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: dir.go
    path of file: ./repos/client/go/kbfs/libfuse
    
    the code of the file until where you have to start completion: // Copyright 2016 Keybase Inc. All rights reserved.
// Use of this source code is governed by a BSD
// license that can be found in the LICENSE file.
//
//go:build !windows
// +build !windows

package libfuse

import (
	"fmt"
	"math"
	"os"
	"strings"
	"sync"
	"syscall"
	"time"

	"bazil.org/fuse"
	"bazil.org/fuse/fs"
	"github.com/keybase/client/go/kbfs/data"
	"github.com/keybase/client/go/kbfs/idutil"
	"github.com/keybase/client/go/kbfs/libcontext"
	"github.com/keybase/client/go/kbfs/libfs"
	"github.com/keybase/client/go/kbfs/libkbfs"
	"github.com/keybase/client/go/kbfs/sysutils"
	"github.com/keybase/client/go/kbfs/tlf"
	"github.com/keybase/client/go/kbfs/tlfhandle"
	"github.com/keybase/client/go/libkb"
	"golang.org/x/net/context"
)

// Folder represents the info shared among all nodes of a KBFS
// top-level folder.
type Folder struct {
	fs   *FS
	list *FolderList

	handleMu       sync.RWMutex
	h              *tlfhandle.Handle
	hPreferredName tlf.PreferredName

	folderBranchMu sync.Mutex
	folderBranch   data.FolderBranch

	// Protects the nodes map.
	nodesMu sync.Mutex
	// Map KBFS nodes to FUSE nodes, to be able to handle multi
    
func (d *Dir) Lookup(ctx context.Context, req *fuse.LookupRequest, resp *fuse.LookupResponse) (node fs.Node, err error) {
	namePPS := d.node.ChildName(req.Name)
	ctx = d.folder.fs.config.MaybeStartTrace(ctx, "Dir.Lookup",
		fmt.Sprintf("%s %s", d.node.GetBasename(), namePPS))
	defer func() { d.folder.fs.config.MaybeFinishTrace(ctx, err) }()

	d.folder.fs.vlog.CLogf(ctx, libkb.VLog1, "Dir Lookup %s", namePPS)
	defer func() { err = d.folder.processError(ctx, libkbfs.ReadMode, err) }()

	// This fits in situation 1 as described in libkbfs/delayed_cancellation.go
	err = libcontext.EnableDelayedCancellationWithGracePeriod(
		ctx, d.folder.fs.config.DelayedCancellationGracePeriod())
	if err != nil {
		return nil, err
	}

	specialNode := handleTLFSpecialFile(
		req.Name, d.folder, &resp.EntryValid)
	if specialNode != nil {
		return specialNode, nil
	}

	// Check if this is a per-file metainformation file, if so
	// return the corresponding SpecialReadFile.
	if strings.HasPrefix(req.Name, libfs.FileInfoPrefix) {
		name := req.Name[len(libfs.FileInfoPrefix):]
		return NewFileInfoFile(d.folder.fs, d.node, name, &resp.EntryValid), nil
	}

	newNode, de, err := d.folder.fs.config.KBFSOps().Lookup(
		ctx, d.node, namePPS)
	if err != nil {
		if _, ok := err.(idutil.NoSuchNameError); ok {
			return nil, fuse.ENOENT
		}
		return nil, err
	}

	// No libkbfs calls after this point!
	d.folder.nodesMu.Lock()
	defer d.folder.nodesMu.Unlock()

	// newNode can be nil even without errors when the KBFS direntry
	// is of a type that doesn't get its own node (is fully contained
	// in the directory); Symlink does this.
	if newNode != nil {
		if n, ok := d.folder.nodes[newNode.GetID()]; ok {
			return n, nil
		}

		newNode.FillCacheDuration(&resp.EntryValid)
	}

	switch de.Type {
	default:
		return nil, fmt.Errorf("unhandled entry type: %v", de.Type)

	case data.File, data.Exec:
		child := d.makeFile(newNode)
		d.folder.nodes[newNode.GetID()] = child
		return child, nil

	case data.Dir:
		child := newDir(d.folder, newNode)
		d.folder.nodes[newNode.GetID()] = child
		return child, nil

	case data.Sym:
		// Give each symlink instance a unique inode.  We don't get
		// enough information about remote renames of syminks to be
		// able to attach a constant inode to a given symlink.
		child := &Symlink{
			parent: d,
			name:   req.Name,
			inode:  d.folder.fs.assignInode(),
		}
		// A Symlink is never included in Folder.nodes, as it doesn't
		// have a libkbfs.Node to keep track of renames.
		return child, nil
	}
}