When looking for a master, if it does exist it should be found
even if it is not connectible. The fact that it is not connectible
should be dealt with when a connection is created.
If a master is found but it is being drained, the connection attempt
is rejected if the master failure mode is fail_instantly.
In that case the logged message makes it plain that it is the draining
that is the reason for the connection attempt to fail.
The schema router now deals with the being drained bit the same way
it deals with the maintenance bit, that is, a server being drained will
simply be ignored.
TODO: This behaviour needs to be document.
TODO: It seems the schema router simply ignores the maintenance bit
once a connection has been establisdhed.
Both the replication lag and the message printing state are saved in SERVER,
although the values are mostly used by readwritesplit. A log message is printed
both when a server goes over the limit and when it comes back below.
Because of concurrency issues, a message may be printed multiple times before
different threads detect the new message state.
Documentation updated to explain the change.
If the connection to the master is lost, knowing what type of an error
caused the call to handleError helps deduce what was the real reason for
it. Logging the idle time of the connection helps detect when the
wait_timeout of a connection is exceeded.
Most of the ones still remaining outside are special cases.
Also, removed locking from status manipulation functions as it
has not been required for quite some time.
By storing the server statistics object in side the session, the lookup
involved in getting a worker-local value is avoided. Since the lookup is
done multiple times for a single query, it is beneficial to store it in
the session.
As the worker-local value is never deleted, it is safe to store a
reference to it in the session. It is also never updated concurrently so
no atomic operations are necessary.
The code now only checks the need for a keepalive ping once every
keepalive interval. Reduced the number of mxs_clock calls to one so that
all servers use the same value.
