MDEV-31403: Server crashes in st_join_table::choose_best_splitting

The code in choose_best_splitting() assumed that the join prefix is
in join->positions[].

This is not necessarily the case. This function might be called when
the join prefix is in join->best_positions[], too.
Follow the approach from best_access_path(), which calls this function:
pass the current join prefix as an argument,
"const POSITION *join_positions" and use that.

mysql-test/main/derived_split_innodb.result

@@ -810,4 +810,19 @@ SELECT t1.* FROM t1 JOIN (SELECT id, COUNT(*) FROM t2 GROUP BY id) sq ON sq.id=
 a
 set optimizer_switch= @tmp1, join_cache_level= @tmp2;
 DROP TABLE t1, t2;
+#
+# MDEV-31403: Server crashes in st_join_table::choose_best_splitting (still)
+#
+CREATE TABLE t1 (a INT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+(1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),(13),(14),(15);
+CREATE TABLE t2 (b INT) ENGINE=InnoDB;
+INSERT INTO t2 VALUES (100),(200);
+CREATE TABLE t3 (c INT, d INT, KEY(c)) ENGINE=InnoDB;
+INSERT INTO t3 VALUES (1,1),(2,2);
+CREATE VIEW v AS SELECT c, d FROM t3 GROUP BY c, d;
+SELECT * FROM t1 JOIN t2 WHERE (t1.a, t2.b) IN (SELECT * FROM v);
+a	b
+DROP VIEW v;
+DROP TABLE t1, t2, t3;
 # End of 10.4 tests

mysql-test/main/derived_split_innodb.test

@@ -462,4 +462,25 @@ set optimizer_switch= @tmp1, join_cache_level= @tmp2;
 # Cleanup
 DROP TABLE t1, t2;
 
+--echo #
+--echo # MDEV-31403: Server crashes in st_join_table::choose_best_splitting (still)
+--echo #
+CREATE TABLE t1 (a INT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+  (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),(13),(14),(15);
+ 
+CREATE TABLE t2 (b INT) ENGINE=InnoDB;
+INSERT INTO t2 VALUES (100),(200);
+ 
+CREATE TABLE t3 (c INT, d INT, KEY(c)) ENGINE=InnoDB;
+INSERT INTO t3 VALUES (1,1),(2,2);
+ 
+CREATE VIEW v AS SELECT c, d FROM t3 GROUP BY c, d;
+ 
+SELECT * FROM t1 JOIN t2 WHERE (t1.a, t2.b) IN (SELECT * FROM v);
+ 
+# Cleanup
+DROP VIEW v;
+DROP TABLE t1, t2, t3;
+
 --echo # End of 10.4 tests

sql/opt_split.cc

@@ -948,6 +948,7 @@ void reset_validity_vars_for_keyuses(KEYUSE_EXT *key_keyuse_ext_start,
 
 SplM_plan_info * JOIN_TAB::choose_best_splitting(uint idx,
                                                  table_map remaining_tables,
+                                                 const POSITION *join_positions,
                                                  table_map *spl_pd_boundary)
 {
   SplM_opt_info *spl_opt_info= table->spl_opt_info;
@@ -1045,7 +1046,7 @@ SplM_plan_info * JOIN_TAB::choose_best_splitting(uint idx,
     else
     {
       table_map last_found= this->table->map;
-      for (POSITION *pos= &this->join->positions[idx - 1]; ; pos--)
+      for (const POSITION *pos= &join_positions[idx - 1]; ; pos--)
       {
         if (pos->table->table->map & excluded_tables)
           continue;

sql/sql_select.cc

@@ -7452,6 +7452,7 @@ best_access_path(JOIN      *join,
   if (s->table->is_splittable())
     spl_plan= s->choose_best_splitting(idx,
                                        remaining_tables,
+                                       join_positions,
                                        &spl_pd_boundary);
 
   Json_writer_array trace_paths(thd, "considered_access_paths");

sql/sql_select.h

@@ -694,6 +694,7 @@ typedef struct st_join_table {
   void add_keyuses_for_splitting();
   SplM_plan_info *choose_best_splitting(uint idx,
                                         table_map remaining_tables,
+                                        const POSITION *join_positions,
                                         table_map *spl_pd_boundary);
   bool fix_splitting(SplM_plan_info *spl_plan, table_map excluded_tables,
                      bool is_const_table);