bufio: fix reading of many blank lines in a row

Fixes #9020.

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/170030043

src/bufio/scan.go

@@ -128,9 +128,10 @@ func (s *Scanner) Scan() bool {
 			}
 			s.token = token
 			if token != nil {
-				if len(token) > 0 {
+				if s.err == nil || advance > 0 {
 					s.empties = 0
 				} else {
+					// Returning tokens not advancing input at EOF.
 					s.empties++
 					if s.empties > 100 {
 						panic("bufio.Scan: 100 empty tokens without progressing")

src/bufio/scan_test.go

@@ -489,6 +489,18 @@ func TestDontLoopForever(t *testing.T) {
 	}
 }
 
+func TestBlankLines(t *testing.T) {
+	s := NewScanner(strings.NewReader(strings.Repeat("\n", 1000)))
+	for count := 0; s.Scan(); count++ {
+		if count > 2000 {
+			t.Fatal("looping")
+		}
+	}
+	if s.Err() != nil {
+		t.Fatal("after scan:", s.Err())
+	}
+}
+
 type countdown int
 
 func (c *countdown) split(data []byte, atEOF bool) (advance int, token []byte, err error) {