cmd/gofmt: return a proper error for empty Go files

I was testing edge cases in gofumpt, a fork of gofmt,
and noticed that gofmt will return a bare io error on empty files,
as demonstrated by the added test case without a fix:

        > ! exec $GOROOT/bin/gofmt empty.go nopackage.go
        [stderr]
        EOF
        nopackage.go:1:1: expected 'package', found not

The problem is the code that detects concurrent modifications.
It relies on ReadFull and correctly deals with io.ErrUnexpectedEOF,
but it did not pay attention to io.EOF, which can happen when size==0.

Change-Id: I6092391721edad4584fb5922d3e3a8fb3da86493
Reviewed-on: https://go-review.googlesource.com/c/go/+/393757
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Trust: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/testdata/script/fmt_load_errors.txt

@@ -16,6 +16,10 @@ stdout 'package x'
 exec $GOROOT/bin/gofmt gofmt-dir
 ! stdout 'package x'
 
+! exec $GOROOT/bin/gofmt empty.go nopackage.go
+stderr -count=1 'empty\.go:1:1: expected .package., found .EOF.'
+stderr -count=1 'nopackage\.go:1:1: expected .package., found not'
+
 -- exclude/empty/x.txt --
 -- exclude/ignore/_x.go --
 package x
@@ -29,3 +33,6 @@ package x
 package x
 -- gofmt-dir/no-extension --
 package x
+-- empty.go --
+-- nopackage.go --
+not the proper start to a Go file

src/cmd/gofmt/gofmt.go

@@ -347,7 +347,12 @@ func readFile(filename string, info fs.FileInfo, in io.Reader) ([]byte, error) {
 	// stop to avoid corrupting it.)
 	src := make([]byte, size+1)
 	n, err := io.ReadFull(in, src)
-	if err != nil && err != io.ErrUnexpectedEOF {
+	switch err {
+	case nil, io.EOF, io.ErrUnexpectedEOF:
+		// io.ReadFull returns io.EOF (for an empty file) or io.ErrUnexpectedEOF
+		// (for a non-empty file) if the file was changed unexpectedly. Continue
+		// with comparing file sizes in those cases.
+	default:
 		return nil, err
 	}
 	if n < size {