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: sourcereader.go
    path of file: ./repos/corteza/server/vendor/github.com/getsentry/sentry-go
    
    the code of the file until where you have to start completion: package sentry

import (
	"bytes"
	"io/ioutil"
	"sync"
)

type sourceReader struct {
	mu    sync.Mutex
	cache map[string][][]byte
}

func newSourceReader() sourceReader {
	return sourceReader{
		cache: make(map[string][][]byte),
	}
}

func (sr *sourceReader) readContextLines(filename
    
func (sr *sourceReader) readContextLines(filename string, line, context int) ([][]byte, int) {
	sr.mu.Lock()
	defer sr.mu.Unlock()

	lines, ok := sr.cache[filename]

	if !ok {
		data, err := ioutil.ReadFile(filename)
		if err != nil {
			sr.cache[filename] = nil
			return nil, 0
		}
		lines = bytes.Split(data, []byte{'\n'})
		sr.cache[filename] = lines
	}

	return sr.calculateContextLines(lines, line, context)
}