text/scanner: avoid further reads after EOF

Fixes #10735.

Change-Id: I5c6e424653657c89da176136ac56597c7565abe5
Reviewed-on: https://go-review.googlesource.com/10039Reviewed-by: Rob Pike <r@golang.org>

src/text/scanner/scanner.go

@@ -314,7 +314,9 @@ func (s *Scanner) Next() rune {
 	s.tokPos = -1 // don't collect token text
 	s.Line = 0    // invalidate token position
 	ch := s.Peek()
+	if ch != EOF {
 		s.ch = s.next()
+	}
 	return ch
 }
 
@@ -597,6 +599,8 @@ redo:
 		}
 	default:
 		switch ch {
+		case EOF:
+			break
 		case '"':
 			if s.Mode&ScanStrings != 0 {
 				s.scanString('"')

src/text/scanner/scanner_test.go

@@ -619,13 +619,12 @@ func TestPos(t *testing.T) {
 
 type countReader int
 
-func (c *countReader) Read([]byte) (int, error) {
-	*c++
-
+func (r *countReader) Read([]byte) (int, error) {
+	*r++
 	return 0, io.EOF
 }
 
-func TestPeekEOFHandling(t *testing.T) {
+func TestNextEOFHandling(t *testing.T) {
 	var r countReader
 
 	// corner case: empty source
@@ -633,15 +632,36 @@ func TestPeekEOFHandling(t *testing.T) {
 
 	tok := s.Next()
 	if tok != EOF {
-		t.Errorf("EOF not reported")
+		t.Error("1) EOF not reported")
+	}
+
+	tok = s.Peek()
+	if tok != EOF {
+		t.Error("2) EOF not reported")
+	}
+
+	if r != 1 {
+		t.Errorf("scanner called Read %d times, not once", r)
+	}
+}
+
+func TestScanEOFHandling(t *testing.T) {
+	var r countReader
+
+	// corner case: empty source
+	s := new(Scanner).Init(&r)
+
+	tok := s.Scan()
+	if tok != EOF {
+		t.Error("1) EOF not reported")
 	}
 
 	tok = s.Peek()
 	if tok != EOF {
-		t.Errorf("EOF not reported")
+		t.Error("2) EOF not reported")
 	}
 
-	if r != 2 {
-		t.Errorf("scanner called Read %d times, not twice", r)
+	if r != 1 {
+		t.Errorf("scanner called Read %d times, not once", r)
 	}
 }