Bug #47930: MATCH IN BOOLEAN MODE returns too many results

inside subquery

Re-setting a fulltext index was a no-operation if not all
the matches of a search were consumed by reading them.
This was preventing a joined table using a fulltext index
in a subquery that requires only 1 row of output (e.g. EXISTS) 
from working correctly because the second execution of the 
sub-query has the fulltext index cursor in a wrong state and
was not finding results.
Fixed by making the re-init code _ftb_init_index_search() 
to re-set open cursors in addition to depleted ones.

mysql-test/t/fulltext.test

@@ -484,3 +484,44 @@ PREPARE s FROM
 EXECUTE s;
 DEALLOCATE PREPARE s;
 DROP TABLE t1;
+
+--echo #
+--echo # Bug #47930: MATCH IN BOOLEAN MODE returns too many results 
+--echo #  inside subquery
+--echo #
+
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+
+CREATE TABLE t2 (a int, b2 char(10), FULLTEXT KEY b2 (b2));
+INSERT INTO t2 VALUES (1,'Scargill');
+
+CREATE TABLE t3 (a int, b int);
+INSERT INTO t3 VALUES (1,1), (2,1);
+
+--echo # t2 should use full text index
+EXPLAIN
+SELECT count(*) FROM t1 WHERE 
+  not exists(
+   SELECT 1 FROM t2, t3
+   WHERE t3.a=t1.a AND MATCH(b2) AGAINST('scargill' IN BOOLEAN MODE)
+  );
+
+--echo # should return 0
+SELECT count(*) FROM t1 WHERE 
+  not exists(
+   SELECT 1 FROM t2, t3
+   WHERE t3.a=t1.a AND MATCH(b2) AGAINST('scargill' IN BOOLEAN MODE)
+  );
+
+--echo # should return 0
+SELECT count(*) FROM t1 WHERE 
+  not exists(
+   SELECT 1 FROM t2 IGNORE INDEX (b2), t3
+   WHERE t3.a=t1.a AND MATCH(b2) AGAINST('scargill' IN BOOLEAN MODE)
+  );
+
+DROP TABLE t1,t2,t3;
+
+
+--echo End of 5.1 tests