After a locking error the open table(s) were not fully
cleaned up for reuse. But they were put into the open table
cache even before the lock was tried. The next statement
reused the table(s) with a wrong lock type set up. This
tricked MyISAM into believing that it don't need to update
the table statistics. Hence CHECK TABLE reported a mismatch
of record count and table size.

Fortunately nothing worse has been detected yet. The effect
of the test case was that the insert worked on a read locked
table. (!)

I added a new function that clears the lock type from all
tables that were prepared for a lock. I call this function
when a lock failes.

No test case. One test would add 50 seconds to the
test suite. Another test requires file mode modifications.
I added a test script to the bug report. It contains three
cases for failing locks. All could reproduce a table
corruption. All are fixed by this patch.

This bug was not lock timeout specific.


sql/lock.cc:
  Bug#18544 - LOCK TABLES timeout causes MyISAM table corruption
  Resetting the lock type in the open table(s) lock data
  after a locking error.