CHECK TABLE did temporarily clear the auto_increment value.
It runs with a read lock, allowing other readers and
conurrent INSERTs. The latter could grab the wrong value
in this moment.

CHECK TABLE does no longer modify the auto_increment value.
Not even for a short moment.


myisam/mi_check.c:
  Bug#19604 - CHECK TABLE with concurrent INSERT can reset auto_increment
  In chk_key() and update_auto_increment_key() in the repair_only
  case, do not touch info->s->state.auto_increment. Especially
  chk_key() can be called from CHECK TABLE with a read lock.
  Concurrent inserts could grab a temporarily changed value.
  Added minor style fixes.
myisam/mi_key.c:
  Bug#19604 - CHECK TABLE with concurrent INSERT can reset auto_increment
  Changed update_auto_increment() to retrieve_auto_increment()
  to reflect that it does not change the auto_increment by
  itself any more. This must now be done externally if needed.
myisam/mi_update.c:
  Bug#19604 - CHECK TABLE with concurrent INSERT can reset auto_increment
  Added explicit update of info->s->state.auto_increment
  after the change from update_auto_increment() to
  retrieve_auto_increment().
myisam/mi_write.c:
  Bug#19604 - CHECK TABLE with concurrent INSERT can reset auto_increment
  Added explicit update of info->s->state.auto_increment
  after the change from update_auto_increment() to
  retrieve_auto_increment().
myisam/myisamdef.h:
  Bug#19604 - CHECK TABLE with concurrent INSERT can reset auto_increment
  Changed update_auto_increment() to retrieve_auto_increment()
  to reflect that it does not change the auto_increment by
  itself any more. This must now be done externally if needed.