Bug#44672: Assertion failed: thd->transaction.xid_state.xid.is_null()

The problem is that when a optimization of read-only transactions
(bypass 2-phase commit) was implemented, it removed the code that
reseted the XID once a transaction wasn't active anymore:

sql/sql_parse.cc:

-  bzero(&thd->transaction.stmt, sizeof(thd->transaction.stmt));
-  if (!thd->active_transaction())
-    thd->transaction.xid_state.xid.null();
+  thd->transaction.stmt.reset();

This mostly worked fine as the transaction commit and rollback
functions (in handler.cc) reset the XID once the transaction is
ended. But those functions wouldn't reset the XID in case of
a empty transaction, leading to a assertion when a new starting
a new XA transaction.

The solution is to ensure that the XID state is reset when empty
transactions are ended (by either commit or rollback). This is
achieved by reorganizing the code so that the transaction cleanup
routine is invoked whenever a transaction is ended.

mysql-test/r/xa.result

@@ -75,3 +75,9 @@ xa rollback 'a','c';
 xa start 'a','c';
 drop table t1;
 End of 5.0 tests
+xa start 'a';
+xa end 'a';
+xa rollback 'a';
+xa start 'a';
+xa end 'a';
+xa rollback 'a';

mysql-test/t/xa.test

@@ -124,6 +124,17 @@ drop table t1;
 
 --echo End of 5.0 tests
 
+#
+# Bug#44672: Assertion failed: thd->transaction.xid_state.xid.is_null()
+#
+
+xa start 'a';
+xa end 'a';
+xa rollback 'a';
+xa start 'a';
+xa end 'a';
+xa rollback 'a';
+
 # Wait till all disconnects are completed
 --source include/wait_until_count_sessions.inc
 

sql/handler.cc

@@ -1073,6 +1073,13 @@ int ha_commit_trans(THD *thd, bool all)
     user, or an implicit commit issued by a DDL.
   */
   THD_TRANS *trans= all ? &thd->transaction.all : &thd->transaction.stmt;
+  /*
+    "real" is a nick name for a transaction for which a commit will
+    make persistent changes. E.g. a 'stmt' transaction inside a 'all'
+    transation is not 'real': even though it's possible to commit it,
+    the changes are not durable as they might be rolled back if the
+    enclosing 'all' transaction is rolled back.
+  */
   bool is_real_trans= all || thd->transaction.all.ha_list == 0;
   Ha_trx_info *ha_info= trans->ha_list;
   my_xid xid= thd->transaction.xid_state.xid.get_my_xid();
@@ -1184,16 +1191,9 @@ end:
     if (rw_trans)
       start_waiting_global_read_lock(thd);
   }
-  else if (all)
-  {
-    /*
-      A COMMIT of an empty transaction. There may be savepoints.
-      Destroy them. If the transaction is not empty
-      savepoints are cleared in ha_commit_one_phase()
-      or ha_rollback_trans().
-    */
+  /* Free resources and perform other cleanup even for 'empty' transactions. */
+  else if (is_real_trans)
     thd->transaction.cleanup();
-  }
 #endif /* USING_TRANSACTIONS */
   DBUG_RETURN(error);
 }
@@ -1206,6 +1206,13 @@ int ha_commit_one_phase(THD *thd, bool all)
 {
   int error=0;
   THD_TRANS *trans=all ? &thd->transaction.all : &thd->transaction.stmt;
+  /*
+    "real" is a nick name for a transaction for which a commit will
+    make persistent changes. E.g. a 'stmt' transaction inside a 'all'
+    transation is not 'real': even though it's possible to commit it,
+    the changes are not durable as they might be rolled back if the
+    enclosing 'all' transaction is rolled back.
+  */
   bool is_real_trans=all || thd->transaction.all.ha_list == 0;
   Ha_trx_info *ha_info= trans->ha_list, *ha_info_next;
   DBUG_ENTER("ha_commit_one_phase");
@@ -1227,8 +1234,6 @@ int ha_commit_one_phase(THD *thd, bool all)
     }
     trans->ha_list= 0;
     trans->no_2pc=0;
-    if (is_real_trans)
-      thd->transaction.xid_state.xid.null();
     if (all)
     {
 #ifdef HAVE_QUERY_CACHE
@@ -1236,8 +1241,9 @@ int ha_commit_one_phase(THD *thd, bool all)
         query_cache.invalidate(thd->transaction.changed_tables);
 #endif
       thd->variables.tx_isolation=thd->session_tx_isolation;
-      thd->transaction.cleanup();
     }
+    if (is_real_trans)
+      thd->transaction.cleanup();
   }
 #endif /* USING_TRANSACTIONS */
   DBUG_RETURN(error);
@@ -1249,6 +1255,13 @@ int ha_rollback_trans(THD *thd, bool all)
   int error=0;
   THD_TRANS *trans=all ? &thd->transaction.all : &thd->transaction.stmt;
   Ha_trx_info *ha_info= trans->ha_list, *ha_info_next;
+  /*
+    "real" is a nick name for a transaction for which a commit will
+    make persistent changes. E.g. a 'stmt' transaction inside a 'all'
+    transation is not 'real': even though it's possible to commit it,
+    the changes are not durable as they might be rolled back if the
+    enclosing 'all' transaction is rolled back.
+  */
   bool is_real_trans=all || thd->transaction.all.ha_list == 0;
   DBUG_ENTER("ha_rollback_trans");
 
@@ -1294,18 +1307,13 @@ int ha_rollback_trans(THD *thd, bool all)
     }
     trans->ha_list= 0;
     trans->no_2pc=0;
-    if (is_real_trans)
-    {
-      if (thd->transaction_rollback_request)
-        thd->transaction.xid_state.rm_error= thd->main_da.sql_errno();
-      else
-        thd->transaction.xid_state.xid.null();
-    }
+    if (is_real_trans && thd->transaction_rollback_request)
+      thd->transaction.xid_state.rm_error= thd->main_da.sql_errno();
     if (all)
       thd->variables.tx_isolation=thd->session_tx_isolation;
   }
   /* Always cleanup. Even if there nht==0. There may be savepoints. */
-  if (all)
+  if (is_real_trans)
     thd->transaction.cleanup();
 #endif /* USING_TRANSACTIONS */
   if (all)

sql/sql_class.h

@@ -1464,6 +1464,14 @@ public:
     {
       changed_tables= 0;
       savepoints= 0;
+      /*
+        If rm_error is raised, it means that this piece of a distributed
+        transaction has failed and must be rolled back. But the user must
+        rollback it explicitly, so don't start a new distributed XA until
+        then.
+      */
+      if (!xid_state.rm_error)
+        xid_state.xid.null();
 #ifdef USING_TRANSACTIONS
       free_root(&mem_root,MYF(MY_KEEP_PREALLOC));
 #endif