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: resolver.go
    path of file: ./repos/cockroach/pkg/util/caller
    
    the code of the file until where you have to start completion: // Copyright 2015 The Cockroach Authors.
//
// Use of this software is governed by the Business Source License
// included in the file licenses/BSL.txt.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0, included in the file
// licenses/APL.txt.

package caller

import (
	"path/filepath"
	"runtime"
	"strings"
	"sync"

	"github.com/cockroachdb/cockroach/pkg/util/syncutil"
)

type cachedLookup struct {
	pkg  string // <logical package name>/<file>.go
	file string // <file>.go
	line int
	fun  string
}

var dummyLookup =
    
func (cr *CallResolver) Lookup(depth int) (_logicalPath string, _line int, _fun string) {
	sl := uintptrSlPool.Get().(*[]uintptr)
	// NB: +2 for Callers, +1 for Caller (historical reasons)
	ok := runtime.Callers(depth+2, *sl) == 1
	pc := (*sl)[0]
	uintptrSlPool.Put(sl)
	sl = nil // prevent reuse
	if !ok || cr == nil {
		return dummyLookup.pkg, dummyLookup.line, dummyLookup.fun
	}

	cr.mu.Lock()
	defer cr.mu.Unlock()
	if v, okCache := cr.cache[pc]; okCache {
		return v.file, v.line, v.fun
	}
	// Now do the expensive thing which we intend to cache.
	frame, _ := runtime.CallersFrames([]uintptr{pc}).Next()

	pkg, fun := parseFQFun(frame.Function)

	cl := &cachedLookup{
		pkg:  pkg,
		file: filepath.Join(pkg, filepath.Base(frame.File)),
		line: frame.Line,
		fun:  fun,
	}
	cr.cache[pc] = cl
	return cl.file, cl.line, cl.fun
}