fmt.Scan: fix %c in the case where the input does not implement ReadRune itself.

While we're at it, clean up and test the code to guarantee we see every byte when
the text is erroneous UTF-8.

Fixes #866.

R=rsc
CC=golang-dev
https://golang.org/cl/1712042

src/pkg/fmt/scan.go

@@ -221,13 +221,34 @@ func (s *ss) Token() (tok string, err os.Error) {
 // not already implement ReadRuner.
 type readRune struct {
 	reader  io.Reader
-	buf    [utf8.UTFMax]byte
+	buf     [utf8.UTFMax]byte // used only inside ReadRune
+	pending int               // number of bytes in pendBuf; only >0 for bad UTF-8
+	pendBuf [utf8.UTFMax]byte // bytes left over
+}
+
+// readByte returns the next byte from the input, which may be
+// left over from a previous read if the UTF-8 was ill-formed.
+func (r *readRune) readByte() (b byte, err os.Error) {
+	if r.pending > 0 {
+		b = r.pendBuf[0]
+		copy(r.pendBuf[0:], r.pendBuf[1:])
+		r.pending--
+		return
+	}
+	_, err = r.reader.Read(r.pendBuf[0:1])
+	return r.pendBuf[0], err
+}
+
+// unread saves the bytes for the next read.
+func (r *readRune) unread(buf []byte) {
+	copy(r.pendBuf[r.pending:], buf)
+	r.pending += len(buf)
 }
 
 // ReadRune returns the next UTF-8 encoded code point from the
 // io.Reader inside r.
-func (r readRune) ReadRune() (rune int, size int, err os.Error) {
+func (r *readRune) ReadRune() (rune int, size int, err os.Error) {
-	_, err = r.reader.Read(r.buf[0:1])
+	r.buf[0], err = r.readByte()
 	if err != nil {
 		return 0, 0, err
 	}
@@ -235,20 +256,22 @@ func (r readRune) ReadRune() (rune int, size int, err os.Error) {
 		rune = int(r.buf[0])
 		return
 	}
-	for size := 1; size < utf8.UTFMax; size++ {
+	var n int
-		_, err = r.reader.Read(r.buf[size : size+1])
+	for n = 1; !utf8.FullRune(r.buf[0:n]); n++ {
+		r.buf[n], err = r.readByte()
 		if err != nil {
+			if err == os.EOF {
+				err = nil
 				break
 			}
-		if !utf8.FullRune(r.buf[0:]) {
-			continue
-		}
-		if c, w := utf8.DecodeRune(r.buf[0:size]); w == size {
-			rune = c
 			return
 		}
 	}
-	return utf8.RuneError, 1, err
+	rune, size = utf8.DecodeRune(r.buf[0:n])
+	if size < n { // an error
+		r.unread(r.buf[size:n])
+	}
+	return
 }
 
 
@@ -264,7 +287,7 @@ func newScanState(r io.Reader, nlIsSpace bool) *ss {
 	if rr, ok := r.(readRuner); ok {
 		s.rr = rr
 	} else {
-		s.rr = readRune{reader: r}
+		s.rr = &readRune{reader: r}
 	}
 	s.nlIsSpace = nlIsSpace
 	s.peekRune = -1

src/pkg/fmt/scan_test.go

@@ -11,6 +11,7 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+	"utf8"
 )
 
 type ScanTest struct {
@@ -108,6 +109,20 @@ func (x *Xs) Scan(state ScanState, verb int) os.Error {
 
 var xVal Xs
 
+// myStringReader implements Read but not ReadRune, allowing us to test our readRune wrapper
+// type that creates something that can read runes given only Read().
+type myStringReader struct {
+	r *strings.Reader
+}
+
+func (s *myStringReader) Read(p []byte) (n int, err os.Error) {
+	return s.r.Read(p)
+}
+
+func newReader(s string) *myStringReader {
+	return &myStringReader{strings.NewReader(s)}
+}
+
 var scanTests = []ScanTest{
 	// Numbers
 	ScanTest{"T\n", &boolVal, true},  // boolean test vals toggle to be sure they are written
@@ -176,6 +191,7 @@ var scanfTests = []ScanfTest{
 	ScanfTest{"%v", "-71\n", &intVal, -71},
 	ScanfTest{"%d", "72\n", &intVal, 72},
 	ScanfTest{"%c", "a\n", &intVal, 'a'},
+	ScanfTest{"%c", "\u5072\n", &intVal, 0x5072},
 	ScanfTest{"%c", "\u1234\n", &intVal, '\u1234'},
 	ScanfTest{"%d", "73\n", &int8Val, int8(73)},
 	ScanfTest{"%d", "+74\n", &int16Val, int16(74)},
@@ -211,6 +227,7 @@ var scanfTests = []ScanfTest{
 	ScanfTest{"%v\n", "true\n", &renamedBoolVal, renamedBool(true)},
 	ScanfTest{"%t\n", "F\n", &renamedBoolVal, renamedBool(false)},
 	ScanfTest{"%v", "101\n", &renamedIntVal, renamedInt(101)},
+	ScanfTest{"%c", "\u0101\n", &renamedIntVal, renamedInt('\u0101')},
 	ScanfTest{"%o", "0146\n", &renamedIntVal, renamedInt(102)},
 	ScanfTest{"%v", "103\n", &renamedUintVal, renamedUint(103)},
 	ScanfTest{"%d", "104\n", &renamedUintVal, renamedUint(104)},
@@ -276,6 +293,7 @@ var multiTests = []ScanfMultiTest{
 	ScanfMultiTest{"%3d22%3d", "33322333", args(&i, &j), args(333, 333), ""},
 	ScanfMultiTest{"%6vX=%3fY", "3+2iX=2.5Y", args(&c, &f), args((3 + 2i), float(2.5)), ""},
 	ScanfMultiTest{"%d%s", "123abc", args(&i, &s), args(123, "abc"), ""},
+	ScanfMultiTest{"%c%c%c", "2\u50c2X", args(&i, &j, &k), args('2', '\u50c2', 'X'), ""},
 
