MDEV-8577: With enforce-storage-engine mysql_upgrade corrupts the schema:

ALTER TABLE should either bypass enforce-storage-engine, or mysql_upgrade
should refuse to run

Allow user to alter contents of existing table without enforcing
storage engine. However, enforce storage engine on ALTER TABLE
x ENGINE=y;

1 files changed, 9 insertions, 2 deletions

diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index da7d144f3d8..61120970ecb 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -9818,14 +9818,21 @@ static bool check_engine(THD *thd, const char *db_name,
   DBUG_ENTER("check_engine");
   handlerton **new_engine= &create_info->db_type;
   handlerton *req_engine= *new_engine;
-  handlerton *enf_engine= thd->variables.enforced_table_plugin ?
-    plugin_hton(thd->variables.enforced_table_plugin) : NULL;
+  handlerton *enf_engine= NULL;
   bool no_substitution= thd->variables.sql_mode & MODE_NO_ENGINE_SUBSTITUTION;
   *new_engine= ha_checktype(thd, req_engine, no_substitution);
   DBUG_ASSERT(*new_engine);
   if (!*new_engine)
     DBUG_RETURN(true);
 
+  /* Enforced storage engine should not be used in
+  ALTER TABLE that does not use explicit ENGINE = x to
+  avoid unwanted unrelated changes.*/
+  if (!(thd->lex->sql_command == SQLCOM_ALTER_TABLE &&
+        !(create_info->used_fields & HA_CREATE_USED_ENGINE)))
+    enf_engine= thd->variables.enforced_table_plugin ?
+       plugin_hton(thd->variables.enforced_table_plugin) : NULL;
+
   if (enf_engine && enf_engine != *new_engine)
   {
     if (no_substitution)