text/scanner: handle non-io.EOF errors

Currently Scan ignores an error returned from source if the number
of bytes source has read is 0.

Fixes #7594.

LGTM=gri
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://golang.org/cl/78120043

src/pkg/text/scanner/scanner.go

@@ -240,6 +240,9 @@ func (s *Scanner) next() rune {
 			s.srcEnd = i + n
 			s.srcBuf[s.srcEnd] = utf8.RuneSelf // sentinel
 			if err != nil {
+				if err != io.EOF {
+					s.error(err.Error())
+				}
 				if s.srcEnd == 0 {
 					if s.lastCharLen > 0 {
 						// previous character was not EOF
@@ -248,9 +251,6 @@ func (s *Scanner) next() rune {
 					s.lastCharLen = 0
 					return EOF
 				}
-				if err != io.EOF {
-					s.error(err.Error())
-				}
 				// If err == EOF, we won't be getting more
 				// bytes; break to avoid infinite loop. If
 				// err is something else, we don't know if

src/pkg/text/scanner/scanner_test.go

@@ -462,6 +462,33 @@ func TestError(t *testing.T) {
 	testError(t, `/*/`, "1:4", "comment not terminated", EOF)
 }
 
+// An errReader returns (0, err) where err is not io.EOF.
+type errReader struct{}
+
+func (errReader) Read(b []byte) (int, error) {
+	return 0, io.ErrNoProgress // some error that is not io.EOF
+}
+
+func TestIOError(t *testing.T) {
+	s := new(Scanner).Init(errReader{})
+	errorCalled := false
+	s.Error = func(s *Scanner, msg string) {
+		if !errorCalled {
+			if want := io.ErrNoProgress.Error(); msg != want {
+				t.Errorf("msg = %q, want %q", msg, want)
+			}
+			errorCalled = true
+		}
+	}
+	tok := s.Scan()
+	if tok != EOF {
+		t.Errorf("tok = %s, want EOF", TokenString(tok))
+	}
+	if !errorCalled {
+		t.Errorf("error handler not called")
+	}
+}
+
 func checkPos(t *testing.T, got, want Position) {
 	if got.Offset != want.Offset || got.Line != want.Line || got.Column != want.Column {
 		t.Errorf("got offset, line, column = %d, %d, %d; want %d, %d, %d",