generate replacement rune when asked to encode a negative rune value.

Fixes #425.

R=rsc
https://golang.org/cl/178043

src/pkg/utf8/utf8.go

@@ -227,6 +227,11 @@ func RuneLen(rune int) int {
 // EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune.
 // It returns the number of bytes written.
 func EncodeRune(rune int, p []byte) int {
+	// Negative values are erroneous.
+	if rune < 0 {
+		rune = RuneError
+	}
+
 	if rune <= _Rune1Max {
 		p[0] = byte(rune);
 		return 1;

src/pkg/utf8/utf8_test.go

@@ -42,6 +42,7 @@ var utf8map = []Utf8Map{
 	Utf8Map{0x10001, "\xf0\x90\x80\x81"},
 	Utf8Map{0x10fffe, "\xf4\x8f\xbf\xbe"},
 	Utf8Map{0x10ffff, "\xf4\x8f\xbf\xbf"},
+	Utf8Map{0xFFFD, "\xef\xbf\xbd"},
 }
 
 // strings.Bytes with one extra byte at end
@@ -81,7 +82,7 @@ func TestEncodeRune(t *testing.T) {
 		n := EncodeRune(m.rune, &buf);
 		b1 := buf[0:n];
 		if !bytes.Equal(b, b1) {
-			t.Errorf("EncodeRune(0x%04x) = %q want %q", m.rune, b1, b)
+			t.Errorf("EncodeRune(%#04x) = %q want %q", m.rune, b1, b)
 		}
 	}
 }
@@ -92,23 +93,23 @@ func TestDecodeRune(t *testing.T) {
 		b := makeBytes(m.str);
 		rune, size := DecodeRune(b);
 		if rune != m.rune || size != len(b) {
-			t.Errorf("DecodeRune(%q) = 0x%04x, %d want 0x%04x, %d", b, rune, size, m.rune, len(b))
+			t.Errorf("DecodeRune(%q) = %#04x, %d want %#04x, %d", b, rune, size, m.rune, len(b))
 		}
 		s := m.str;
 		rune, size = DecodeRuneInString(s);
 		if rune != m.rune || size != len(b) {
-			t.Errorf("DecodeRune(%q) = 0x%04x, %d want 0x%04x, %d", s, rune, size, m.rune, len(b))
+			t.Errorf("DecodeRune(%q) = %#04x, %d want %#04x, %d", s, rune, size, m.rune, len(b))
 		}
 
 		// there's an extra byte that bytes left behind - make sure trailing byte works
 		rune, size = DecodeRune(b[0:cap(b)]);
 		if rune != m.rune || size != len(b) {
-			t.Errorf("DecodeRune(%q) = 0x%04x, %d want 0x%04x, %d", b, rune, size, m.rune, len(b))
+			t.Errorf("DecodeRune(%q) = %#04x, %d want %#04x, %d", b, rune, size, m.rune, len(b))
 		}
 		s = m.str + "\x00";
 		rune, size = DecodeRuneInString(s);
 		if rune != m.rune || size != len(b) {
-			t.Errorf("DecodeRuneInString(%q) = 0x%04x, %d want 0x%04x, %d", s, rune, size, m.rune, len(b))
+			t.Errorf("DecodeRuneInString(%q) = %#04x, %d want %#04x, %d", s, rune, size, m.rune, len(b))
 		}
 
 		// make sure missing bytes fail
@@ -118,12 +119,12 @@ func TestDecodeRune(t *testing.T) {
 		}
 		rune, size = DecodeRune(b[0 : len(b)-1]);
 		if rune != RuneError || size != wantsize {
-			t.Errorf("DecodeRune(%q) = 0x%04x, %d want 0x%04x, %d", b[0:len(b)-1], rune, size, RuneError, wantsize)
+			t.Errorf("DecodeRune(%q) = %#04x, %d want %#04x, %d", b[0:len(b)-1], rune, size, RuneError, wantsize)
 		}
 		s = m.str[0 : len(m.str)-1];
 		rune, size = DecodeRuneInString(s);
 		if rune != RuneError || size != wantsize {
-			t.Errorf("DecodeRuneInString(%q) = 0x%04x, %d want 0x%04x, %d", s, rune, size, RuneError, wantsize)
+			t.Errorf("DecodeRuneInString(%q) = %#04x, %d want %#04x, %d", s, rune, size, RuneError, wantsize)
 		}
 
 		// make sure bad sequences fail
@@ -134,16 +135,27 @@ func TestDecodeRune(t *testing.T) {
 		}
 		rune, size = DecodeRune(b);
 		if rune != RuneError || size != 1 {
-			t.Errorf("DecodeRune(%q) = 0x%04x, %d want 0x%04x, %d", b, rune, size, RuneError, 1)
+			t.Errorf("DecodeRune(%q) = %#04x, %d want %#04x, %d", b, rune, size, RuneError, 1)
 		}
 		s = string(b);
 		rune, size = DecodeRune(b);
 		if rune != RuneError || size != 1 {
-			t.Errorf("DecodeRuneInString(%q) = 0x%04x, %d want 0x%04x, %d", s, rune, size, RuneError, 1)
+			t.Errorf("DecodeRuneInString(%q) = %#04x, %d want %#04x, %d", s, rune, size, RuneError, 1)
 		}
 	}
 }
 
+// Check that negative runes encode as U+FFFD.
+func TestNegativeRune(t *testing.T) {
+	errorbuf := make([]byte, UTFMax);
+	errorbuf = errorbuf[0:EncodeRune(RuneError, errorbuf)];
+	buf := make([]byte, UTFMax);
+	buf = buf[0:EncodeRune(-1, buf)];
+	if !bytes.Equal(buf, errorbuf) {
+		t.Errorf("incorrect encoding [% x] for -1; expected [% x]", buf, errorbuf)
+	}
+}
+
 type RuneCountTest struct {
 	in	string;
 	out	int;