A spider_conn may outlive its associated ha_spider (in the field
queued_ping_spider) used for connecting to and pinging the data
node (a call to spider_db_ping(), guarded by the boolean field
queued_ping). In a call to ha_spider::close() (which is often preceded
with the deletion of the ha_spider itself), many cleanups happen,
including freeing the associated spider_share, which is used by the
spider_conn in spider_db_ping. Therefore it is necessary to reset both
the queued_ping_spider and queued_ping fields, so that any further
spider interaction with the data node will not trigger the call using
the ha_spider including its freed spider_share.
Also out of caution added an assert and internal error in case a
connection has not been established (the db_conn field of type
MYSQL * is NULL), and attempt to connect is skipped because both
queued_connect and queued_ping are false. Note that this unlikely (if
not impossible) scenario would not be a regression caused by this
change, as it strictly falls under the scenario of this bug.
18a44b4298