 	// Custom scanner.
 	ScanfMultiTest{"%2e%f", "eefffff", args(&x, &y), args(Xs("ee"), Xs("fffff")), ""},
@@ -285,18 +303,26 @@ var multiTests = []ScanfMultiTest{
 	ScanfMultiTest{"%d %d %d", "23 18", args(&i, &j), args(23, 18), "too few operands"},
 	ScanfMultiTest{"%d %d", "23 18 27", args(&i, &j, &k), args(23, 18), "too many operands"},
 	ScanfMultiTest{"%c", "\u0100", args(&int8Val), nil, "overflow"},
+
+	// Bad UTF-8: should see every byte.
+	ScanfMultiTest{"%c%c%c", "\xc2X\xc2", args(&i, &j, &k), args(utf8.RuneError, 'X', utf8.RuneError), ""},
 }
 
-func testScan(t *testing.T, scan func(r io.Reader, a ...interface{}) (int, os.Error)) {
+func testScan(name string, t *testing.T, scan func(r io.Reader, a ...interface{}) (int, os.Error)) {
 	for _, test := range scanTests {
-		r := strings.NewReader(test.text)
+		var r io.Reader
+		if name == "StringReader" {
+			r = strings.NewReader(test.text)
+		} else {
+			r = newReader(test.text)
+		}
 		n, err := scan(r, test.in)
 		if err != nil {
-			t.Errorf("got error scanning %q: %s", test.text, err)
+			t.Errorf("%s got error scanning %q: %s", name, test.text, err)
 			continue
 		}
 		if n != 1 {
-			t.Errorf("count error on entry %q: got %d", test.text, n)
+			t.Errorf("%s count error on entry %q: got %d", name, test.text, n)
 			continue
 		}
 		// The incoming value may be a pointer
@@ -306,17 +332,25 @@ func testScan(t *testing.T, scan func(r io.Reader, a ...interface{}) (int, os.Er
 		}
 		val := v.Interface()
 		if !reflect.DeepEqual(val, test.out) {
-			t.Errorf("scanning %q: expected %v got %v, type %T", test.text, test.out, val, val)
+			t.Errorf("%s scanning %q: expected %v got %v, type %T", name, test.text, test.out, val, val)
 		}
 	}
 }
 
 func TestScan(t *testing.T) {
-	testScan(t, Fscan)
+	testScan("StringReader", t, Fscan)
+}
+
+func TestMyReaderScan(t *testing.T) {
+	testScan("myStringReader", t, Fscan)
 }
 
 func TestScanln(t *testing.T) {
-	testScan(t, Fscanln)
+	testScan("StringReader", t, Fscanln)
+}
+
+func TestMyReaderScanln(t *testing.T) {
+	testScan("myStringReader", t, Fscanln)
 }
 
 func TestScanf(t *testing.T) {
@@ -359,17 +393,23 @@ func TestScanOverflow(t *testing.T) {
 
 // TODO: there's no conversion from []T to ...T, but we can fake it.  These
 // functions do the faking.  We index the table by the length of the param list.
-var scanf = []func(string, string, []interface{}) (int, os.Error){
+var fscanf = []func(io.Reader, string, []interface{}) (int, os.Error){
-	0: func(s, f string, i []interface{}) (int, os.Error) { return Sscanf(s, f) },
+	0: func(r io.Reader, f string, i []interface{}) (int, os.Error) { return Fscanf(r, f) },
-	1: func(s, f string, i []interface{}) (int, os.Error) { return Sscanf(s, f, i[0]) },
+	1: func(r io.Reader, f string, i []interface{}) (int, os.Error) { return Fscanf(r, f, i[0]) },
-	2: func(s, f string, i []interface{}) (int, os.Error) { return Sscanf(s, f, i[0], i[1]) },
+	2: func(r io.Reader, f string, i []interface{}) (int, os.Error) { return Fscanf(r, f, i[0], i[1]) },
-	3: func(s, f string, i []interface{}) (int, os.Error) { return Sscanf(s, f, i[0], i[1], i[2]) },
+	3: func(r io.Reader, f string, i []interface{}) (int, os.Error) { return Fscanf(r, f, i[0], i[1], i[2]) },
 }
 
-func TestScanfMulti(t *testing.T) {
+func testScanfMulti(name string, t *testing.T) {
 	sliceType := reflect.Typeof(make([]interface{}, 1)).(*reflect.SliceType)
 	for _, test := range multiTests {
-		n, err := scanf[len(test.in)](test.text, test.format, test.in)
+		var r io.Reader
+		if name == "StringReader" {
+			r = strings.NewReader(test.text)
+		} else {
+			r = newReader(test.text)
+		}
+		n, err := fscanf[len(test.in)](r, test.format, test.in)
 		if err != nil {
 			if test.err == "" {
 				t.Errorf("got error scanning (%q, %q): %q", test.format, test.text, err)
@@ -398,6 +438,14 @@ func TestScanfMulti(t *testing.T) {
 	}
 }
 
+func TestScanfMulti(t *testing.T) {
+	testScanfMulti("StringReader", t)
+}
+
+func TestMyReaderScanfMulti(t *testing.T) {
+	testScanfMulti("myStringReader", t)
+}
+
 func TestScanMultiple(t *testing.T) {
 	var a int
 	var s string