[t:4953], add some comments

git-svn-id: file:///svn/toku/tokudb@44205 c7de825b-a66e-492c-adef-691d508d4ae1

ft/log-internal.h

@@ -172,7 +172,8 @@ struct tokutxn {
     // Protected by the txn manager lock:
     TOKUTXN_STATE state;
     struct toku_list prepared_txns_link; // list of prepared transactions
-    uint32_t num_pin;
+    uint32_t num_pin; // number of threads (all hot indexes) that want this
+                      // txn to not transition to commit or abort
 };
 
 static inline int

ft/txn_manager.c

@@ -774,11 +774,17 @@ exit:
 }
 
 // needed for hot indexing
+
+// prevents a client thread from transitioning txn from LIVE|PREPAREING -> COMMITTING|ABORTING
+// hot indexing may need a transactions to stay in the LIVE|PREPARING state while it processes
+// a leafentry.
 void toku_txn_manager_pin_live_txn_unlocked(TXN_MANAGER UU(txn_manager), TOKUTXN txn) {
     assert(txn->state == TOKUTXN_LIVE || txn->state == TOKUTXN_PREPARING);
     txn->num_pin++;
 }
 
+// allows a client thread to go back to being able to transition txn 
+// from LIVE|PREPAREING -> COMMITTING|ABORTING
 void toku_txn_manager_unpin_live_txn_unlocked(TXN_MANAGER txn_manager, TOKUTXN txn) {
     assert(txn->state == TOKUTXN_LIVE || txn->state == TOKUTXN_PREPARING);
     assert(txn->num_pin > 0);