fmt: Scanf misses last character with io.EOF

[dsnet]

Commit 6520da6 caused a regression where the last character in a string would be dropped if it was accompanied by an io.EOF.

Consider the following test:

func main() {
    r := iotest.DataErrReader(strings.NewReader("hello"))
    var s string
    fmt.Fscanf(r, "%s", &s)
    fmt.Println(s)
}

On 1.6.2, this prints "hello".
On 1.7rc1, this prints "hell"

[gopherbot]

CL https://golang.org/cl/24981 mentions this issue.

[pi]

It is correct to return both data and EOF in one call? From io.EOF doc:
EOF is the error returned by Read when no more input is available. Functions should return EOF only to signal a graceful end of input. If the EOF occurs unexpectedly in a structured data stream, the appropriate error is either ErrUnexpectedEOF or some other error giving more detail.

[robpike]

Yes, it is correct to return data and EOF in one call, although the situation is unfamiliar to many and is often mishandled.

[pi]

Thanks for important clarification. I think that should be clearly stated in EOF's doc to reduce number of mishandles.

[ianlancetaylor]

CC @martisch