fix for BUG#1331: "Unexistent user variable is not replicated".

When an update query is to be written to the binlog, and it reads unset
user variables (example: INSERT INTO t VALUEs(@A) where @A does not exist),
we create the variable like if it had been set with SET before, and we loop
(i.e. we redo the steps of logging, now that the variable exists).

mysql-test/r/rpl_user_variables.result

@@ -21,6 +21,7 @@ set @q:='abc';
 insert t1 values (@q), (@q:=concat(@q, 'n1')), (@q:=concat(@q, 'n2'));
 set @a:=5;
 insert into t1 values (@a),(@a);
+insert into t1 values (@a),(@a),(@a*5);
 select * from t1;
 n
 12345678901234
@@ -45,6 +46,36 @@ abcn1
 abcn1n2
 5
 5
+NULL
+NULL
+NULL
+select * from t1;
+n
+12345678901234
+-12345678901234
+0
+-1
+12.5
+-12.5
+This is a test
+
+abc'def
+abc\def
+abc'def
+NULL
+NULL
+0
+1
+2
+5
+abc
+abcn1
+abcn1n2
+5
+5
+NULL
+NULL
+NULL
 show binlog events from 141;
 Log_name	Pos	Event_type	Server_id	Orig_log_pos	Info
 slave-bin.000001	141	User var	2	141	@i1=12345678901234
@@ -63,13 +94,16 @@ slave-bin.000001	719	User var	2	719	@s5='abc'def'
 slave-bin.000001	761	Query	1	761	use `test`; insert into t1 values (@s1), (@s2), (@s3), (@s4), (@s5)
 slave-bin.000001	851	User var	2	851	@n1=NULL
 slave-bin.000001	877	Query	1	877	use `test`; insert into t1 values (@n1)
-slave-bin.000001	939	Query	1	939	use `test`; insert into t1 values (@n2)
-slave-bin.000001	1001	Query	1	1001	use `test`; insert into t1 values (@a:=0), (@a:=@a+1), (@a:=@a+1)
-slave-bin.000001	1089	User var	2	1089	@a=2
-slave-bin.000001	1131	Query	1	1131	use `test`; insert into t1 values (@a+(@b:=@a+1))
-slave-bin.000001	1203	User var	2	1203	@q='abc'
-slave-bin.000001	1240	Query	1	1240	use `test`; insert t1 values (@q), (@q:=concat(@q, 'n1')), (@q:=concat(@q, 'n2'))
-slave-bin.000001	1344	User var	2	1344	@a=5
-slave-bin.000001	1386	Query	1	1386	use `test`; insert into t1 values (@a),(@a)
+slave-bin.000001	939	User var	2	939	@n2=NULL
+slave-bin.000001	965	Query	1	965	use `test`; insert into t1 values (@n2)
+slave-bin.000001	1027	Query	1	1027	use `test`; insert into t1 values (@a:=0), (@a:=@a+1), (@a:=@a+1)
+slave-bin.000001	1115	User var	2	1115	@a=2
+slave-bin.000001	1157	Query	1	1157	use `test`; insert into t1 values (@a+(@b:=@a+1))
+slave-bin.000001	1229	User var	2	1229	@q='abc'
+slave-bin.000001	1266	Query	1	1266	use `test`; insert t1 values (@q), (@q:=concat(@q, 'n1')), (@q:=concat(@q, 'n2'))
+slave-bin.000001	1370	User var	2	1370	@a=5
+slave-bin.000001	1412	Query	1	1412	use `test`; insert into t1 values (@a),(@a)
+slave-bin.000001	1478	User var	2	1478	@a=NULL
+slave-bin.000001	1503	Query	1	1503	use `test`; insert into t1 values (@a),(@a),(@a*5)
 drop table t1;
 stop slave;

mysql-test/t/rpl_user_variables.test

@@ -29,13 +29,16 @@ insert into t1 values (@i1), (@i2), (@i3), (@i4);
 insert into t1 values (@r1), (@r2);
 insert into t1 values (@s1), (@s2), (@s3), (@s4), (@s5);
 insert into t1 values (@n1);
-insert into t1 values (@n2);
+insert into t1 values (@n2); # not explicitely set before
 insert into t1 values (@a:=0), (@a:=@a+1), (@a:=@a+1);
 insert into t1 values (@a+(@b:=@a+1));
 set @q:='abc';
 insert t1 values (@q), (@q:=concat(@q, 'n1')), (@q:=concat(@q, 'n2'));
 set @a:=5;
 insert into t1 values (@a),(@a);
+connection master1; # see if variable is reset in binlog when thread changes
+insert into t1 values (@a),(@a),(@a*5);
+select * from t1;
 save_master_pos;
 connection slave;
 sync_with_master;

