sandbox: table_print prints function signatures instead of pointers

This does not work with C functions etc. but it seems that we do not
expose them directly in Lua interface for users.

daemon/lua/sandbox.lua.in

@@ -354,6 +354,44 @@ function eval_cmd(line, raw)
 end
 
 -- Pretty printing
+
+local function funcsign(f)
+-- thanks to AnandA777 from StackOverflow! Function funcsign is adapted version of
+-- https://stackoverflow.com/questions/51095022/inspect-function-signature-in-lua-5-1
+	assert(type(f) == 'function', "bad argument #1 to 'funcsign' (function expected)")
+	local func_args = {}
+	pcall(function()
+		local oldhook
+		local delay = 2
+		local function hook(event, line)
+			delay = delay - 1
+			if delay == 0 then  -- call this only for the introspected function
+				for i = 1, math.huge do
+					-- stack depth 2 is the introspected function
+					local k, v = debug.getlocal(2, i)
+					if (k or '('):sub(1, 1) == '(' then
+						break  -- internal variable, skip
+					else
+						table.insert(func_args, k)
+					end
+				end
+			if debug.getlocal(2, -1) then
+				-- vararg function
+				table.insert(func_args, "...")
+			end
+			debug.sethook(oldhook)
+			error('aborting the call to introspected function')
+		end
+	end
+	oldhook = debug.sethook(hook, "c")  -- invoke hook() on function call
+	-- fake arguments, necessary to detect vararg functions
+	fakearg = {}
+	for j = 1, 64 do fakearg[#fakearg + 1] = true end
+	f(unpack(fakearg)) -- huh?
+	end)
+	return "(" .. table.concat(func_args, ", ") .. ")"
+end
+
 function table_print (tt, indent, done)
 	done = done or {}
 	indent = indent or 0
@@ -394,13 +432,21 @@ function table_print (tt, indent, done)
 				result = result .. table_print (value, indent + 4, done)
 				result = result .. string.rep (" ", indent)
 				result = result .. "}\n"
+			elseif type (value) == "function" then
+				result = result .. string.format("[%s] => function %s%s\n",
+				         tostring(key), tostring(key), funcsign(value))
 			else
 				result = result .. string.format("[%s] => %s\n",
 				         tostring (key), printable(value))
 			end
 		end
-	else
-		result = result .. tostring(tt) .. "\n"
+	else  -- not a table
+		if type(tt) == "function" then
+			tt_str = string.format("function%s\n", funcsign(tt))
+		else
+			tt_str = tostring(tt)
+		end
+		result = result .. tt_str .. "\n"
 	end
 	return result
 end