Apply BIP30 checks to all blocks except the two historic violations.

Matt pointed out some time ago that there existed a minor DOS
attack where a node in its initial block download could be wedged
by an overwrite attack in a fork created between checkpoints before
a time where BIP30 was enforced. Now that the BIP30 timestamp
is irreversibly past the check can be more aggressive and apply to
all blocks except the two historic violations.

src/main.cpp

@@ -1360,9 +1360,12 @@ bool CBlock::ConnectBlock(CTxDB& txdb, CBlockIndex* pindex, bool fJustCheck)
     // See BIP30 and http://r6.ca/blog/20120206T005236Z.html for more information.
     // This logic is not necessary for memory pool transactions, as AcceptToMemoryPool
     // already refuses previously-known transaction ids entirely.
-    // This rule applies to all blocks whose timestamp is after March 15, 2012, 0:00 UTC.
+    // This rule was originally applied all blocks whose timestamp was after March 15, 2012, 0:00 UTC.
-    int64 nBIP30SwitchTime = 1331769600;
+    // Now that the whole chain is irreversibly beyond that time it is applied to all blocks except the
-    bool fEnforceBIP30 = (pindex->nTime > nBIP30SwitchTime);
+    // two in the chain that violate it. This prevents exploiting the issue against nodes in their
+    // initial block download.
+    bool fEnforceBIP30 = !((pindex->nHeight==91842 && pindex->GetBlockHash() == uint256("0x00000000000a4d0a398161ffc163c503763b1f4360639393e0e4c8e300e0caec")) ||
+                           (pindex->nHeight==91880 && pindex->GetBlockHash() == uint256("0x00000000000743f190a18c5577a3c2d2a1f610ae9601ac046a38084ccb7cd721")));
 
     // BIP16 didn't become active until Apr 1 2012
     int64 nBIP16SwitchTime = 1333238400;