sql/item_func.cc

@@ -2071,6 +2071,16 @@ static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
     entry->value=0;
     entry->length=0;
     entry->update_query_id=0;
+    /*
+      If we are here, we were called from a SET or a query which sets a
+      variable. Imagine it is this:
+      INSERT INTO t SELECT @a:=10, @a:=@a+1.
+      Then when we have a Item_func_get_user_var (because of the @a+1) so we
+      think we have to write the value of @a to the binlog. But before that,
+      we have a Item_func_set_user_var to create @a (@a:=10), in this we mark
+      the variable as "already logged" (line below) so that it won't be logged
+      by Item_func_get_user_var (because that's not necessary).
+    */
     entry->used_query_id=current_thd->query_id;
     entry->type=STRING_RESULT;
     memcpy(entry->name.str, name.str, name.length+1);
@@ -2083,7 +2093,10 @@ static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
   return entry;
 }
 
-
+/*
+  When a user variable is updated (in a SET command or a query like SELECT @a:=
+  ).
+*/
 
 bool Item_func_set_user_var::fix_fields(THD *thd, TABLE_LIST *tables,
 					Item **ref)
@@ -2093,6 +2106,11 @@ bool Item_func_set_user_var::fix_fields(THD *thd, TABLE_LIST *tables,
       !(entry= get_variable(&thd->user_vars, name, 1)))
     return 1;
   entry->type= cached_result_type;
+  /* 
+     Remember the last query which updated it, this way a query can later know
+     if this variable is a constant item in the query (it is if update_query_id
+     is different from query_id).
+  */
   entry->update_query_id=thd->query_id;
   return 0;
 }
@@ -2315,19 +2333,60 @@ longlong Item_func_get_user_var::val_int()
 }
 
 
+/*
+  When a user variable is invoked from an update query (INSERT, UPDATE etc),
+  stores this variable and its value in thd->user_var_events, so that it can be
+  written to the binlog (will be written just before the query is written, see
+  log.cc).
+*/
+
 void Item_func_get_user_var::fix_length_and_dec()
 {
-  BINLOG_USER_VAR_EVENT *user_var_event;
   THD *thd=current_thd;
+
+  if (!(opt_bin_log && is_update_query(thd->lex.sql_command)))
+       return;
+
+  BINLOG_USER_VAR_EVENT *user_var_event;
   maybe_null=1;
   decimals=NOT_FIXED_DEC;
   max_length=MAX_BLOB_WIDTH;
 
-  if ((var_entry= get_variable(&thd->user_vars, name, 0)))
-  {
-    if (opt_bin_log && is_update_query(thd->lex.sql_command) &&
-	var_entry->used_query_id != thd->query_id)
+  /*
+    If the variable does not exist, it's NULL, but we want to create it so
+    that it gets into the binlog (if it didn't, the slave could be
+    influenced by a variable of the same name previously set by another
+    thread).
+  */
+  
+  if (!(var_entry= get_variable(&thd->user_vars, name, 0)))
   {
+    /*
+      We create it like if it had been explicitely set with SET before.
+      The 'new' mimicks what sql_yacc.yy does when 'SET @a=10;'.
+      sql_set_variables() is what is called from 'case SQLCOM_SET_OPTION'
+      in dispatch_command()). Instead of building a one-element list to pass to
+      sql_set_variables(), we could instead manually call check() and update();
+      this would save memory and time; but calling sql_set_variables() makes one
+      unique place to maintain (sql_set_variables()). 
+    */
+
+    List<set_var_base> tmp_var_list;
+    tmp_var_list.push_back(new set_var_user(new Item_func_set_user_var(name,
+                                                                       new Item_null())));
+    if (sql_set_variables(thd, &tmp_var_list)) /* this will create the variable */
+      goto err;
+    if (!(var_entry= get_variable(&thd->user_vars, name, 0)))
+      goto err;
+  }
+  /* 
+     If this variable was already stored in user_var_events by this query
+     (because it's used in more than one place in the query), don't store
+     it.
+  */
+  else if (var_entry->used_query_id == thd->query_id)
+    return;
+
   uint size;
   /*
     First we need to store value of var_entry, when the next situation
@@ -2357,11 +2416,11 @@ void Item_func_get_user_var::fix_length_and_dec()
     memcpy(user_var_event->value, var_entry->value,
            var_entry->length);
   }
+  /* Mark that this variable has been used by this query */
   var_entry->used_query_id= thd->query_id;
   if (insert_dynamic(&thd->user_var_events, (gptr) &user_var_event))
     goto err;
-    }
-  }
+
   return;
 
 err: