don't wait forever for the iptables lock

if iptables version supports wait, we use `-w` without any additional
argument, so it keeps waiting forever trying to acquire the lock
at a 1 second interval rate.

This can cause issues on busy environments by software that depends
on this library, because they can be waiting forever.

We can make the timeout configurable, just only for iptables versions
that have the --wait flag.

Signed-off-by: Antonio Ojea <[email protected]>

iptables/iptables.go

@@ -73,6 +73,7 @@ type IPTables struct {
 	v2             int
 	v3             int
 	mode           string // the underlying iptables operating mode, e.g. nf_tables
+	timeout        int    // time to wait for the iptables lock, default is 0, that means wait forever
 }
 
 // Stat represents a structured statistic entry.
@@ -132,6 +133,17 @@ func (ipt *IPTables) Proto() Protocol {
 	return ipt.proto
 }
 
+// Timeout returns the timeout used by this IPTables.
+func (ipt *IPTables) Timeout() int {
+	return ipt.timeout
+}
+
+// SetTimeout sets the timeout used by this IPTables.
+// 0 means wait forever
+func (ipt *IPTables) SetTimeout(timeout int) {
+	ipt.timeout = timeout
+}
+
 // Exists checks if given rulespec in specified table/chain exists
 func (ipt *IPTables) Exists(table, chain string, rulespec ...string) (bool, error) {
 	if !ipt.hasCheck {
@@ -426,6 +438,9 @@ func (ipt *IPTables) runWithOutput(args []string, stdout io.Writer) error {
 	args = append([]string{ipt.path}, args...)
 	if ipt.hasWait {
 		args = append(args, "--wait")
+		if ipt.timeout != 0 {
+			args = append(args, strconv.Itoa(ipt.timeout))
+		}
 	} else {
 		fmu, err := newXtablesFileLock()
 		if err != nil {

iptables/iptables_test.go

@@ -50,6 +50,22 @@ func TestProto(t *testing.T) {
 	}
 }
 
+func TestTimeout(t *testing.T) {
+	ipt, err := New()
+	if err != nil {
+		t.Fatalf("New failed: %v", err)
+	}
+	if ipt.Timeout() != 0 {
+		t.Fatalf("Expected timeout 0 (wait forever), got %v", ipt.Timeout())
+	}
+
+	ipt.SetTimeout(5)
+	if ipt.Timeout() != 5 {
+		t.Fatalf("Expected timeout 5, got %v", ipt.Timeout())
+	}
+
+}
+
 func randChain(t *testing.T) string {
 	n, err := rand.Int(rand.Reader, big.NewInt(1000000))
 	if err != nil {