[t:2811], add some comments

git-svn-id: file:///svn/toku/tokudb@26097 c7de825b-a66e-492c-adef-691d508d4ae1

newbrt/rollback.c

@@ -241,6 +241,13 @@ void toku_rollback_txn_close (TOKUTXN txn) {
             r = toku_omt_delete_at(txn->logger->live_root_txns, idx);
             assert(r==0);
         }
+        //
+        // if this txn created a snapshot, make necessary modifications to list of snapshot txnids and live_list_reverse
+        // the order of operations is important. We first remove the txnid from the list of snapshot txnids. This is
+        // necessary because root snapshot transactions are in their own live lists. If we do not remove 
+        // the txnid from the snapshot txnid list first, then when we go to make the modifications to 
+        // live_list_reverse, we have trouble. We end up never removing (id, id) from live_list_reverse
+        //
         if (txn->snapshot_type != TXN_SNAPSHOT_NONE && (txn->parent==NULL || txn->snapshot_type == TXN_SNAPSHOT_CHILD)) {
             {
                 u_int32_t idx;

newbrt/txn.c

@@ -196,6 +196,21 @@ int toku_txn_begin_with_xid (
         r = toku_omt_insert_at(logger->live_txns, result, toku_omt_size(logger->live_txns));
         if (r!=0) goto died;
 
+        //
+        // maintain the data structures necessary for MVCC:
+        //  1. add txn to list of live_root_txns if this is a root transaction
+        //  2. if the transaction is creating a snapshot:
+        //    - create a live list for the transaction
+        //    - add the id to the list of snapshot ids
+        //    - make the necessary modifications to the live_list_reverse
+        //
+        // The order of operations is important here, and must be taken
+        // into account when the transaction is closed. The txn is added
+        // to the live_root_txns first (if it is a root txn). This has the implication
+        // that a root level snapshot transaction is in its own live list. This fact
+        // is taken into account when the transaction is closed.
+        //
+
         // add ancestor information, and maintain global live root txn list
         if (parent_tokutxn==NULL) {
             //Add txn to list (omt) of live root txns