bytes: panic in ReadFrom with more information with negative Read counts

This is only to aid in human debugging, and for that reason we maintain a panic, and not return an error.

Fixes #22097

Change-Id: If72e4d1e47ec9125ca7bc97d5fe4cedb7f76ae72
Reviewed-on: https://go-review.googlesource.com/67970
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/buffer.go

@@ -41,6 +41,7 @@ const (
 
 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
 var ErrTooLarge = errors.New("bytes.Buffer: too large")
+var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read")
 
 const maxInt = int(^uint(0) >> 1)
 
@@ -198,6 +199,10 @@ func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
 	for {
 		i := b.grow(MinRead)
 		m, e := r.Read(b.buf[i:cap(b.buf)])
+		if m < 0 {
+			panic(errNegativeRead)
+		}
+
 		b.buf = b.buf[:i+m]
 		n += int64(m)
 		if e == io.EOF {

src/bytes/buffer_test.go

@@ -17,6 +17,10 @@ const N = 10000       // make this bigger for a larger (and slower) test
 var testString string // test data for write tests
 var testBytes []byte  // test data; same as testString but as a slice.
 
+type negativeReader struct{}
+
+func (r *negativeReader) Read([]byte) (int, error) { return -1, nil }
+
 func init() {
 	testBytes = make([]byte, N)
 	for i := 0; i < N; i++ {
@@ -265,6 +269,26 @@ func TestReadFrom(t *testing.T) {
 	}
 }
 
+func TestReadFromNegativeReader(t *testing.T) {
+	var b Buffer
+	defer func() {
+		switch err := recover().(type) {
+		case nil:
+			t.Fatal("bytes.Buffer.ReadFrom didn't panic")
+		case error:
+			// this is the error string of errNegativeRead
+			wantError := "bytes.Buffer: reader returned negative count from Read"
+			if err.Error() != wantError {
+				t.Fatalf("recovered panic: got %v, want %v", err.Error(), wantError)
+			}
+		default:
+			t.Fatalf("unexpected panic value: %#v", err)
+		}
+	}()
+
+	b.ReadFrom(new(negativeReader))
+}
+
 func TestWriteTo(t *testing.T) {
 	var buf Buffer
 	for i := 3; i < 30; i += 3 {