fmt: don't unread eof scanning %x

robpike · robpike · commit fe3d8d4db04d · 2015-08-10T03:34:48.000Z
When scanning a hex byte at EOF, the code was ungetting the eof, which backed up the input and caused double-scanning of a byte. Delete the call to UnreadRune. This line appeared in 1.5 for some reason; it was not in 1.4 and should be removed again for 1.5 Fixes golang#12090. Change-Id: Iad1ce8e7db8ec26615c5271310f4b0228cca7d78 Reviewed-on: https://go-review.googlesource.com/13461 Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/src/fmt/scan.go b/src/fmt/scan.go
@@ -888,7 +888,6 @@ func hexDigit(d rune) (int, bool) {
 func (s *ss) hexByte() (b byte, ok bool) {
 	rune1 := s.getRune()
 	if rune1 == eof {
-		s.UnreadRune()
 		return
 	}
 	value1, ok := hexDigit(rune1)
diff --git a/src/fmt/scan_test.go b/src/fmt/scan_test.go
@@ -1128,3 +1128,31 @@ func TestScanfNewlineMatchFormat(t *testing.T) {
 		}
 	}
 }
+
+// Test for issue 12090: Was unreading at EOF, double-scanning a byte.
+
+type hexBytes [2]byte
+
+func (h *hexBytes) Scan(ss ScanState, verb rune) error {
+	var b []byte
+	_, err := Fscanf(ss, "%4x", &b)
+	if err != nil {
+		panic(err) // Really shouldn't happen.
+	}
+	copy((*h)[:], b)
+	return err
+}
+
+func TestHexByte(t *testing.T) {
+	var h hexBytes
+	n, err := Sscanln("0123\n", &h)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 1 {
+		t.Fatalf("expected 1 item; scanned %d", n)
+	}
+	if h[0] != 0x01 || h[1] != 0x23 {
+		t.Fatalf("expected 0123 got %x", h)
+	}
+}

Original file line number	Diff line number	Diff line change
`@@ -888,7 +888,6 @@ func hexDigit(d rune) (int, bool) {`
`888`	`888`	`func (s *ss) hexByte() (b byte, ok bool) {`
`889`	`889`	`rune1 := s.getRune()`
`890`	`890`	`if rune1 == eof {`
`891`		`- s.UnreadRune()`
`892`	`891`	`return`
`893`	`892`	`}`
`894`	`893`	`value1, ok := hexDigit(rune1)`