ha_innodb.cc:

  write_row(): document the ALTER TABLE tweak better,
  and commit every n*10000 rows, not n*10000-1 rows.

sql/ha_innodb.cc

@@ -2314,10 +2314,8 @@ ha_innobase::write_row(
         if (table->timestamp_field_type & TIMESTAMP_AUTO_SET_ON_INSERT)
                 table->timestamp_field->set_time();
 
-	num_write_row++;
-
 	if (user_thd->lex->sql_command == SQLCOM_ALTER_TABLE
-	    && num_write_row > 10000) {
+	    && num_write_row >= 10000) {
 		/* ALTER TABLE is COMMITted at every 10000 copied rows.
 		The IX table lock for the original table has to be re-issued.
 		As this method will be called on a temporary table where the
@@ -2331,14 +2329,20 @@ ha_innobase::write_row(
 		dict_table_t* table = lock_get_ix_table(
 				UT_LIST_GET_FIRST(prebuilt->trx->trx_locks));
 		num_write_row = 0;
+		/* Commit the transaction.  This will release the table
+		locks, so they have to be acquired again. */
 		innobase_commit(user_thd, prebuilt->trx);
+		/* Note that this transaction is still active. */
 		user_thd->transaction.all.innodb_active_trans = 1;
+		/* Re-acquire the IX table lock on the source table. */
 		row_lock_table_for_mysql(prebuilt, table);
-		goto new_trx;
+		/* We will need an IX lock on the destination table. */
+	        prebuilt->sql_stat_start = TRUE;
 	}
 
+	num_write_row++;
+
 	if (last_query_id != user_thd->query_id) {
-	new_trx:
 	        prebuilt->sql_stat_start = TRUE;
                 last_query_id = user_thd->query_id;