[aggregator] Add ActivePlacement method to TCP client (#2971)

src/aggregator/client/tcp_client.go

@@ -222,6 +222,23 @@ func (c *TCPClient) WriteForwarded(
 	return c.write(metric.ID, metric.TimeNanos, payload)
 }
 
+// ActivePlacement returns a copy of the currently active placement and its version.
+func (c *TCPClient) ActivePlacement() (placement.Placement, int, error) {
+	stagedPlacement, onStagedPlacementDoneFn, err := c.placementWatcher.ActiveStagedPlacement()
+	if err != nil {
+		return nil, 0, err
+	}
+	defer onStagedPlacementDoneFn()
+
+	placement, onPlacementDoneFn, err := stagedPlacement.ActivePlacement()
+	if err != nil {
+		return nil, 0, err
+	}
+	defer onPlacementDoneFn()
+
+	return placement.Clone(), stagedPlacement.Version(), nil
+}
+
 // Flush flushes any remaining data buffered by the client.
 func (c *TCPClient) Flush() error {
 	c.metrics.flush.Inc(1)

src/aggregator/client/tcp_client_test.go

@@ -29,6 +29,7 @@ import (
 	"time"
 
 	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"github.com/m3db/m3/src/cluster/kv/mem"
@@ -786,6 +787,30 @@ func TestTCPClientWriteTimeRangeFor(t *testing.T) {
 	}
 }
 
+func TestTCPClientActivePlacement(t *testing.T) {
+	var (
+		c               = mustNewTestTCPClient(t, testOptions())
+		emptyPl         = placement.NewPlacement()
+		ctrl            = gomock.NewController(t)
+		mockPl          = placement.NewMockPlacement(ctrl)
+		stagedPlacement = placement.NewMockActiveStagedPlacement(ctrl)
+		watcher         = placement.NewMockStagedPlacementWatcher(ctrl)
+		doneCalls       int
+	)
+
+	c.placementWatcher = watcher
+	watcher.EXPECT().ActiveStagedPlacement().Return(stagedPlacement, func() { doneCalls++ }, nil)
+	stagedPlacement.EXPECT().Version().Return(42)
+	stagedPlacement.EXPECT().ActivePlacement().Return(mockPl, func() { doneCalls++ }, nil)
+	mockPl.EXPECT().Clone().Return(emptyPl)
+
+	pl, v, err := c.ActivePlacement()
+	assert.NoError(t, err)
+	assert.Equal(t, 42, v)
+	assert.Equal(t, 2, doneCalls)
+	assert.Equal(t, emptyPl, pl)
+}
+
 func TestTCPClientInitAndClose(t *testing.T) {
 	c := mustNewTestTCPClient(t, testOptions())
 	require.NoError(t, c.Init())