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.

client/mysqldump.c

@@ -3726,11 +3726,12 @@ static int dump_all_tables_in_db(char *database)
   {
     DYNAMIC_STRING query;
     init_dynamic_string_checked(&query, "LOCK TABLES ", 256, 1024);
-    for (numrows= 0 ; (table= getTableName(1)) ; numrows++)
+    for (numrows= 0 ; (table= getTableName(1)) ; )
     {
       char *end= strmov(afterdot, table);
       if (include_table(hash_key,end - hash_key))
       {
+        numrows++;
         dynstr_append_checked(&query, quote_name(table, table_buff, 1));
         dynstr_append_checked(&query, " READ /*!32311 LOCAL */,");
       }
@@ -3804,6 +3805,11 @@ static my_bool dump_all_views_in_db(char *database)
   char *table;
   uint numrows;
   char table_buff[NAME_LEN*2+3];
+  char hash_key[2*NAME_LEN+2];  /* "db.tablename" */
+  char *afterdot;
+
+  afterdot= strmov(hash_key, database);
+  *afterdot++= '.';
 
   if (init_dumping(database, init_dumping_views))
     return 1;
@@ -3813,10 +3819,15 @@ static my_bool dump_all_views_in_db(char *database)
   {
     DYNAMIC_STRING query;
     init_dynamic_string_checked(&query, "LOCK TABLES ", 256, 1024);
-    for (numrows= 0 ; (table= getTableName(1)); numrows++)
+    for (numrows= 0 ; (table= getTableName(1)); )
     {
-      dynstr_append_checked(&query, quote_name(table, table_buff, 1));
-      dynstr_append_checked(&query, " READ /*!32311 LOCAL */,");
+      char *end= strmov(afterdot, table);
+      if (include_table(hash_key,end - hash_key))
+      {
+        numrows++;
+        dynstr_append_checked(&query, quote_name(table, table_buff, 1));
+        dynstr_append_checked(&query, " READ /*!32311 LOCAL */,");
+      }
     }
     if (numrows && mysql_real_query(mysql, query.str, query.length-1))
       DB_error(mysql, "when using LOCK TABLES");
@@ -3830,7 +3841,11 @@ static my_bool dump_all_views_in_db(char *database)
            /* We shall continue here, if --force was given */
   }
   while ((table= getTableName(0)))
-     get_view_structure(table, database);
+  {
+    char *end= strmov(afterdot, table);
+    if (include_table(hash_key, end - hash_key))
+      get_view_structure(table, database);
+  }
   if (opt_xml)
   {
     fputs("</database>\n", md_result_file);