MDEV-24422 Server crashes in ha_connect::GetRealType upon ALTER TABLE

The problem was that the CONNECT engine is trying to open the .frm file
during drop_table(), which the code did not take into account.
Fixed by adding the HA_REUSES_FILE_NAMES table flag to CONNECT.

Other things:
- Fixed a wrong test of HA_REUSE_FILE_NAMES of in mysql_alter_table()
  (Comment was correct, no the code)
- Added a test in the connect engine that if the .frm it tries to use in
  delete is not made for connect, it will generate an error instead of
  crash.

Author: montywi

sql/handler.h

@@ -307,7 +307,11 @@ enum chf_create_flags {
 
 #define HA_PERSISTENT_TABLE              (1ULL << 48)
 
-/* If storage engine uses another engine as a base */
+/*
+  If storage engine uses another engine as a base
+  This flag is also needed if the table tries to open the .frm file
+  as part of drop table.
+*/
 #define HA_REUSES_FILE_NAMES             (1ULL << 49)
 
 /*

sql/sql_table.cc

@@ -10900,12 +10900,11 @@ do_continue:;
       The above is mainly true for the sequence and the partition engine.
   */
   engine_changed= ((new_table->file->ht != table->file->ht) &&
-                   (((!(new_table->file->ha_table_flags() & HA_FILE_BASED) ||
-                      !(table->file->ha_table_flags() & HA_FILE_BASED))) ||
-                    (!(table->file->ha_table_flags() & HA_REUSES_FILE_NAMES) &&
-                     !(new_table->file->ha_table_flags() &
-                       HA_REUSES_FILE_NAMES))));
-
+                   ((!(new_table->file->ha_table_flags() & HA_FILE_BASED) ||
+                     !(table->file->ha_table_flags() & HA_FILE_BASED))) &&
+                   !(table->file->ha_table_flags() & HA_REUSES_FILE_NAMES) &&
+                   !(new_table->file->ha_table_flags() &
+                     HA_REUSES_FILE_NAMES));
   /*
     Close the intermediate table that will be the new table, but do
     not delete it! Even though MERGE tables do not have their children

storage/connect/ha_connect.cc

@@ -1169,7 +1169,8 @@ ulonglong ha_connect::table_flags() const
 //                   HA_NULL_IN_KEY |    not implemented yet
 //                   HA_FAST_KEY_READ |  causes error when sorting (???)
                      HA_NO_TRANSACTIONS | HA_DUPLICATE_KEY_NOT_IN_ORDER |
-                     HA_NO_BLOBS | HA_MUST_USE_TABLE_CONDITION_PUSHDOWN;
+                     HA_NO_BLOBS | HA_MUST_USE_TABLE_CONDITION_PUSHDOWN |
+                     HA_REUSES_FILE_NAMES;
   ha_connect *hp= (ha_connect*)this;
   PTOS        pos= hp->GetTableOptionStruct();
 
@@ -5244,6 +5245,14 @@ int ha_connect::delete_or_rename_table(const char *name, const char *to)
     thd->push_internal_handler(&error_handler);
     bool got_error= open_table_def(thd, share);
     thd->pop_internal_handler();
+    if (!got_error && share->db_type() != connect_hton)
+    {
+      /* The .frm file is not for the connect engine. Something is wrong! */
+      got_error= 1;
+      rc= HA_ERR_INTERNAL_ERROR;
+      my_error(HA_ERR_INTERNAL_ERROR, MYF(0),
+               "TABLE_SHARE is not for the CONNECT engine");
+    }
     if (!got_error) {
       // Now we can work
       if ((pos= share->option_struct)) {

storage/connect/mysql-test/connect/r/alter_engine.result

@@ -0,0 +1,11 @@
+#
+# MDEV-24422 Server crashes in GetTypeID / ha_connect::GetRealType upon
+# altering table engine
+#
+CREATE TABLE t1 (f INT) ENGINE=CONNECT;
+Warnings:
+Warning	1105	No table_type. Will be set to DOS
+Warning	1105	No file name. Table will use t1.dos
+ALTER TABLE t1 ENGINE InnoDB;
+ALTER TABLE t1 ENGINE CONNECT;
+DROP TABLE t1;

storage/connect/mysql-test/connect/t/alter_engine.test

@@ -0,0 +1,11 @@
+--source include/have_innodb.inc
+
+--echo #
+--echo # MDEV-24422 Server crashes in GetTypeID / ha_connect::GetRealType upon
+--echo # altering table engine
+--echo #
+
+CREATE TABLE t1 (f INT) ENGINE=CONNECT;
+ALTER TABLE t1 ENGINE InnoDB;
+ALTER TABLE t1 ENGINE CONNECT;
+DROP TABLE t1;