diff --git a/vendor/google.golang.org/grpc/picker_wrapper.go b/vendor/google.golang.org/grpc/picker_wrapper.go
new file mode 100644
index 0000000..14f915d
--- /dev/null
+++ b/vendor/google.golang.org/grpc/picker_wrapper.go
@@ -0,0 +1,180 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package grpc
+
+import (
+	"context"
+	"io"
+	"sync"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/internal/channelz"
+	"google.golang.org/grpc/internal/transport"
+	"google.golang.org/grpc/status"
+)
+
+// pickerWrapper is a wrapper of balancer.Picker. It blocks on certain pick
+// actions and unblock when there's a picker update.
+type pickerWrapper struct {
+	mu         sync.Mutex
+	done       bool
+	blockingCh chan struct{}
+	picker     balancer.Picker
+
+	// The latest connection happened.
+	connErrMu sync.Mutex
+	connErr   error
+}
+
+func newPickerWrapper() *pickerWrapper {
+	bp := &pickerWrapper{blockingCh: make(chan struct{})}
+	return bp
+}
+
+func (bp *pickerWrapper) updateConnectionError(err error) {
+	bp.connErrMu.Lock()
+	bp.connErr = err
+	bp.connErrMu.Unlock()
+}
+
+func (bp *pickerWrapper) connectionError() error {
+	bp.connErrMu.Lock()
+	err := bp.connErr
+	bp.connErrMu.Unlock()
+	return err
+}
+
+// updatePicker is called by UpdateBalancerState. It unblocks all blocked pick.
+func (bp *pickerWrapper) updatePicker(p balancer.Picker) {
+	bp.mu.Lock()
+	if bp.done {
+		bp.mu.Unlock()
+		return
+	}
+	bp.picker = p
+	// bp.blockingCh should never be nil.
+	close(bp.blockingCh)
+	bp.blockingCh = make(chan struct{})
+	bp.mu.Unlock()
+}
+
+func doneChannelzWrapper(acw *acBalancerWrapper, done func(balancer.DoneInfo)) func(balancer.DoneInfo) {
+	acw.mu.Lock()
+	ac := acw.ac
+	acw.mu.Unlock()
+	ac.incrCallsStarted()
+	return func(b balancer.DoneInfo) {
+		if b.Err != nil && b.Err != io.EOF {
+			ac.incrCallsFailed()
+		} else {
+			ac.incrCallsSucceeded()
+		}
+		if done != nil {
+			done(b)
+		}
+	}
+}
+
+// pick returns the transport that will be used for the RPC.
+// It may block in the following cases:
+// - there's no picker
+// - the current picker returns ErrNoSubConnAvailable
+// - the current picker returns other errors and failfast is false.
+// - the subConn returned by the current picker is not READY
+// When one of these situations happens, pick blocks until the picker gets updated.
+func (bp *pickerWrapper) pick(ctx context.Context, failfast bool, opts balancer.PickOptions) (transport.ClientTransport, func(balancer.DoneInfo), error) {
+	var (
+		p  balancer.Picker
+		ch chan struct{}
+	)
+
+	for {
+		bp.mu.Lock()
+		if bp.done {
+			bp.mu.Unlock()
+			return nil, nil, ErrClientConnClosing
+		}
+
+		if bp.picker == nil {
+			ch = bp.blockingCh
+		}
+		if ch == bp.blockingCh {
+			// This could happen when either:
+			// - bp.picker is nil (the previous if condition), or
+			// - has called pick on the current picker.
+			bp.mu.Unlock()
+			select {
+			case <-ctx.Done():
+				return nil, nil, ctx.Err()
+			case <-ch:
+			}
+			continue
+		}
+
+		ch = bp.blockingCh
+		p = bp.picker
+		bp.mu.Unlock()
+
+		subConn, done, err := p.Pick(ctx, opts)
+
+		if err != nil {
+			switch err {
+			case balancer.ErrNoSubConnAvailable:
+				continue
+			case balancer.ErrTransientFailure:
+				if !failfast {
+					continue
+				}
+				return nil, nil, status.Errorf(codes.Unavailable, "%v, latest connection error: %v", err, bp.connectionError())
+			default:
+				// err is some other error.
+				return nil, nil, toRPCErr(err)
+			}
+		}
+
+		acw, ok := subConn.(*acBalancerWrapper)
+		if !ok {
+			grpclog.Infof("subconn returned from pick is not *acBalancerWrapper")
+			continue
+		}
+		if t, ok := acw.getAddrConn().getReadyTransport(); ok {
+			if channelz.IsOn() {
+				return t, doneChannelzWrapper(acw, done), nil
+			}
+			return t, done, nil
+		}
+		grpclog.Infof("blockingPicker: the picked transport is not ready, loop back to repick")
+		// If ok == false, ac.state is not READY.
+		// A valid picker always returns READY subConn. This means the state of ac
+		// just changed, and picker will be updated shortly.
+		// continue back to the beginning of the for loop to repick.
+	}
+}
+
+func (bp *pickerWrapper) close() {
+	bp.mu.Lock()
+	defer bp.mu.Unlock()
+	if bp.done {
+		return
+	}
+	bp.done = true
+	close(bp.blockingCh)
+}