You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: picker_wrapper.go
    path of file: ./repos/octant/vendor/google.golang.org/grpc
    
    the code of the file until where you have to start completion: /*
 *
 * 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/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
}

func newPickerWrapper() *pickerWrapper {
	return &pickerWrapper{blockingCh: make(chan struct{})}
}

// updatePicker is called by UpdateBalancerState. It unblocks all blo
    
func (pw *pickerWrapper) pick(ctx context.Context, failfast bool, info balancer.PickInfo) (transport.ClientTransport, func(balancer.DoneInfo), error) {
	var ch chan struct{}

	var lastPickErr error
	for {
		pw.mu.Lock()
		if pw.done {
			pw.mu.Unlock()
			return nil, nil, ErrClientConnClosing
		}

		if pw.picker == nil {
			ch = pw.blockingCh
		}
		if ch == pw.blockingCh {
			// This could happen when either:
			// - pw.picker is nil (the previous if condition), or
			// - has called pick on the current picker.
			pw.mu.Unlock()
			select {
			case <-ctx.Done():
				var errStr string
				if lastPickErr != nil {
					errStr = "latest balancer error: " + lastPickErr.Error()
				} else {
					errStr = ctx.Err().Error()
				}
				switch ctx.Err() {
				case context.DeadlineExceeded:
					return nil, nil, status.Error(codes.DeadlineExceeded, errStr)
				case context.Canceled:
					return nil, nil, status.Error(codes.Canceled, errStr)
				}
			case <-ch:
			}
			continue
		}

		ch = pw.blockingCh
		p := pw.picker
		pw.mu.Unlock()

		pickResult, err := p.Pick(info)

		if err != nil {
			if err == balancer.ErrNoSubConnAvailable {
				continue
			}
			if _, ok := status.FromError(err); ok {
				// Status error: end the RPC unconditionally with this status.
				return nil, nil, err
			}
			// For all other errors, wait for ready RPCs should block and other
			// RPCs should fail with unavailable.
			if !failfast {
				lastPickErr = err
				continue
			}
			return nil, nil, status.Error(codes.Unavailable, err.Error())
		}

		acw, ok := pickResult.SubConn.(*acBalancerWrapper)
		if !ok {
			logger.Errorf("subconn returned from pick is type %T, not *acBalancerWrapper", pickResult.SubConn)
			continue
		}
		if t := acw.getAddrConn().getReadyTransport(); t != nil {
			if channelz.IsOn() {
				return t, doneChannelzWrapper(acw, pickResult.Done), nil
			}
			return t, pickResult.Done, nil
		}
		if pickResult.Done != nil {
			// Calling done with nil error, no bytes sent and no bytes received.
			// DoneInfo with default value works.
			pickResult.Done(balancer.DoneInfo{})
		}
		logger.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.
	}
}