MDEV-23447 SIGSEGV in fil_system_t::keyrotate_next()

fil_system_t::keyrotate_next(): If space && space->is_in_rotation_list
does not hold, iterate from the start of the list.

In debug builds, we would typically have hit SIGSEGV because the
iterator would have wrapped a null pointer. It might also be that
we are dereferencing a stale pointer.

There is no test case, because the encryption is very nondeterministic
in nature, due to the use of background threads.

This scenario can be hit by setting the following:

SET GLOBAL innodb_encryption_threads=5;
SET GLOBAL innodb_encryption_rotate_key_age=0;

storage/innobase/fil/fil0crypt.cc

@@ -1388,29 +1388,34 @@ the encryption parameters were changed
 inline fil_space_t *fil_system_t::keyrotate_next(fil_space_t *space,
                                                  bool recheck, bool encrypt)
 {
-  ut_ad(mutex_own(&fil_system->mutex));
+  ut_ad(mutex_own(&mutex));
 
   sized_ilist<fil_space_t, rotation_list_tag_t>::iterator it=
-    space ? space : fil_system->rotation_list.begin();
+    space && space->is_in_rotation_list ? space : rotation_list.begin();
   const sized_ilist<fil_space_t, rotation_list_tag_t>::iterator end=
-    fil_system->rotation_list.end();
+    rotation_list.end();
 
   if (space)
   {
-    while (++it != end && (!UT_LIST_GET_LEN(it->chain) || it->is_stopping()));
+    const bool released= !--space->n_pending_ops;
 
-    /* If one of the encryption threads already started the encryption
-    of the table then don't remove the unencrypted spaces from rotation list
-
-    If there is a change in innodb_encrypt_tables variables value then
-    don't remove the last processed tablespace from the rotation list. */
-    if (!--space->n_pending_ops &&
-        (!recheck || space->crypt_data) && !encrypt == !srv_encrypt_tables &&
-        space->is_in_rotation_list)
+    if (space->is_in_rotation_list)
     {
-      ut_a(!fil_system->rotation_list.empty());
-      fil_system->rotation_list.remove(*space);
-      space->is_in_rotation_list= false;
+      while (++it != end &&
+             (!UT_LIST_GET_LEN(it->chain) || it->is_stopping()));
+
+      /* If one of the encryption threads already started the encryption
+      of the table then don't remove the unencrypted spaces from rotation list
+
+      If there is a change in innodb_encrypt_tables variables value then
+      don't remove the last processed tablespace from the rotation list. */
+      if (released && (!recheck || space->crypt_data) &&
+          !encrypt == !srv_encrypt_tables)
+      {
+        ut_a(!rotation_list.empty());
+        rotation_list.remove(*space);
+        space->is_in_rotation_list= false;
+      }
     }
   }