WL#3984 (Revise locking of mysql.general_log and mysql.slow_log)

Bug#25422 (Hang with log tables)
Bug 17876 (Truncating mysql.slow_log in a SP after using cursor locks the
          thread)
Bug 23044 (Warnings on flush of a log table)
Bug 29129 (Resetting general_log while the GLOBAL READ LOCK is set causes
           a deadlock)

Prior to this fix, the server would hang when performing concurrent
ALTER TABLE or TRUNCATE TABLE statements against the LOG TABLES,
which are mysql.general_log and mysql.slow_log.

The root cause traces to the following code:
in sql_base.cc, open_table()
  if (table->in_use != thd)
  {
    /* wait_for_condition will unlock LOCK_open for us */
    wait_for_condition(thd, &LOCK_open, &COND_refresh);
  }
The problem with this code is that the current implementation of the
LOGGER creates 'fake' THD objects, like
- Log_to_csv_event_handler::general_log_thd
- Log_to_csv_event_handler::slow_log_thd
which are not associated to a real thread running in the server,
so that waiting for these non-existing threads to release table locks
cause the dead lock.

In general, the design of Log_to_csv_event_handler does not fit into the
general architecture of the server, so that the concept of general_log_thd
and slow_log_thd has to be abandoned:
- this implementation does not work with table locking
- it will not work with commands like SHOW PROCESSLIST
- having the log tables always opened does not integrate well with DDL
operations / FLUSH TABLES / SET GLOBAL READ_ONLY

With this patch, the fundamental design of the LOGGER has been changed to:
- always open and close a log table when writing a log
- remove totally the usage of fake THD objects
- clarify how locking of log tables is implemented in general.

See WL#3984 for details related to the new locking design.

Additional changes (misc bugs exposed and fixed):

1)

mysqldump which would ignore some tables in dump_all_tables_in_db(),
 but forget to ignore the same in dump_all_views_in_db().

2)

mysqldump would also issue an empty "LOCK TABLE" command when all the tables
to lock are to be ignored (numrows == 0), instead of not issuing the query.

3)

Internal errors handlers could intercept errors but not warnings
(see sql_error.cc).

4)

Implementing a nested call to open tables, for the performance schema tables,
exposed an existing bug in remove_table_from_cache(), which would perform:
  in_use->some_tables_deleted=1;
against another thread, without any consideration about thread locking.
This call inside remove_table_from_cache() was not required anyway,
since calling mysql_lock_abort() takes care of aborting -- cleanly -- threads
that might hold a lock on a table.
This line (in_use->some_tables_deleted=1) has been removed.

sql/handler.cc

@@ -1516,34 +1516,6 @@ THD *handler::ha_thd(void) const
 }
 
 
-bool handler::check_if_log_table_locking_is_allowed(uint sql_command,
-                                                    ulong type, TABLE *table)
-{
-  /*
-    Deny locking of the log tables, which is incompatible with
-    concurrent insert. The routine is not called if the table is
-    being locked from a logger THD (general_log_thd or slow_log_thd)
-    or from a privileged thread (see log.cc for details)
-  */
-  if (table->s->log_table &&
-      sql_command != SQLCOM_TRUNCATE &&
-      sql_command != SQLCOM_ALTER_TABLE &&
-      !(sql_command == SQLCOM_FLUSH &&
-        type & REFRESH_LOG) &&
-      (table->reginfo.lock_type >= TL_READ_NO_INSERT))
-  {
-    /*
-      The check  >= TL_READ_NO_INSERT denies all write locks
-      plus the only read lock (TL_READ_NO_INSERT itself)
-    */
-    table->reginfo.lock_type == TL_READ_NO_INSERT ?
-      my_error(ER_CANT_READ_LOCK_LOG_TABLE, MYF(0)) :
-        my_error(ER_CANT_WRITE_LOCK_LOG_TABLE, MYF(0));
-    return FALSE;
-  }
-  return TRUE;
-}
-
 /** @brief
   Open database-handler.
 
@@ -3681,6 +3653,18 @@ int handler::ha_write_row(uchar *buf)
   return 0;
 }
 
+
+/**
+  Write a record to the engine bypassing row-level binary logging.
+  This method is used internally by the server for writing to
+  performance schema tables, which are never replicated.
+*/
+int handler::ha_write_row_no_binlog(uchar *buf)
+{
+  return write_row(buf);
+}
+
+
 int handler::ha_update_row(const uchar *old_data, uchar *new_data)
 {
   int error;