WL#1339 "Add per account max_user_connections limit (maximum number

of concurrent connections for the same account)"

Added support of account specific max_user_connections limit. Made all
user limits to be counted per account instead of the old behavior,
which was per user/host accounting. Added option which enables the old
behavior. Added testing of these to the test suite.

(After review version).


client/mysqltest.c:
  Extended mysqltest to be able to handle connect() statements for which
  error is expected.
  
  - Added replace_dynstr_append() utility function.
  - added connect_n_handle_errors() function which connects with server
    without retries and handles errors as if "connect" was usual
    statement.
  - do_connect(): added ability to handle connects which are expected
    to return an error.
  - run_query_normal(): Moved all expected-error-handling code to separate
    normal_handle_error()/normal_handle_no_error() functions to be able
    to reuse them in connect_n_handle_errors().
mysql-test/r/grant.result:
  Fixed test results since one more column to mysql.user was added.
mysql-test/r/system_mysql_db.result:
  Fixed test results since one more column to mysql.user was added.
scripts/mysql_create_system_tables.sh:
  Added max_user_connections column to mysql.user table for storing
  new maximal concurrent connections per account limit.
scripts/mysql_fix_privilege_tables.sql:
  Added max_user_connections column to mysql.user table for storing
  new maximal concurrent connections per account limit.
sql/lex.h:
  Added MAX_USER_CONNECTIONS symbol used for specifying
  maximum number of concurrent connections per account.
sql/mysql_priv.h:
  Added declaration of opt_old_style_user_limits variable which is defined
  in sql/mysqld.cc used in sql/sql_parse.cc.
sql/mysqld.cc:
  Added "old-style-user-limits" option which forces user limits to behave
  in old way i.e. to be counted per user/host pair instead of per account.
  Added comment describing mqh_used variable.
sql/set_var.cc:
  Added sys_var_max_user_conn class which implements support for the new 
  behavior of max_user_connections variable. Now the global instance of
  this variable holds default maximum number of concurrent connections per
  account (as it was before) and the session instance gives read-only
  access to account-specific version of this limit.
sql/set_var.h:
  Added sys_var_max_user_conn class which implements support for the new 
  behavior of max_user_connections variable. Now the global instance of
  this variable holds default maximum number of concurrent connections per
  account (as it was before) and the session instance gives read-only
  access to account-specific version of this limit.
sql/sql_acl.cc:
  Added support for account-specific MAX_USER_CONNECTIONS limit.
  Tweaked USER_RESOURCES and their handling for better clarity.
sql/sql_parse.cc:
  Added support for account-specific MAX_USER_CONNECTIONS (maximum number
  of concurrent connections per account) limit. Changed default behavior
  of all user limits to be per account instead of per user+host.
  '--old-style-user-limits' option was added to enable the old behavior.
  Made maximum number of connections per hour to be independant on the
  value of global max_user_connections variable.
sql/sql_yacc.yy:
  Added support of new MAX_USER_CONNECTIONS limit to grammar.
  Renamed USER_RESOURCES::connections member to conn_per_hour and bits
  member to specified_limits. Also enum is used instead of naked numbers
  when we are working with specified_limits.
sql/structs.h:
  USER_RESOURCES struct:
   - Added user_conn member to store the maximum number of concurrent 
     connections for an account. Renamed connections member to 
     conn_per_hour for less ambiguity.
   - Renamed member 'bits' to 'specified_limits' for the sake of clarity.
     The member was used as a flag indicating which limits were mentioned
     in GRANT clause.
   - Added comments.
  USER_CONN struct:
   - Removed unused user_len member.
   - Added comments.

sql/structs.h

@@ -205,16 +205,65 @@ typedef struct	st_lex_user {
 } LEX_USER;
 
 
+/*
+  This structure specifies the maximum amount of resources which
+  can be consumed by each account. Zero value of a member means
+  there is no limit.
+*/
 typedef struct user_resources {
-  uint questions, updates, connections, bits;
+  /* Maximum number of queries/statements per hour. */
+  uint questions;
+  /*
+     Maximum number of updating statements per hour (which statements are
+     updating is defined by uc_update_queries array).
+  */
+  uint updates;
+  /* Maximum number of connections established per hour. */
+  uint conn_per_hour;
+  /* Maximum number of concurrent connections. */
+  uint user_conn;
+  /*
+     Values of this enum and specified_limits member are used by the
+     parser to store which user limits were specified in GRANT statement.
+  */
+  enum {QUERIES_PER_HOUR= 1, UPDATES_PER_HOUR= 2, CONNECTIONS_PER_HOUR= 4,
+        USER_CONNECTIONS= 8};
+  uint specified_limits;
 } USER_RESOURCES;
 
+
+/*
+  This structure is used for counting resources consumed and for checking
+  them against specified user limits.
+*/
 typedef struct  user_conn {
-  char *user, *host;
-  uint len, connections, conn_per_hour, updates, questions, user_len;
+  /*
+     Pointer to user+host key (pair separated by '\0') defining the entity
+     for which resources are counted (By default it is user account thus
+     priv_user/priv_host pair is used. If --old-style-user-limits option
+     is enabled, resources are counted for each user+host separately).
+  */
+  char *user;
+  /* Pointer to host part of the key. */
+  char *host;
+  /* Total length of the key. */
+  uint len;
+  /* Current amount of concurrent connections for this account. */
+  uint connections;
+  /*
+     Current number of connections per hour, number of updating statements
+     per hour and total number of statements per hour for this account.
+  */
+  uint conn_per_hour, updates, questions;
+  /* Maximum amount of resources which account is allowed to consume. */
   USER_RESOURCES user_resources;
+  /*
+     The moment of time when per hour counters were reset last time
+     (i.e. start of "hour" for conn_per_hour, updates, questions counters).
+  */
   time_t intime;
 } USER_CONN;
+
 	/* Bits in form->update */
 #define REG_MAKE_DUPP		1	/* Make a copy of record when read */
 #define REG_NEW_RECORD		2	/* Write a new record if not found */