fix minor interoperability issues.

varint.go

@@ -8,12 +8,25 @@ import (
 )
 
 var (
-	ErrOverflow   = errors.New("varints larger than uint64 not yet supported")
+	ErrOverflow   = errors.New("varints larger than uint63 not supported")
 	ErrUnderflow  = errors.New("varints malformed, could not reach the end")
 	ErrNotMinimal = errors.New("varint not minimally encoded")
 )
 
+const (
+	// MaxLenUvarint63 is the maximum number of bytes representing an uvarint in
+	// this encoding, supporting a maximum value of 2^63 (uint63), aka
+	// MaxValueUvarint63.
+	MaxLenUvarint63 = 9
+
+	// MaxValueUvarint63 is the maximum encodable uint63 value.
+	MaxValueUvarint63 = (1 << 63) - 1
+)
+
 // UvarintSize returns the size (in bytes) of `num` encoded as a unsigned varint.
+//
+// This may return a size greater than MaxUvarintLen63, which would be an
+// illegal value, and would be rejected by readers.
 func UvarintSize(num uint64) int {
 	bits := bits.Len64(num)
 	q, r := bits/7, bits%7
@@ -39,10 +52,14 @@ func FromUvarint(buf []byte) (uint64, int, error) {
 	var x uint64
 	var s uint
 	for i, b := range buf {
+		if (i == 8 && b >= 0x80) || i >= MaxLenUvarint63 {
+			// this is the 9th and last byte we're willing to read, but it
+			// signals there's more (1 in MSB).
+			// or this is the >= 10th byte, and for some reason we're still here.
+			return 0, 0, ErrOverflow
+		}
 		if b < 0x80 {
-			if i > 9 || i == 9 && b > 1 {
-				return 0, 0, ErrOverflow
-			} else if b == 0 && s > 0 {
+			if b == 0 && s > 0 {
 				return 0, 0, ErrNotMinimal
 			}
 			return x | uint64(b)<<s, i + 1, nil
@@ -70,12 +87,14 @@ func ReadUvarint(r io.ByteReader) (uint64, error) {
 			}
 			return 0, err
 		}
+		if (i == 8 && b >= 0x80) || i >= MaxLenUvarint63 {
+			// this is the 9th and last byte we're willing to read, but it
+			// signals there's more (1 in MSB).
+			// or this is the >= 10th byte, and for some reason we're still here.
+			return 0, ErrOverflow
+		}
 		if b < 0x80 {
-			if i > 9 || i == 9 && b > 1 {
-				return 0, ErrOverflow
-			} else if b == 0 && s > 0 {
-				// we should never _finish_ on a 0 byte if we
-				// have more than one byte.
+			if b == 0 && s > 0 {
 				return 0, ErrNotMinimal
 			}
 			return x | uint64(b)<<s, nil
@@ -89,6 +108,9 @@ func ReadUvarint(r io.ByteReader) (uint64, error) {
 //
 // This is provided for convenience so users of this library can avoid built-in
 // varint functions and easily audit code for uses of those functions.
+//
+// Make sure that x is smaller or equal to MaxValueUvarint63, otherwise this
+// function will produce values that may be rejected by readers.
 func PutUvarint(buf []byte, x uint64) int {
 	return binary.PutUvarint(buf, x)
 }