If the protocol routes a COM_QUIT packet to the backend, it must not
generate a packet when it is shutting down. This could cause unexpected
write errors if the backend server managed to close the socket before the
write was done.
The backend didn't expect AuthSwitchRequest packets in response to the
handshake response packets. This is allowed by the protocol and appears to
happen with at least MySQL 8.0.
When a query returns a WSREP error, most of the time it is not something
the client application is expecting. To prevent this from affecting the
client, it can be treated the same way a transaction rollback is treated:
ignore the error and try again.
If a query returned multiple resultsets and the connection was broken
between the resultsets, the backend would not know that parts of the
response were already sent. This is caused by the cyclic nature of the
state machine when multi-result responses are being processed.
To fix the problem, the result size is tracked to know how many bytes have
been sent to the client. This is a backport of the
MySQLProtocol::Result::size from 2.5(develop).
If an error is generated while a COM_CHANGE_USER is being done, it would
always use the sequence number 1. To properly handle this case and send
the correct sequence number, the COM_CHANGE_USER progress needs to be
tracked at the session level.
The information needs to be shared between the backend and client
protocols as the final OK to the COM_CHANGE_USER, with the sequence number
3, is the one that the backend server returns. Only after this response
has been received and routed to the client can the COM_CHANGE_USER
processing stop.
If a server fails mid-resultset, there's not a lot we can do to recover
the situation. A few cases could be handled (e.g. generate an ERR if the
resultset has proceeded to the row processing stage) but these fall
outside the scope of the original issue.
