encoding/binary: ReadUvarint return io.ErrUnexpectedEOF when read at least 1 byte

Fixes #54139

Change-Id: Ifc73bd7f181b13970ee6a08968f9d8f6e55d7ff3
GitHub-Last-Rev: 1e0a79bd3e
GitHub-Pull-Request: golang/go#54143
Reviewed-on: https://go-review.googlesource.com/c/go/+/420274
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/encoding/binary/varint.go

@@ -126,12 +126,18 @@ func Varint(buf []byte) (int64, int) {
 var overflow = errors.New("binary: varint overflows a 64-bit integer")
 
 // ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
+// The error is EOF only if no bytes were read.
+// If an EOF happens after reading some but not all the bytes,
+// ReadUvarint returns io.ErrUnexpectedEOF.
 func ReadUvarint(r io.ByteReader) (uint64, error) {
 	var x uint64
 	var s uint
 	for i := 0; i < MaxVarintLen64; i++ {
 		b, err := r.ReadByte()
 		if err != nil {
+			if i > 0 && err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
 			return x, err
 		}
 		if b < 0x80 {
@@ -147,6 +153,9 @@ func ReadUvarint(r io.ByteReader) (uint64, error) {
 }
 
 // ReadVarint reads an encoded signed integer from r and returns it as an int64.
+// The error is EOF only if no bytes were read.
+// If an EOF happens after reading some but not all the bytes,
+// ReadVarint returns io.ErrUnexpectedEOF.
 func ReadVarint(r io.ByteReader) (int64, error) {
 	ux, err := ReadUvarint(r) // ok to continue in presence of error
 	x := int64(ux >> 1)

src/encoding/binary/varint_test.go

@@ -128,7 +128,11 @@ func TestBufferTooSmall(t *testing.T) {
 		}
 
 		x, err := ReadUvarint(bytes.NewReader(buf))
-		if x != 0 || err != io.EOF {
+		wantErr := io.EOF
+		if i > 0 {
+			wantErr = io.ErrUnexpectedEOF
+		}
+		if x != 0 || err != wantErr {
 			t.Errorf("ReadUvarint(%v): got x = %d, err = %s", buf, x, err)
 		}
 	}