middleware/proxy: add read/writeDeadline (#477)

Add deadline to break the connection. We use the default of 5 seconds.
After this the backend is marked unhealthy and not used for some time.

Fixes #467

test/proxy_health_test.go

@@ -0,0 +1,43 @@
+package test
+
+import (
+	"io/ioutil"
+	"log"
+	"testing"
+
+	"github.com/miekg/coredns/middleware/proxy"
+	"github.com/miekg/coredns/middleware/test"
+	"github.com/miekg/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+func TestProxyErratic(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+
+	corefile := `example.org:0 {
+		erratic {
+			drop 2
+		}
+	}
+`
+
+	backend, err := CoreDNSServer(corefile)
+	if err != nil {
+		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
+	}
+
+	udp, _ := CoreDNSServerPorts(backend, 0)
+	if udp == "" {
+		t.Fatalf("Could not get UDP listening port")
+	}
+	defer backend.Stop()
+
+	p := proxy.New([]string{udp})
+	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
+
+	// We do one lookup that should not time out.
+	// After this the backend is marked unhealthy anyway. So basically this
+	// tests that it times out.
+	p.Lookup(state, "example.org.", dns.TypeA)
+}