Fix bug#54583. This change reverses r1530 by getting rid of a bogus assertion

and clarifies the invariant in dict_table_get_on_id().
  
In Mar 2007 Marko observed a crash during recovery, the crash resulted from
an UNDO operation on a system table. His solution was to acquire an X lock on
the data dictionary, this in hindsight was an overkill. It is unclear what
caused the crash, current hypothesis is that it was a memory corruption.
  
The X lock results in performance issues by when undoing changes due to
rollback during normal operation on regular tables.
  
Why the change is safe:
======================
The InnoDB code has changed since the original X lock change was made. In the
new code we always lock the data dictionary in X mode during startup when
UNDOing operations on the system tables (this is a given). This ensures that
the crash Marko observed cannot happen as long as all transactions that update
the system tables follow the standard rules by setting the appropriate DICT_OP
flag when writing the log records when they make the changes.
  
If transactions violate the above mentioned rule then during recovery (at
startup) the rollback code (see trx0roll.c) will not acquire the X lock
and we will see the crash again.  This will however be a different bug.

1 files changed, 4 insertions, 6 deletions

diff --git a/storage/innobase/dict/dict0dict.c b/storage/innobase/dict/dict0dict.c
index 064a5a34960..802f0bd8b6f 100644
--- a/storage/innobase/dict/dict0dict.c
+++ b/storage/innobase/dict/dict0dict.c
@@ -581,13 +581,11 @@ dict_table_get_on_id(
 
 	if (table_id <= DICT_FIELDS_ID
 	    || trx->dict_operation_lock_mode == RW_X_LATCH) {
-		/* It is a system table which will always exist in the table
-		cache: we avoid acquiring the dictionary mutex, because
-		if we are doing a rollback to handle an error in TABLE
-		CREATE, for example, we already have the mutex! */
 
-		ut_ad(mutex_own(&(dict_sys->mutex))
-		      || trx->dict_operation_lock_mode == RW_X_LATCH);
+		/* Note: An X latch implies that the transaction
+		already owns the dictionary mutex. */
+
+		ut_ad(mutex_own(&dict_sys->mutex));
 
 		return(dict_table_get_on_id_low(table_id));
 	}