Improve safety of ID allocation (#307)

There was an existing mechanism to allocate IDs for sets, but this was
using a global counter without any synchronization to prevent data
races. I replaced this by a new mechanism which uses a connection-scoped
counter, protected by the Conn.mu Mutex. This can then also be used in
other places where IDs need to be allocated.

As an additional safeguard, it will panic instead of allocating the same
ID twice in a transaction. Most likely, your program will run out of
memory before reaching this point.

conn.go

@@ -17,6 +17,7 @@ package nftables
 import (
 	"errors"
 	"fmt"
+	"math"
 	"os"
 	"sync"
 	"syscall"
@@ -44,6 +45,8 @@ type Conn struct {
 	err          error
 	nlconn       *netlink.Conn // netlink socket using NETLINK_NETFILTER protocol.
 	sockOptions  []SockOption
+	lastID       uint32
+	allocatedIDs uint32
 }
 
 // ConnOption is an option to change the behavior of the nftables Conn returned by Open.
@@ -244,6 +247,7 @@ func (cc *Conn) Flush() error {
 	cc.mu.Lock()
 	defer func() {
 		cc.messages = nil
+		cc.allocatedIDs = 0
 		cc.mu.Unlock()
 	}()
 	if len(cc.messages) == 0 {
@@ -369,3 +373,20 @@ func batch(messages []netlink.Message) []netlink.Message {
 
 	return batch
 }
+
+// allocateTransactionID allocates an identifier which is only valid in the
+// current transaction.
+func (cc *Conn) allocateTransactionID() uint32 {
+	if cc.allocatedIDs == math.MaxUint32 {
+		panic(fmt.Sprintf("trying to allocate more than %d IDs in a single nftables transaction", math.MaxUint32))
+	}
+	// To make it more likely to catch when a transaction ID is erroneously used
+	// in a later transaction, cc.lastID is not reset after each transaction;
+	// instead it is only reset once it rolls over from math.MaxUint32 to 0.
+	cc.allocatedIDs++
+	cc.lastID++
+	if cc.lastID == 0 {
+		cc.lastID = 1
+	}
+	return cc.lastID
+}

set.go

@@ -46,8 +46,6 @@ const (
 	NFTA_SET_ELEM_EXPRESSIONS = 0x11
 )
 
-var allocSetID uint32
-
 // SetDatatype represents a datatype declared by nft.
 type SetDatatype struct {
 	Name  string
@@ -532,8 +530,7 @@ func (cc *Conn) AddSet(s *Set, vals []SetElement) error {
 	}
 
 	if s.ID == 0 {
-		allocSetID++
+		s.ID = cc.allocateTransactionID()
-		s.ID = allocSetID
 		if s.Anonymous {
 			s.Name = "__set%d"
 			if s.IsMap {