Fix menubar under Lua 5.1 (#1838)

LGI's async support was trying to yield inside a protected call. Lua 5.1
cannot do that. Work around this by reverting to the behaviour before
commit 50cfa6c: Only call the callback in a protected context.

Fixes: https://github.com/awesomeWM/awesome/issues/1837
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/menubar/utils.lua

@@ -43,6 +43,28 @@ utils.wm_name = "awesome"
 
 -- Private section
 
+local do_protected_call, call_callback
+do
+    -- Lua 5.1 cannot yield across a protected call. Instead of hardcoding a
+    -- check, we check for this problem: The following coroutine yields true on
+    -- success (so resume() returns true, true). On failure, pcall returns
+    -- false and a message, so resume() returns true, false, message.
+    local _, has_yieldable_pcall = coroutine.resume(coroutine.create(function()
+        return pcall(coroutine.yield, true)
+    end))
+    if has_yieldable_pcall then
+        do_protected_call = protected_call.call
+        call_callback = function(callback, ...)
+            return callback(...)
+        end
+    else
+        do_protected_call = function(f, ...)
+            return f(...)
+        end
+        call_callback = protected_call.call
+    end
+end
+
 local all_icon_sizes = {
     '128x128' ,
     '96x96',
@@ -298,10 +320,10 @@ function utils.parse_dir(dir_path, callback)
         enum:async_close()
     end
 
-    gio.Async.start(protected_call.call)(function()
+    gio.Async.start(do_protected_call)(function()
         local result = {}
         parser(gio.File.new_for_path(dir_path), result)
-        callback(result)
+        call_callback(callback, result)
     end)
 end