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: initialize vcols before key initialization

Also key initialization is moved to a function.

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

@@ -3034,6 +3034,66 @@ static bool check_vcol_forward_refs(Field *field, Virtual_column_info *vcol)
   return res;
 }
 
+/**
+ Copy keys from share to a table, so that fields in key_parts would match
+ fields in a table.
+
+ Also if key_part uses a field prefix, clone a field and make its length match
+ that prefix.
+
+ @retval true if success
+ @retval false if memory allocation fails
+ */
+static bool initialize_keys(TABLE_SHARE *share, TABLE *outparam)
+{
+  if (share->key_parts == 0)
+    return true;
+  uint n_length= share->keys * sizeof(KEY)
+                 + share->ext_key_parts * sizeof(KEY_PART_INFO);
+
+  KEY *key_info= (KEY*) alloc_root(&outparam->mem_root, n_length);
+  if (!key_info)
+    return false;
+
+  outparam->key_info= key_info;
+
+  KEY *key_info_end= key_info + share->keys;
+  KEY_PART_INFO *key_part= reinterpret_cast<KEY_PART_INFO*>(key_info_end);
+
+  memcpy(key_info, share->key_info, sizeof(*key_info) * share->keys);
+  memcpy(key_part, share->key_info[0].key_part, (sizeof(*key_part) *
+                                                 share->ext_key_parts));
+
+  for ( ; key_info < key_info_end; key_info++)
+  {
+    key_info->table= outparam;
+    key_info->key_part= key_part;
+
+    KEY_PART_INFO *key_part_end= key_part + (share->use_ext_keys
+                                            ? key_info->ext_key_parts
+                                            : key_info->user_defined_key_parts);
+    for ( ; key_part < key_part_end; key_part++)
+    {
+      Field *field= key_part->field= outparam->field[key_part->fieldnr - 1];
+
+      if (field->key_length() != key_part->length &&
+          !(field->flags & BLOB_FLAG))
+      {
+        /*
+          We are using only a prefix of the column as a key:
+          Create a new field for the key part that matches the index
+        */
+        field= key_part->field=field->make_new_field(&outparam->mem_root,
+                                                     outparam, 0);
+        field->field_length= key_part->length;
+      }
+    }
+    if (!share->use_ext_keys)
+      key_part+= key_info->ext_key_parts - key_info->user_defined_key_parts;
+  }
+  return true;
+}
+
 /*
   Open a table based on a TABLE_SHARE
 
@@ -3073,6 +3133,7 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
   Field **field_ptr;
   uint8 save_context_analysis_only= thd->lex->context_analysis_only;
   TABLE_SHARE::enum_v_keys check_set_initialized= share->check_set_initialized;
+  bool success;
   DBUG_ENTER("open_table_from_share");
   DBUG_PRINT("enter",("name: '%s.%s'  form: %p", share->db.str,
                       share->table_name.str, outparam));
@@ -3182,54 +3243,6 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
     outparam->found_next_number_field=
       outparam->field[(uint) (share->found_next_number_field - share->field)];
 
-  /* Fix key->name and key_part->field */
-  if (share->key_parts)
-  {
-    KEY	*key_info, *key_info_end;
-    KEY_PART_INFO *key_part;
-    uint n_length;
-    n_length= share->keys*sizeof(KEY) + share->ext_key_parts*sizeof(KEY_PART_INFO);
-    if (!(key_info= (KEY*) alloc_root(&outparam->mem_root, n_length)))
-      goto err;
-    outparam->key_info= key_info;
-    key_part= (reinterpret_cast<KEY_PART_INFO*>(key_info+share->keys));
-
-    memcpy(key_info, share->key_info, sizeof(*key_info)*share->keys);
-    memcpy(key_part, share->key_info[0].key_part, (sizeof(*key_part) *
-                                                   share->ext_key_parts));
-
-    for (key_info_end= key_info + share->keys ;
-         key_info < key_info_end ;
-         key_info++)
-    {
-      KEY_PART_INFO *key_part_end;
-
-      key_info->table= outparam;
-      key_info->key_part= key_part;
-
-      key_part_end= key_part + (share->use_ext_keys ? key_info->ext_key_parts :
-			                              key_info->user_defined_key_parts) ;
-      for ( ; key_part < key_part_end; key_part++)
-      {
-        Field *field= key_part->field= outparam->field[key_part->fieldnr - 1];
-
-        if (field->key_length() != key_part->length &&
-            !(field->flags & BLOB_FLAG))
-        {
-          /*
-            We are using only a prefix of the column as a key:
-            Create a new field for the key part that matches the index
-          */
-          field= key_part->field=field->make_new_field(&outparam->mem_root,
-                                                       outparam, 0);
-          field->field_length= key_part->length;
-        }
-      }
-      if (!share->use_ext_keys)
-	key_part+= key_info->ext_key_parts - key_info->user_defined_key_parts;
-    }
-  }
-
   /*
     Process virtual and default columns, if any.
   */
@@ -3286,6 +3299,10 @@ enum open_frm_error open_table_from_share(THD *thd, TABLE_SHARE *share,
     switch_defaults_to_nullable_trigger_fields(outparam);
   }
 
+  success= initialize_keys(share, outparam);
+  if (!success)
+    goto err;
+
 #ifdef WITH_PARTITION_STORAGE_ENGINE
   if (share->partition_info_str_len && outparam->file)
   {