by KILL or shutdown, do not mark the table as corrupted".
It is indeed more logical to leave the corruption flag unchanged.
This cannot be extended to REPAIR/OPTIMIZE as they make no backup copy
of the MYI. This patch was tested with KILL and mysqladmin shutdown
while a CHECK TABLE was running. Without the patch, the table becomes
unusable (can't INSERT to it, error 145). With the patch, no.


sql/ha_myisam.cc:
  When CHECK TABLE or ANALYZE TABLE is killed by KILL,
  do not mark the table corrupted; leave its corruption flag
  unchanged.