_store[primary] then
+            return original_store[primary]
+        end
+        local success, result, err = pcall(original_metatype, primary, ...)
+        if not success then
+            -- hard error was thrown
+            error(result, 2)
+        end
+        if not result then
+            -- soft error was returned
+            return result, err
+        end
+        -- it worked, store and return
+        original_store[primary] = result
+        return result
+    end
 end
 
 require "defaults"