From 2a0fdf6ea05dc31526e95990aadc2b327933cce1 Mon Sep 17 00:00:00 2001
From: Rob Pike <r@golang.org>
Date: Mon, 11 Jun 2012 17:52:09 -0400
Subject: [PATCH] fmt.Fscanf: don't read past newline Makes interactive uses
 work line-by-line. Fixes #3481.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/6297075
---
 src/pkg/fmt/scan.go      |  3 ++-
 src/pkg/fmt/scan_test.go | 27 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/pkg/fmt/scan.go b/src/pkg/fmt/scan.go
index 0b3e04069a..d69911c7d7 100644
--- a/src/pkg/fmt/scan.go
+++ b/src/pkg/fmt/scan.go
@@ -1090,7 +1090,8 @@ func (s *ss) advance(format string) (i int) {
 			// There was space in the format, so there should be space (EOF)
 			// in the input.
 			inputc := s.getRune()
-			if inputc == eof {
+			if inputc == eof || inputc == '\n' {
+				// If we've reached a newline, stop now; don't read ahead.
 				return
 			}
 			if !isSpace(inputc) {
diff --git a/src/pkg/fmt/scan_test.go b/src/pkg/fmt/scan_test.go
index 320857b73e..cc09e910aa 100644
--- a/src/pkg/fmt/scan_test.go
+++ b/src/pkg/fmt/scan_test.go
@@ -810,6 +810,33 @@ func TestMultiLine(t *testing.T) {
 	}
 }
 
+// simpleReader is a strings.Reader that implements only Read, not ReadRune.
+// Good for testing readahead.
+type simpleReader struct {
+	sr *strings.Reader
+}
+
+func (s *simpleReader) Read(b []byte) (n int, err error) {
+	return s.sr.Read(b)
+}
+
+// Test that Fscanf does not read past newline. Issue 3481.
+func TestLineByLineFscanf(t *testing.T) {
+	r := &simpleReader{strings.NewReader("1\n2\n")}
+	var i, j int
+	n, err := Fscanf(r, "%v\n", &i)
+	if n != 1 || err != nil {
+		t.Fatalf("first read: %d %q", n, err)
+	}
+	n, err = Fscanf(r, "%v\n", &j)
+	if n != 1 || err != nil {
+		t.Fatalf("second read: %d %q", n, err)
+	}
+	if i != 1 || j != 2 {
+		t.Errorf("wrong values; wanted 1 2 got %d %d", i, j)
+	}
+}
+
 // RecursiveInt accepts a string matching %d.%d.%d....
 // and parses it into a linked list.
 // It allows us to benchmark recursive descent style scanners.
-- 
2.48.1