cbi.lua: Implement Flag.validate function

cbi.lua
- Implement Flag.validate function to be overwritable
- rewritten if clause for easier reading ;-)

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

modules/luci-base/luasrc/cbi.lua

@@ -1528,17 +1528,25 @@ function Flag.__init__(self, ...)
 end
 
 -- A flag can only have two states: set or unset
-function Flag.parse(self, section)
+function Flag.parse(self, section, novld)
 	local fexists = self.map:formvalue(
 		FEXIST_PREFIX .. self.config .. "." .. section .. "." .. self.option)
 
 	if fexists then
 		local fvalue = self:formvalue(section) and self.enabled or self.disabled
 		local cvalue = self:cfgvalue(section)
-		if fvalue ~= self.default or (not self.optional and not self.rmempty) then
+		local val_err
-			self:write(section, fvalue)
+		fvalue, val_err = self:validate(fvalue, section)
-		else
+		if not fvalue then
+			if not novld then
+				self:add_error(section, "invalid", val_err)
+			end
+			return
+		end
+		if fvalue == self.default and (self.optional or self.rmempty) then
 			self:remove(section)
+		else
+			self:write(section, fvalue)
 		end
 		if (fvalue ~= cvalue) then self.section.changed = true end
 	else
@@ -1550,7 +1558,9 @@ end
 function Flag.cfgvalue(self, section)
 	return AbstractValue.cfgvalue(self, section) or self.default
 end
-
+function Flag.validate(self, value)
+	return value
+end
 
 --[[
 ListValue - A one-line value predefined in a list