modules: change the "indexing" syntax sugar for C modules

In particular, throw errors when used in weird ways,
instead of the usual "return nil" semantics.
That might be surprising to some lua users.

NEWS

@@ -5,6 +5,8 @@ Improvements
 ------------
 - DNS-over-HTTPS: answers include `access-control-allow-origin: *` (!823)
 - support named AF_UNIX stream sockets for the http module (again)
+- lua tables for C modules are more strict by default, e.g. `nsid.foo`
+  will throw an error instead of returning `nil` (!797)
 
 Bugfixes
 --------

daemon/lua/sandbox.lua.in

@@ -179,7 +179,8 @@ function modules_create_table_for_c(kr_module_ud)
 		return
 	end
 	local module = {}
-	_G[ffi.string(kr_module.name)] = module
+	local module_name = ffi.string(kr_module.name)
+	_G[module_name] = module
 
 	--- Construct lua functions for properties.
 	if kr_module.props ~= nil then
@@ -229,19 +230,33 @@ function modules_create_table_for_c(kr_module_ud)
 
 	--- Add syntactic sugar for get() and set() properties.
 	--- That also "catches" any commands like `moduleName.foo = bar`.
-	setmetatable(module, {
-		__index = function (t, k)
-			local  v = rawget(t, k)
-			if     v     then return v
-			elseif rawget(t, 'get') then return t.get(k)
-			end
-		end,
-		__newindex = function (t, k, v)
-			local  old_v = rawget(t, k)
-			if not old_v and rawget(t, 'set') then
-				t.set(k..' '..v)
-			end
+	local m_index, m_newindex
+	local get_f = rawget(module, 'get')
+	if get_f ~= nil then
+		m_index = function (_, key)
+			return get_f(key)
+		end
+	else
+		m_index = function ()
+			error('module ' .. module_name .. ' does not support indexing syntax sugar')
 		end
+	end
+	local set_f = rawget(module, 'set')
+	if set_f ~= nil then
+		m_newindex = function (_, key, value)
+			-- This will produce a nasty error on some non-string parameters.
+			-- Still, we already use it with integer values, e.g. in predict module :-/
+			return set_f(key .. ' ' .. value)
+		end
+	else
+		m_newindex = function ()
+			error('module ' .. module_name .. ' does not support assignment syntax sugar')
+		end
+	end
+	setmetatable(module, {
+		-- note: the two functions only get called for *missing* indices
+		__index = m_index,
+		__newindex = m_newindex,
 	})
 end