Allow dns query to fail when set "spray" option.

When the "spray" parameter is set, dnsredir will retry indefinitely
when it encounters an error. When the network is temporarily
interrupted or misconfigured, dnsredir will eat all the CPU.

This patch fixes this issue.

Signed-off-by: yuguorui <[email protected]>

dnsredir.go

@@ -76,12 +76,15 @@ func (r *Dnsredir) ServeDNS(ctx context.Context, w dns.ResponseWriter, req *dns.
 
 	var reply *dns.Msg
 	var upstreamErr error
+	var tryCount int32
 	deadline := time.Now().Add(defaultTimeout)
+	tryCount = 0
 	for time.Now().Before(deadline) {
+		tryCount += 1
 		start := time.Now()
 
 		host := upstream.Select()
-		if host == nil {
+		if host == nil || tryCount > upstream.maxRetry {
 			log.Debug(errNoHealthy)
 			return dns.RcodeServerFailure, errNoHealthy
 		}

upstream.go

@@ -34,6 +34,7 @@ type reloadableUpstream struct {
 	ipset     interface{}
 	pf        interface{}
 	noIPv6    bool
+	maxRetry  int32
 }
 
 // reloadableUpstream implements Upstream interface
@@ -124,8 +125,9 @@ func newReloadableUpstream(c *caddy.Controller) (Upstream, error) {
 			urlReadTimeout: defaultUrlReadTimeout,
 			stopUrlReload:  make(chan struct{}),
 		},
-		ignored: make(domainSet),
-		inline:  make(domainSet),
+		ignored:  make(domainSet),
+		inline:   make(domainSet),
+		maxRetry: defaultMaxRetry,
 		HealthCheck: &HealthCheck{
 			stop:          make(chan struct{}),
 			maxFails:      defaultMaxFails,
@@ -362,6 +364,13 @@ func parseBlock(c *caddy.Controller, u *reloadableUpstream) error {
 		}
 		u.maxFails = n
 		log.Infof("%v: %v", dir, n)
+	case "max_retry":
+		n, err := parseInt32(c)
+		if err != nil {
+			return err
+		}
+		u.maxRetry = n
+		log.Infof("%v: %v", dir, n)
 	case "health_check":
 		args := c.RemainingArgs()
 		n := len(args)
@@ -585,6 +594,7 @@ func parseBootstrap(c *caddy.Controller, u *reloadableUpstream) error {
 
 const (
 	defaultMaxFails = 3
+	defaultMaxRetry = 10
 
 	defaultPathReloadInterval = 2 * time.Second
 	defaultUrlReloadInterval  = 30 * time.Minute