middleware/proxy: make it scale (#287)

* middleware/proxy

Use connection pooling for communicating with an upstream, instead of
opening a new socket every time.

This makes the proxy more efficient and allowed for some cleanups.

* Some cleanups

* Some fixes

* more

* Kill pool

* Add nil check

* remove pool

middleware/proxy/client.go

@@ -0,0 +1,97 @@
+package proxy
+
+import (
+	"net"
+	"time"
+
+	"github.com/miekg/coredns/middleware/pkg/singleflight"
+	"github.com/miekg/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+type Client struct {
+	Timeout time.Duration
+
+	group *singleflight.Group
+}
+
+func NewClient() *Client {
+	return &Client{Timeout: defaultTimeout, group: new(singleflight.Group)}
+}
+
+// ServeDNS does not satisfy middleware.Handler, instead it interacts with the upstream
+// and returns the respons or an error.
+func (c *Client) ServeDNS(w dns.ResponseWriter, r *dns.Msg, u *UpstreamHost) (*dns.Msg, error) {
+	co, err := net.DialTimeout(request.Proto(w), u.Name, c.Timeout)
+	if err != nil {
+		return nil, err
+	}
+
+	reply, _, err := c.Exchange(r, co)
+
+	co.Close()
+
+	if reply != nil && reply.Truncated {
+		// Suppress proxy error for truncated responses
+		err = nil
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	reply.Compress = true
+	reply.Id = r.Id
+
+	return reply, nil
+}
+
+func (c *Client) Exchange(m *dns.Msg, co net.Conn) (*dns.Msg, time.Duration, error) {
+	t := "nop"
+	if t1, ok := dns.TypeToString[m.Question[0].Qtype]; ok {
+		t = t1
+	}
+	cl := "nop"
+	if cl1, ok := dns.ClassToString[m.Question[0].Qclass]; ok {
+		cl = cl1
+	}
+
+	start := time.Now()
+
+	// Name needs to be normalized! Bug in go dns.
+	r, err := c.group.Do(m.Question[0].Name+t+cl, func() (interface{}, error) {
+		ret, e := c.exchange(m, co)
+		return ret, e
+	})
+
+	rtt := time.Since(start)
+	if err != nil {
+		return &dns.Msg{}, rtt, err
+	}
+
+	r1 := r.(dns.Msg)
+	return &r1, rtt, nil
+}
+
+// exchange does *not* return a pointer to dns.Msg because that leads to buffer reuse when
+// group.Do is used in Exchange.
+func (c *Client) exchange(m *dns.Msg, co net.Conn) (dns.Msg, error) {
+	opt := m.IsEdns0()
+
+	udpsize := uint16(dns.MinMsgSize)
+	// If EDNS0 is used use that for size.
+	if opt != nil && opt.UDPSize() >= dns.MinMsgSize {
+		udpsize = opt.UDPSize()
+	}
+
+	dnsco := &dns.Conn{Conn: co, UDPSize: udpsize}
+
+	dnsco.WriteMsg(m)
+	r, err := dnsco.ReadMsg()
+	dnsco.Close()
+	if r == nil {
+		return dns.Msg{}, err
+	}
+	return *r, err
+}