Adjust consensus rule to accept genesis block without height in coinbase

Closes #1753

src/gtest/test_checkblock.cpp

@@ -29,3 +29,46 @@ TEST(CheckBlock, VersionTooLow) {
     EXPECT_CALL(state, DoS(100, false, REJECT_INVALID, "version-too-low", false)).Times(1);
     EXPECT_FALSE(CheckBlock(block, state, false, false));
 }
+
+TEST(ContextualCheckBlock, BadCoinbaseHeight) {
+    SelectParams(CBaseChainParams::MAIN);
+
+    // Create a block with no height in scriptSig
+    CMutableTransaction mtx;
+    mtx.vin.resize(1);
+    mtx.vin[0].prevout.SetNull();
+    mtx.vin[0].scriptSig = CScript() << OP_0;
+    mtx.vout.resize(1);
+    mtx.vout[0].scriptPubKey = CScript() << OP_TRUE;
+    mtx.vout[0].nValue = 0;
+    CTransaction tx {mtx};
+    CBlock block;
+    block.vtx.push_back(tx);
+
+    // Treating block as genesis should pass
+    MockCValidationState state;
+    EXPECT_TRUE(ContextualCheckBlock(block, state, NULL));
+
+    // Treating block as non-genesis should fail
+    mtx.vout.push_back(CTxOut(GetBlockSubsidy(1, Params().GetConsensus())/5, Params().GetFoundersRewardScriptAtHeight(1)));
+    CTransaction tx2 {mtx};
+    block.vtx[0] = tx2;
+    CBlock prev;
+    CBlockIndex indexPrev {prev};
+    indexPrev.nHeight = 0;
+    EXPECT_CALL(state, DoS(100, false, REJECT_INVALID, "bad-cb-height", false)).Times(1);
+    EXPECT_FALSE(ContextualCheckBlock(block, state, &indexPrev));
+
+    // Setting to an incorrect height should fail
+    mtx.vin[0].scriptSig = CScript() << 2 << OP_0;
+    CTransaction tx3 {mtx};
+    block.vtx[0] = tx3;
+    EXPECT_CALL(state, DoS(100, false, REJECT_INVALID, "bad-cb-height", false)).Times(1);
+    EXPECT_FALSE(ContextualCheckBlock(block, state, &indexPrev));
+
+    // After correcting the scriptSig, should pass
+    mtx.vin[0].scriptSig = CScript() << 1 << OP_0;
+    CTransaction tx4 {mtx};
+    block.vtx[0] = tx4;
+    EXPECT_TRUE(ContextualCheckBlock(block, state, &indexPrev));
+}

src/main.cpp

@@ -3090,7 +3090,10 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
 
     // Enforce block.nVersion=2 rule that the coinbase starts with serialized block height
     // if 750 of the last 1,000 blocks are version 2 or greater (51/100 if testnet):
-    if (block.nVersion >= 2)
+    // Since MIN_BLOCK_VERSION = 4 all blocks with nHeight > 0 should satisfy this.
+    // This rule is not applied to the genesis block, which didn't include the height
+    // in the coinbase.
+    if (nHeight > 0)
     {
         CScript expect = CScript() << nHeight;
         if (block.vtx[0].vin[0].scriptSig.size() < expect.size() ||