plugin/forward: Allow Proxy to be used outside of forward plugin. (#5951)

* plugin/forward: Move Proxy into pkg/plugin/proxy, to allow forward.Proxy to be used outside of forward plugin.

Signed-off-by: Patrick Downey <patrick.downey@dioadconsulting.com>

plugin/pkg/proxy/connect.go

@@ -0,0 +1,152 @@
+// Package proxy implements a forwarding proxy. It caches an upstream net.Conn for some time, so if the same
+// client returns the upstream's Conn will be precached. Depending on how you benchmark this looks to be
+// 50% faster than just opening a new connection for every client. It works with UDP and TCP and uses
+// inband healthchecking.
+package proxy
+
+import (
+	"context"
+	"io"
+	"strconv"
+	"sync/atomic"
+	"time"
+
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// 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
+}
+
+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) (*persistConn, bool, error) {
+	// If tls has been configured; use it.
+	if t.tlsConfig != nil {
+		proto = "tcp-tls"
+	}
+
+	t.dial <- proto
+	pc := <-t.ret
+
+	if pc != nil {
+		ConnCacheHitsCount.WithLabelValues(t.addr, proto).Add(1)
+		return pc, true, nil
+	}
+	ConnCacheMissesCount.WithLabelValues(t.addr, proto).Add(1)
+
+	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 &persistConn{c: conn}, false, err
+	}
+	conn, err := dns.DialTimeout(proto, t.addr, timeout)
+	t.updateDialTimeout(time.Since(reqTime))
+	return &persistConn{c: conn}, false, err
+}
+
+// Connect selects an upstream, sends the request and waits for a response.
+func (p *Proxy) Connect(ctx context.Context, state request.Request, opts Options) (*dns.Msg, error) {
+	start := time.Now()
+
+	proto := ""
+	switch {
+	case opts.ForceTCP: // TCP flag has precedence over UDP flag
+		proto = "tcp"
+	case opts.PreferUDP:
+		proto = "udp"
+	default:
+		proto = state.Proto()
+	}
+
+	pc, cached, err := p.transport.Dial(proto)
+	if err != nil {
+		return nil, err
+	}
+
+	// Set buffer size correctly for this client.
+	pc.c.UDPSize = uint16(state.Size())
+	if pc.c.UDPSize < 512 {
+		pc.c.UDPSize = 512
+	}
+
+	pc.c.SetWriteDeadline(time.Now().Add(maxTimeout))
+	// records the origin Id before upstream.
+	originId := state.Req.Id
+	state.Req.Id = dns.Id()
+	defer func() {
+		state.Req.Id = originId
+	}()
+
+	if err := pc.c.WriteMsg(state.Req); err != nil {
+		pc.c.Close() // not giving it back
+		if err == io.EOF && cached {
+			return nil, ErrCachedClosed
+		}
+		return nil, err
+	}
+
+	var ret *dns.Msg
+	pc.c.SetReadDeadline(time.Now().Add(p.readTimeout))
+	for {
+		ret, err = pc.c.ReadMsg()
+		if err != nil {
+			pc.c.Close() // not giving it back
+			if err == io.EOF && cached {
+				return nil, ErrCachedClosed
+			}
+			// recovery the origin Id after upstream.
+			if ret != nil {
+				ret.Id = originId
+			}
+			return ret, err
+		}
+		// drop out-of-order responses
+		if state.Req.Id == ret.Id {
+			break
+		}
+	}
+	// recovery the origin Id after upstream.
+	ret.Id = originId
+
+	p.transport.Yield(pc)
+
+	rc, ok := dns.RcodeToString[ret.Rcode]
+	if !ok {
+		rc = strconv.Itoa(ret.Rcode)
+	}
+
+	RequestCount.WithLabelValues(p.addr).Add(1)
+	RcodeCount.WithLabelValues(rc, p.addr).Add(1)
+	RequestDuration.WithLabelValues(p.addr, rc).Observe(time.Since(start).Seconds())
+
+	return ret, nil
+}
+
+const cumulativeAvgWeight = 4