MDEV-26220 Server crashes with indexed by prefix virtual column

Server crashes in Field::register_field_in_read_map upon select from
partitioned table with indexed by prefix virtual column.

After several read-mark fixes a problem has surfaced:
Since KEY (c(10),a) uses only a prefix of c, a new field is created,
duplicated from table->field[3], with a new length. However,
vcol_inco->expr is not copied.

Therefore, (*key_info)->key_part[i].field->vcol_info->expr was left NULL
in ha_partition::index_init().

Solution: copy vcol_info from table field when it's set up.

mysql-test/suite/gcol/inc/gcol_partition.inc

@@ -169,3 +169,15 @@ CREATE TABLE t1 (
 INSERT INTO t1 () VALUES (),();
 UPDATE t1 SET a = 0 WHERE b IS NULL ORDER BY pk;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-26220 Server crashes with indexed by prefix virtual column
+--echo #
+
+CREATE TABLE t1 (pk INT PRIMARY KEY, a INT, b CHAR(20), c CHAR(20) AS (b),
+                 KEY (c(10),a)) PARTITION BY HASH(pk);
+INSERT INTO t1 (pk,a,b) VALUES (1,10,'foo'),(2,11,'baz');
+SELECT a FROM t1;
+
+# Cleanup
+DROP TABLE t1;

mysql-test/suite/gcol/r/gcol_partition_innodb.result

@@ -104,6 +104,17 @@ INSERT INTO t1 () VALUES (),();
 UPDATE t1 SET a = 0 WHERE b IS NULL ORDER BY pk;
 DROP TABLE t1;
 #
+# MDEV-26220 Server crashes with indexed by prefix virtual column
+#
+CREATE TABLE t1 (pk INT PRIMARY KEY, a INT, b CHAR(20), c CHAR(20) AS (b),
+KEY (c(10),a)) PARTITION BY HASH(pk);
+INSERT INTO t1 (pk,a,b) VALUES (1,10,'foo'),(2,11,'baz');
+SELECT a FROM t1;
+a
+11
+10
+DROP TABLE t1;
+#
 # MDEV-16980 Wrongly set tablename len while opening the
 #                table for purge thread
 #

mysql-test/suite/gcol/r/gcol_partition_myisam.result

@@ -101,6 +101,17 @@ KEY (b,d)
 INSERT INTO t1 () VALUES (),();
 UPDATE t1 SET a = 0 WHERE b IS NULL ORDER BY pk;
 DROP TABLE t1;
+#
+# MDEV-26220 Server crashes with indexed by prefix virtual column
+#
+CREATE TABLE t1 (pk INT PRIMARY KEY, a INT, b CHAR(20), c CHAR(20) AS (b),
+KEY (c(10),a)) PARTITION BY HASH(pk);
+INSERT INTO t1 (pk,a,b) VALUES (1,10,'foo'),(2,11,'baz');
+SELECT a FROM t1;
+a
+11
+10
+DROP TABLE t1;
 DROP VIEW  IF EXISTS v1,v2;
 DROP TABLE IF EXISTS t1,t2,t3;
 DROP PROCEDURE IF EXISTS p1;

sql/table.cc

@@ -3284,6 +3284,21 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
 
     /* Update to use trigger fields */
     switch_defaults_to_nullable_trigger_fields(outparam);
+
+    for (uint k= 0; k < share->keys; k++)
+    {
+      KEY &key_info= outparam->key_info[k];
+      uint parts = (share->use_ext_keys ? key_info.ext_key_parts :
+                    key_info.user_defined_key_parts);
+      for (uint p= 0; p < parts; p++)
+      {
+        KEY_PART_INFO &kp= key_info.key_part[p];
+        if (kp.field != outparam->field[kp.fieldnr - 1])
+        {
+          kp.field->vcol_info = outparam->field[kp.fieldnr - 1]->vcol_info;
+        }
+      }
+    }
   }
 
 #ifdef WITH_PARTITION_STORAGE_ENGINE