leave the table unusable".
 
Failing ALTER statement on partitioned table could have left
this table in an unusable state. This has happened in cases
when ALTER was executed using "fast" algorithm, which doesn't 
involve copying of data between old and new versions of table, 
and the resulting new table was incompatible with partitioning
function in some way.
 
The problem stems from the fact that discrepancies between new 
table definition and partitioning function are discovered only 
when the table is opened. In case of "fast" algorithm this has
happened too late during ALTER's execution, at the moment when
all changes were already done and couldn't have been reverted.
 
In the cases when "slow" algorithm, which copies data, is used 
such discrepancies are detected at the moment new table
definition is opened implicitly when new version of table is
created in storage engine. As result ALTER is aborted before 
any changes to table were done.
 
This fix tries to address this issue by ensuring that "fast"
algorithm behaves similarly to "slow" algorithm and checks
compatibility between new definition and partitioning function 
by trying to open new definition after .FRM file for it has 
been created.
 
Long term we probably should implement some way to check
compatibility between partitioning function and new table
definition which won't involve opening it, as this should
allow much cleaner fix for this problem.

mysql-test/r/partition_innodb.result:
  Added test for bug #57985 "ONLINE/FAST ALTER PARTITION can
  fail and leave the table unusable".
mysql-test/t/partition_innodb.test:
  Added test for bug #57985 "ONLINE/FAST ALTER PARTITION can
  fail and leave the table unusable".
sql/sql_table.cc:
  Ensure that in cases when .FRM for partitioned table is
  created without creating table in storage engine (e.g.
  during "fast" ALTER TABLE) we still open table definition. 
  This allows to check that definition of created table/.FRM 
  is compatible with its partitioning function.