slirp: fix setup on ipv6 disabled systems

libpod/networking_slirp4netns.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 
@@ -302,11 +303,15 @@ func (r *Runtime) setupSlirp4netns(ctr *Container, netns ns.NetNS) error {
 	cmd.Stdout = logFile
 	cmd.Stderr = logFile
 
-	var slirpReadyChan (chan struct{})
-
+	var slirpReadyWg, netnsReadyWg *sync.WaitGroup
 	if netOptions.enableIPv6 {
-		slirpReadyChan = make(chan struct{})
-		defer close(slirpReadyChan)
+		// use two wait groups to make sure we set the sysctl before
+		// starting slirp and reset it only after slirp is ready
+		slirpReadyWg = &sync.WaitGroup{}
+		netnsReadyWg = &sync.WaitGroup{}
+		slirpReadyWg.Add(1)
+		netnsReadyWg.Add(1)
+
 		go func() {
 			err := ns.WithNetNSPath(netnsPath, func(_ ns.NetNS) error {
 				// Duplicate Address Detection slows the ipv6 setup down for 1-2 seconds.
@@ -318,23 +323,37 @@ func (r *Runtime) setupSlirp4netns(ctr *Container, netns ns.NetNS) error {
 				// is ready in case users rely on this sysctl.
 				orgValue, err := ioutil.ReadFile(ipv6ConfDefaultAcceptDadSysctl)
 				if err != nil {
+					netnsReadyWg.Done()
+					// on ipv6 disabled systems the sysctl does not exists
+					// so we should not error
+					if errors.Is(err, os.ErrNotExist) {
+						return nil
+					}
 					return err
 				}
 				err = ioutil.WriteFile(ipv6ConfDefaultAcceptDadSysctl, []byte("0"), 0644)
+				netnsReadyWg.Done()
 				if err != nil {
 					return err
 				}
-				// wait for slirp to finish setup
-				<-slirpReadyChan
+
+				// wait until slirp4nets is ready before reseting this value
+				slirpReadyWg.Wait()
 				return ioutil.WriteFile(ipv6ConfDefaultAcceptDadSysctl, orgValue, 0644)
 			})
 			if err != nil {
 				logrus.Warnf("failed to set net.ipv6.conf.default.accept_dad sysctl: %v", err)
 			}
 		}()
+
+		// wait until we set the sysctl
+		netnsReadyWg.Wait()
 	}
 
 	if err := cmd.Start(); err != nil {
+		if netOptions.enableIPv6 {
+			slirpReadyWg.Done()
+		}
 		return errors.Wrapf(err, "failed to start slirp4netns process")
 	}
 	defer func() {
@@ -344,11 +363,12 @@ func (r *Runtime) setupSlirp4netns(ctr *Container, netns ns.NetNS) error {
 		}
 	}()
 
-	if err := waitForSync(syncR, cmd, logFile, 1*time.Second); err != nil {
-		return err
+	err = waitForSync(syncR, cmd, logFile, 1*time.Second)
+	if netOptions.enableIPv6 {
+		slirpReadyWg.Done()
 	}
-	if slirpReadyChan != nil {
-		slirpReadyChan <- struct{}{}
+	if err != nil {
+		return err
 	}
 
 	// Set a default slirp subnet. Parsing a string with the net helper is easier than building the struct myself