Plugin/Forward - autotune the dialTimeout for connection (#1852)

* - implement an auto-tunable dialTimeout for fallback.

* - fix gofmt

* - factorized timeout computation with readTimeout / updated readme /

* - fix comment

plugin/forward/connect.go

@@ -16,21 +16,65 @@ import (
 	"github.com/miekg/dns"
 )
 
-func (p *Proxy) readTimeout() time.Duration {
-	rtt := time.Duration(atomic.LoadInt64(&p.avgRtt))
+// limitTimeout is a utility function to auto-tune timeout values
+// average observed time is moved towards the last observed delay moderated by a weight
+// next timeout to use will be the double of the computed average, limited by min and max frame.
+func limitTimeout(currentAvg *int64, minValue time.Duration, maxValue time.Duration) time.Duration {
+	rt := time.Duration(atomic.LoadInt64(currentAvg))
+	if rt < minValue {
+		return minValue
+	}
+	if rt < maxValue/2 {
+		return 2 * rt
+	}
+	return maxValue
+}
 
-	if rtt < minTimeout {
-		return minTimeout
+func averageTimeout(currentAvg *int64, observedDuration time.Duration, weight int64) {
+	dt := time.Duration(atomic.LoadInt64(currentAvg))
+	atomic.AddInt64(currentAvg, int64(observedDuration-dt)/weight)
+}
+
+func (t *transport) dialTimeout() time.Duration {
+	return limitTimeout(&t.avgDialTime, minDialTimeout, maxDialTimeout)
+}
+
+func (t *transport) updateDialTimeout(newDialTime time.Duration) {
+	averageTimeout(&t.avgDialTime, newDialTime, cumulativeAvgWeight)
+}
+
+// Dial dials the address configured in transport, potentially reusing a connection or creating a new one.
+func (t *transport) Dial(proto string) (*dns.Conn, bool, error) {
+	// If tls has been configured; use it.
+	if t.tlsConfig != nil {
+		proto = "tcp-tls"
 	}
-	if rtt < maxTimeout/2 {
-		return 2 * rtt
+
+	t.dial <- proto
+	c := <-t.ret
+
+	if c != nil {
+		return c, true, nil
 	}
-	return maxTimeout
+
+	reqTime := time.Now()
+	timeout := t.dialTimeout()
+	if proto == "tcp-tls" {
+		conn, err := dns.DialTimeoutWithTLS("tcp", t.addr, t.tlsConfig, timeout)
+		t.updateDialTimeout(time.Since(reqTime))
+		return conn, false, err
+	}
+	conn, err := dns.DialTimeout(proto, t.addr, timeout)
+	t.updateDialTimeout(time.Since(reqTime))
+	return conn, false, err
+}
+
+func (p *Proxy) readTimeout() time.Duration {
+	return limitTimeout(&p.avgRtt, minTimeout, maxTimeout)
 }
 
 func (p *Proxy) updateRtt(newRtt time.Duration) {
-	rtt := time.Duration(atomic.LoadInt64(&p.avgRtt))
-	atomic.AddInt64(&p.avgRtt, int64((newRtt-rtt)/rttCount))
+	averageTimeout(&p.avgRtt, newRtt, cumulativeAvgWeight)
 }
 
 // Connect selects an upstream, sends the request and waits for a response.
@@ -92,4 +136,4 @@ func (p *Proxy) Connect(ctx context.Context, state request.Request, forceTCP, me
 	return ret, nil
 }
 
-const rttCount = 4
+const cumulativeAvgWeight = 4