The problem is that a SELECT .. FOR UPDATE statement might open
a table and later wait for a impeding global read lock without
noticing whether it is holding a table that is being waited upon
the the flush phase of the process that took the global read
lock.

The same problem also affected the following statements:

LOCK TABLES .. WRITE
UPDATE .. SET (update and multi-table update)
TRUNCATE TABLE ..
LOAD DATA ..

The solution is to make the above statements wait for a impending
global read lock before opening the tables. If there is no
impending global read lock, the statement raises a temporary
protection against global read locks and progresses smoothly
towards completion.

Important notice: the patch does not try to address all possible
cases, only those which are common and can be fixed unintrusively
enough for 5.0.

mysql-test/r/lock_multi.result:
  Add test case result for Bug#43230
mysql-test/t/lock_multi.test:
  Add test case for Bug#43230
sql/sql_lex.cc:
  Initialize flag.
sql/sql_lex.h:
  Add a flag to the lexer.
sql/sql_parse.cc:
  Wait for the global read lock is a write lock is going to be
  taken. The wait is done before opening tables.
sql/sql_yacc.yy:
  Protect against the GRL if its a SELECT .. FOR UPDATE or LOCK TABLES
  .. WRITE statement.