math/big: Rat.SetString to report error if input is not consumed entirely

Also, document behavior explicitly for all SetString implementations.

Fixes #17001.

Change-Id: Iccc882b4bc7f8b61b6092f330e405c146a80dc98
Reviewed-on: https://go-review.googlesource.com/30472Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/math/big/floatconv.go

@@ -14,7 +14,9 @@ import (
 
 // SetString sets z to the value of s and returns z and a boolean indicating
 // success. s must be a floating-point number of the same format as accepted
-// by Parse, with base argument 0.
+// by Parse, with base argument 0. The entire string (not just a prefix) must
+// be valid for success. If the operation failed, the value of z is undefined
+// but the returned value is nil.
 func (z *Float) SetString(s string) (*Float, bool) {
 	if f, _, err := z.Parse(s, 0); err == nil {
 		return f, true
@@ -212,6 +214,7 @@ func (z *Float) pow5(n uint64) *Float {
 //
 // It sets z to the (possibly rounded) value of the corresponding floating-
 // point value, and returns z, the actual base b, and an error err, if any.
+// The entire string (not just a prefix) must be consumed for success.
 // If z's precision is 0, it is changed to 64 before rounding takes effect.
 // The number must be of the form:
 //

src/math/big/int.go

@@ -361,7 +361,8 @@ func (x *Int) Uint64() uint64 {
 }
 
 // SetString sets z to the value of s, interpreted in the given base,
-// and returns z and a boolean indicating success. If SetString fails,
+// and returns z and a boolean indicating success. The entire string
+// (not just a prefix) must be valid for success. If SetString fails,
 // the value of z is undefined but the returned value is nil.
 //
 // The base argument must be 0 or a value between 2 and MaxBase. If the base
@@ -371,12 +372,11 @@ func (x *Int) Uint64() uint64 {
 //
 func (z *Int) SetString(s string, base int) (*Int, bool) {
 	r := strings.NewReader(s)
-	_, _, err := z.scan(r, base)
-	if err != nil {
+	if _, _, err := z.scan(r, base); err != nil {
 		return nil, false
 	}
-	_, err = r.ReadByte()
-	if err != io.EOF {
+	// entire string must have been consumed
+	if _, err := r.ReadByte(); err != io.EOF {
 		return nil, false
 	}
 	return z, true // err == io.EOF => scan consumed all of s

src/math/big/ratconv.go

@@ -36,8 +36,9 @@ func (z *Rat) Scan(s fmt.ScanState, ch rune) error {
 
 // SetString sets z to the value of s and returns z and a boolean indicating
 // success. s can be given as a fraction "a/b" or as a floating-point number
-// optionally followed by an exponent. If the operation failed, the value of
-// z is undefined but the returned value is nil.
+// optionally followed by an exponent. The entire string (not just a prefix)
+// must be valid for success. If the operation failed, the value of z is un-
+// defined but the returned value is nil.
 func (z *Rat) SetString(s string) (*Rat, bool) {
 	if len(s) == 0 {
 		return nil, false
@@ -49,9 +50,13 @@ func (z *Rat) SetString(s string) (*Rat, bool) {
 		if _, ok := z.a.SetString(s[:sep], 0); !ok {
 			return nil, false
 		}
-		s = s[sep+1:]
+		r := strings.NewReader(s[sep+1:])
 		var err error
-		if z.b.abs, _, _, err = z.b.abs.scan(strings.NewReader(s), 0, false); err != nil {
+		if z.b.abs, _, _, err = z.b.abs.scan(r, 0, false); err != nil {
+			return nil, false
+		}
+		// entire string must have been consumed
+		if _, err = r.ReadByte(); err != io.EOF {
 			return nil, false
 		}
 		if len(z.b.abs) == 0 {

src/math/big/ratconv_test.go

@@ -50,6 +50,10 @@ var setStringTests = []StringTest{
 	{"204211327800791583.81095", "4084226556015831676219/20000", true},
 	{"0e9999999999", "0", true}, // issue #16176
 	{in: "1/0"},
+	{in: "4/3/2"}, // issue 17001
+	{in: "4/3/"},
+	{in: "4/3."},
+	{in: "4/"},
 }
 
 // These are not supported by fmt.Fscanf.
@@ -59,6 +63,7 @@ var setStringTests2 = []StringTest{
 	{"-010.", "-10", true},
 	{"0x10/0x20", "1/2", true},
 	{"0b1000/3", "8/3", true},
+	{in: "4/3x"},
 	// TODO(gri) add more tests
 }