Set rule handle during flush

This change makes it possible to delete rules after inserting them,
without needing to query the rules first. Additionally, this allows
positioning a new rule next to an existing rule.

There are two ways to refer to a rule: Either by ID or by handle. The ID
is assigned by userspace, and is only valid within a transaction, so it
can only be used before the flush. The handle is assigned by the kernel
when the transaction is committed, and can thus only be used after the
flush. We thus need to set an ID on each newly created rule, and
retrieve the handle of the rule during the flush.

I extended the message struct with a pointer to the Rule which the
message creates. This allows calling the reply handler callback which
sets the handle.

I updated tests to add a handle to generated replies for the
NFT_MSG_NEWRULE messages.

conn.go

@@ -52,7 +52,7 @@ type Conn struct {
 type netlinkMessage struct {
 	Header netlink.Header
 	Data   []byte
-	handleReply func(reply netlink.Message) error
+	rule   *Rule
 }
 
 // ConnOption is an option to change the behavior of the nftables Conn returned by Open.
@@ -274,7 +274,8 @@ func (cc *Conn) Flush() error {
 		} else if replyIndex < len(cc.messages) {
 			msg := messages[replyIndex+1]
 			if msg.Header.Sequence == reply.Header.Sequence && msg.Header.Type == reply.Header.Type {
-				err := cc.messages[replyIndex].handleReply(reply)
+				// The only messages which set the echo flag are rule create messages.
+				err := cc.messages[replyIndex].rule.handleCreateReply(reply)
 				if err != nil {
 					errs = errors.Join(errs, err)
 				}

rule.go

@@ -174,14 +174,14 @@ func (cc *Conn) newRule(r *Rule, op ruleOperation) *Rule {
 	}
 
 	var flags netlink.HeaderFlags
-	var handleReply func(reply netlink.Message) error
+	var ruleRef *Rule
 	switch op {
 	case operationAdd:
 		flags = netlink.Request | netlink.Acknowledge | netlink.Create | netlink.Echo | netlink.Append
-		handleReply = r.handleCreateReply
+		ruleRef = r
 	case operationInsert:
 		flags = netlink.Request | netlink.Acknowledge | netlink.Create | netlink.Echo
-		handleReply = r.handleCreateReply
+		ruleRef = r
 	case operationReplace:
 		flags = netlink.Request | netlink.Acknowledge | netlink.Replace
 	}
@@ -202,7 +202,7 @@ func (cc *Conn) newRule(r *Rule, op ruleOperation) *Rule {
 			Flags: flags,
 		},
 		Data: append(extraHeader(uint8(r.Table.Family), 0), msgData...),
-		handleReply: handleReply,
+		rule: ruleRef,
 	})
 
 	return r