Partial Line returned from Reader.ReadLine()

[jason-baker]

What version of Go are you using (go version)?

$ go version
go version go1.10.2 windows/amd64

Does this issue reproduce with the latest release?

Yes

$ go version
go version go1.11.2 windows/amd64

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\\AppData\Local\go-build
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\\go
set GORACE=
set GOROOT=C:\Go
set GOTMPDIR=
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\\AppData\Local\Temp\go-build034252242=/tmp/go-build -gno-record-gcc-switches

What did you do?

func main() {
	reader := bufio.NewReaderSize(os.Stdin, 1024*1024)

	qTemp, err := strconv.ParseInt(readLine(reader), 10, 64)
	checkError(err)
	q := int32(qTemp)

	for qItr := 0; qItr < int(q); qItr++ {
		nm := strings.Split(readLine(reader), " ")

		nTemp, err := strconv.ParseInt(nm[0], 10, 64)
		checkError(err)
		n := int32(nTemp)

		m, err := strconv.ParseInt(nm[1], 10, 64)
		checkError(err)

		println("n: ", n, "m: ", m)

		aTemp := strings.Split(readLine(reader), " ")
		println(len(aTemp))
		if int32(len(aTemp)) != n {
			println(fmt.Sprintf("%v", strings.Split(readLine(reader), " ")))
		}

		var a []int64

		for i := 0; i < int(n); i++ {
			aItem, err := strconv.ParseInt(aTemp[i], 10, 64)
			checkError(err)
			a = append(a, aItem)
		}

		result := maximumSum(a, m)

		println(fmt.Sprintf("%d\n", result))
	}
}

func readLine(reader *bufio.Reader) string {
	str, _, err := reader.ReadLine()
	println(err)
	if err == io.EOF {
		return ""
	}

	return strings.TrimRight(string(str), "\r\n")
}

test.txt

cat test.txt | go run test.go

What did you expect to see?

All bytes returned on one line

What did you see instead?

cat test.txt | go run test.go
n: 100000 m: 10002143548612
100000
n: 100000 m: 10000418366442
100000
n: 100000 m: 10001238194497
100000
n: 100000 m: 10000855912503
99993
panic: Unexpeced return from readline

goroutine 1 [running]:
main.main()
C:/Users//go/src/test.go:33 +0x51b
exit status 2

[ianlancetaylor]

Please read the docs for Reader.ReadLine (https://golang.org/pkg/bufio/#Reader.ReadLine). They are clear that ReadLine does not always return a complete line. You need to either check the isPrefix result, which you are currently ignoring, or use ReadBytes.

[jason-baker]

Mea culpa; you are correct this is a non-issue.