Allow to set daemon and server configurations in a file.

Read configuration after flags making this the priority:

1- Apply configuration from file.
2- Apply configuration from flags.

Reload configuration when a signal is received, USR2 in Linux:

- Reload router if the debug configuration changes.
- Reload daemon labels.
- Reload cluster discovery.

Signed-off-by: David Calavera <david.calavera@gmail.com>

daemon/daemon_test.go

@@ -4,9 +4,13 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"reflect"
 	"testing"
+	"time"
 
 	"github.com/docker/docker/container"
+	"github.com/docker/docker/pkg/discovery"
+	_ "github.com/docker/docker/pkg/discovery/memory"
 	"github.com/docker/docker/pkg/registrar"
 	"github.com/docker/docker/pkg/truncindex"
 	"github.com/docker/docker/volume"
@@ -371,3 +375,118 @@ func TestMerge(t *testing.T) {
 		}
 	}
 }
+
+func TestDaemonReloadLabels(t *testing.T) {
+	daemon := &Daemon{}
+	daemon.configStore = &Config{
+		CommonConfig: CommonConfig{
+			Labels: []string{"foo:bar"},
+		},
+	}
+
+	newConfig := &Config{
+		CommonConfig: CommonConfig{
+			Labels: []string{"foo:baz"},
+		},
+	}
+
+	daemon.Reload(newConfig)
+	label := daemon.configStore.Labels[0]
+	if label != "foo:baz" {
+		t.Fatalf("Expected daemon label `foo:baz`, got %s", label)
+	}
+}
+
+func TestDaemonDiscoveryReload(t *testing.T) {
+	daemon := &Daemon{}
+	daemon.configStore = &Config{
+		CommonConfig: CommonConfig{
+			ClusterStore:     "memory://127.0.0.1",
+			ClusterAdvertise: "127.0.0.1:3333",
+		},
+	}
+
+	if err := daemon.initDiscovery(daemon.configStore); err != nil {
+		t.Fatal(err)
+	}
+
+	expected := discovery.Entries{
+		&discovery.Entry{Host: "127.0.0.1", Port: "3333"},
+	}
+
+	stopCh := make(chan struct{})
+	defer close(stopCh)
+	ch, errCh := daemon.discoveryWatcher.Watch(stopCh)
+
+	select {
+	case <-time.After(1 * time.Second):
+		t.Fatal("failed to get discovery advertisements in time")
+	case e := <-ch:
+		if !reflect.DeepEqual(e, expected) {
+			t.Fatalf("expected %v, got %v\n", expected, e)
+		}
+	case e := <-errCh:
+		t.Fatal(e)
+	}
+
+	newConfig := &Config{
+		CommonConfig: CommonConfig{
+			ClusterStore:     "memory://127.0.0.1:2222",
+			ClusterAdvertise: "127.0.0.1:5555",
+		},
+	}
+
+	expected = discovery.Entries{
+		&discovery.Entry{Host: "127.0.0.1", Port: "5555"},
+	}
+
+	if err := daemon.Reload(newConfig); err != nil {
+		t.Fatal(err)
+	}
+	ch, errCh = daemon.discoveryWatcher.Watch(stopCh)
+
+	select {
+	case <-time.After(1 * time.Second):
+		t.Fatal("failed to get discovery advertisements in time")
+	case e := <-ch:
+		if !reflect.DeepEqual(e, expected) {
+			t.Fatalf("expected %v, got %v\n", expected, e)
+		}
+	case e := <-errCh:
+		t.Fatal(e)
+	}
+}
+
+func TestDaemonDiscoveryReloadFromEmptyDiscovery(t *testing.T) {
+	daemon := &Daemon{}
+	daemon.configStore = &Config{}
+
+	newConfig := &Config{
+		CommonConfig: CommonConfig{
+			ClusterStore:     "memory://127.0.0.1:2222",
+			ClusterAdvertise: "127.0.0.1:5555",
+		},
+	}
+
+	expected := discovery.Entries{
+		&discovery.Entry{Host: "127.0.0.1", Port: "5555"},
+	}
+
+	if err := daemon.Reload(newConfig); err != nil {
+		t.Fatal(err)
+	}
+	stopCh := make(chan struct{})
+	defer close(stopCh)
+	ch, errCh := daemon.discoveryWatcher.Watch(stopCh)
+
+	select {
+	case <-time.After(1 * time.Second):
+		t.Fatal("failed to get discovery advertisements in time")
+	case e := <-ch:
+		if !reflect.DeepEqual(e, expected) {
+			t.Fatalf("expected %v, got %v\n", expected, e)
+		}
+	case e := <-errCh:
+		t.Fatal(e)
+	}
+}