Fixed bug #51855. Race condition in XA START. If several threads

concurrently execute the statement XA START 'x', then mysqld
server could crash.

sql/sql_class.cc:
  xid_cache_insert: added checking for element in cache before
  insert it, return TRUE if such element already exists.
sql/sql_parse.cc:
  mysql_execute_command modified:
  * sequence of calls to xid_cache_search(..)/xid_cache_insert(...)
  replaced by call to xid_cache_insert(...) in alternative
  'case SQLCOM_XA_START:'
  * added comment to alternative 'case SQLCOM_XA_COMMIT:'.

1 files changed, 7 insertions, 3 deletions

diff --git a/sql/sql_class.cc b/sql/sql_class.cc
index 93aa6a8268c..99792f2b262 100644
--- a/sql/sql_class.cc
+++ b/sql/sql_class.cc
@@ -3365,9 +3365,13 @@ bool xid_cache_insert(XID *xid, enum xa_states xa_state)
 bool xid_cache_insert(XID_STATE *xid_state)
 {
   pthread_mutex_lock(&LOCK_xid_cache);
-  DBUG_ASSERT(hash_search(&xid_cache, xid_state->xid.key(),
-                          xid_state->xid.key_length())==0);
-  my_bool res=my_hash_insert(&xid_cache, (uchar*)xid_state);
+  if (hash_search(&xid_cache, xid_state->xid.key(), xid_state->xid.key_length()))
+  {
+    pthread_mutex_unlock(&LOCK_xid_cache);
+    my_error(ER_XAER_DUPID, MYF(0));
+    return TRUE;
+  }
+  my_bool res= my_hash_insert(&xid_cache, (uchar*)xid_state);
   pthread_mutex_unlock(&LOCK_xid_cache);
   return res;
 }