Deep-copying prevents subsequent modifications done by the caller from
affecting the data that can be potentially stored in the write queue of
the backend's DCB.
mxs::Buffer::iterator is not a random-access iterator and hence will have
linear cost. This is too costly to do on every packet with even moderately
sized resultsets.
RWBackend did not expect that a resultset and an unexpected ERR packet
could be stored in the same buffer. This can happen for example if a
server shuts down immediately after the resultset is sent.
This makes iterating over packets in buffers faster while still
maintaining the requirements for forward iterators. Not using operator+=
makes it clear that this is not a random access iterator.
The largest part of the code deals with the start of a response. Moving
this into a subfunction makes the function clearer as the switch statement
inside a switch statement is removed.
By processing the packets one at a time, the reply state is updated
correctly regardless of how many packets are received. This removes the
need for the clunky code that used modutil_count_signal_packets to detect
the end of the result set.
If a result consists of only OK packets, they would be processed
recursively which most of the time leads to a stack overflow. This can be
prevented by consuming all OK packets in the result in one go.
By splitting the processing and state querying into two separate
functions, the result can be inspected multiple times without triggering
the result processing.
