GODRIVER-2311 Improve the BSON unmarshal buffer reuse fix to reduce memory allocations. (#891)

Author: matthewdale

bson/bsonrw/value_reader.go

@@ -384,9 +384,13 @@ func (vr *valueReader) ReadBinary() (b []byte, btype byte, err error) {
 	if err != nil {
 		return nil, 0, err
 	}
+	// Make a copy of the returned byte slice because it's just a subslice from the valueReader's
+	// buffer and is not safe to return in the unmarshaled value.
+	cp := make([]byte, len(b))
+	copy(cp, b)
 
 	vr.pop()
-	return b, btype, nil
+	return cp, btype, nil
 }
 
 func (vr *valueReader) ReadBoolean() (bool, error) {
@@ -737,6 +741,9 @@ func (vr *valueReader) ReadValue() (ValueReader, error) {
 	return vr, nil
 }
 
+// readBytes reads length bytes from the valueReader starting at the current offset. Note that the
+// returned byte slice is a subslice from the valueReader buffer and must be converted or copied
+// before returning in an unmarshaled value.
 func (vr *valueReader) readBytes(length int32) ([]byte, error) {
 	if length < 0 {
 		return nil, fmt.Errorf("invalid length: %d", length)
@@ -749,10 +756,7 @@ func (vr *valueReader) readBytes(length int32) ([]byte, error) {
 	start := vr.offset
 	vr.offset += int64(length)
 
-	b := make([]byte, length)
-	copy(b, vr.d[start:start+int64(length)])
-
-	return b, nil
+	return vr.d[start : start+int64(length)], nil
 }
 
 func (vr *valueReader) appendBytes(dst []byte, length int32) ([]byte, error) {