Update BIP68 implementation to match spec

bip-0068.mediawiki

@@ -65,9 +65,9 @@ enum {
  * disables nLockTime. */
 static const uint32_t SEQUENCE_FINAL = 0xffffffff;
 
+/* Below flags apply in the context of BIP 68*/
 /* If this flag set, CTxIn::nSequence is NOT interpreted as a
- * relative lock-time. Setting the most significant bit of a
- * sequence number disabled relative lock-time. */
+ * relative lock-time. */
 static const uint32_t SEQUENCE_LOCKTIME_DISABLE_FLAG = (1 << 31);
 
 /* If CTxIn::nSequence encodes a relative lock-time and this flag
@@ -174,16 +174,17 @@ bool SequenceLocks(const CTransaction &tx, int flags, std::vector<int>* prevHeig
 bool CheckSequenceLocks(const CTransaction &tx, int flags)
 {
     AssertLockHeld(cs_main);
+    AssertLockHeld(mempool.cs);
 
     CBlockIndex* tip = chainActive.Tip();
     CBlockIndex index;
     index.pprev = tip;
     // CheckSequenceLocks() uses chainActive.Height()+1 to evaluate
     // height based locks because when SequenceLocks() is called within
-    // CBlock::AcceptBlock(), the height of the block *being*
-    // evaluated is what is used. Thus if we want to know if a
-    // transaction can be part of the *next* block, we need to call
-    // SequenceLocks() with one more than chainActive.Height().
+    // ConnectBlock(), the height of the block *being*
+    // evaluated is what is used.
+    // Thus if we want to know if a transaction can be part of the
+    // *next* block, we need to use one more than chainActive.Height()
     index.nHeight = tip->nHeight + 1;
 
     // pcoinsTip contains the UTXO set for chainActive.Tip()