Add support to gracefully shutdown the server

This is done by draining the connections for configured time after registry receives a SIGTERM signal.
This adds a `draintimeout` setting under `HTTP`. Registry doesn't drain
if draintimeout is not provided.

Signed-off-by: Manish Tomar <manish.tomar@docker.com>

registry/registry_test.go

@@ -1,10 +1,19 @@
 package registry
 
 import (
+	"bufio"
+	"context"
+	"fmt"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"os"
 	"reflect"
 	"testing"
+	"time"
 
 	"github.com/docker/distribution/configuration"
+	_ "github.com/docker/distribution/registry/storage/driver/inmemory"
 )
 
 // Tests to ensure nextProtos returns the correct protocols when:
@@ -28,3 +37,64 @@ func TestNextProtos(t *testing.T) {
 		t.Fatalf("expected protos to equal [http/1.1], got %s", protos)
 	}
 }
+
+func setupRegistry() (*Registry, error) {
+	config := &configuration.Configuration{}
+	// TODO: this needs to change to something ephemeral as the test will fail if there is any server
+	// already listening on port 5000
+	config.HTTP.Addr = ":5000"
+	config.HTTP.DrainTimeout = time.Duration(10) * time.Second
+	config.Storage = map[string]configuration.Parameters{"inmemory": map[string]interface{}{}}
+	return NewRegistry(context.Background(), config)
+}
+
+func TestGracefulShutdown(t *testing.T) {
+	registry, err := setupRegistry()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// run registry server
+	var errchan chan error
+	go func() {
+		errchan <- registry.ListenAndServe()
+	}()
+	select {
+	case err = <-errchan:
+		t.Fatalf("Error listening: %v", err)
+	default:
+	}
+
+	// Wait for some unknown random time for server to start listening
+	time.Sleep(3 * time.Second)
+
+	// send incomplete request
+	conn, err := net.Dial("tcp", "localhost:5000")
+	if err != nil {
+		t.Fatal(err)
+	}
+	fmt.Fprintf(conn, "GET /v2/ ")
+
+	// send stop signal
+	quit <- os.Interrupt
+	time.Sleep(100 * time.Millisecond)
+
+	// try connecting again. it shouldn't
+	_, err = net.Dial("tcp", "localhost:5000")
+	if err == nil {
+		t.Fatal("Managed to connect after stopping.")
+	}
+
+	// make sure earlier request is not disconnected and response can be received
+	fmt.Fprintf(conn, "HTTP/1.1\r\nHost: 127.0.0.1\r\n\r\n")
+	resp, err := http.ReadResponse(bufio.NewReader(conn), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.Status != "200 OK" {
+		t.Error("response status is not 200 OK: ", resp.Status)
+	}
+	if body, err := ioutil.ReadAll(resp.Body); err != nil || string(body) != "{}" {
+		t.Error("Body is not {}; ", string(body))
+	}
+}