Use proper meta input prevout (#1099)

A coinbase transaction in both bitcoin as well as in unit-e includes a meta input which does not spend anything and is skipped in validation. It's scriptSig contains block height and snapshot hash.

The prevout of that input should be `0x00000` (which it is) and `0xFFFF` as index (currently uses 0). This is not correct and changed in this pull request. By using the default constructor of `COutPoint`, which in turn invokes `SetNull` (which sets the afore mentioned values), the right values are being filled in + delegated to the definition of `SetNull (as it should).

The problem otherwise: Theoretically it is allowed for a transaction to have the hash `0x00...00` (although this will lead to other problems in the wallet...), but it is super unlikely that any transaction would ever have 2^16 number of inputs. That both happen is astronomically super black hole merger unlikely.

Signed-off-by: Julian Fleischer <julian@thirdhash.com>

src/proposer/block_builder.cpp

@@ -114,7 +114,7 @@ class BlockBuilderImpl : public BlockBuilder {
     {
       CScript script_sig = CScript() << CScriptNum::serialize(eligible_coin.target_height)
                                      << ToByteVector(snapshot_hash);
-      tx.vin.emplace_back(uint256(), 0, script_sig);
+      tx.vin.emplace_back(COutPoint(), script_sig);
     }
 
     // add stake

src/staking/block_validator.cpp

@@ -81,6 +81,10 @@ class BlockValidatorImpl : public AbstractBlockValidator {
 
     const CScript &script_sig = in.scriptSig;
 
+    if (!in.prevout.IsNull()) {
+      return BlockValidationResult(Error::INVALID_META_INPUT_PREVOUT);
+    }
+
     bool check;
     opcodetype op;
     std::vector<uint8_t> buf;

src/staking/validation_error.cpp

@@ -91,6 +91,10 @@ const ValidationError &GetValidationErrorFor(const staking::BlockValidationError
       static ValidationError err("bad-blk-public-key");
       return err;
     }
+    case staking::BlockValidationError::INVALID_META_INPUT_PREVOUT: {
+      static ValidationError err("bad-cb-meta-input-prevout");
+      return err;
+    }
     case staking::BlockValidationError::MISMATCHING_HEIGHT: {
       static ValidationError err("bad-cb-height");
       return err;

src/staking/validation_error.h

@@ -30,6 +30,7 @@ BETTER_ENUM(
     INVALID_BLOCK_HEIGHT,
     INVALID_BLOCK_TIME,
     INVALID_BLOCK_PUBLIC_KEY,
+    INVALID_META_INPUT_PREVOUT,
     MERKLE_ROOT_MISMATCH,
     MERKLE_ROOT_DUPLICATE_TRANSACTIONS,
     MISMATCHING_HEIGHT,

src/test/staking/block_validator_tests.cpp

@@ -45,7 +45,7 @@ CTransactionRef MakeCoinbaseTransaction(const KeyFixture &key_fixture = MakeKeyF
   // meta input: block height, snapshot hash, terminator
   CScript script_sig = CScript() << CScriptNum::serialize(4711)
                                  << ToByteVector(uint256S("689dae90b6913ff34a64750dd537177afa58b3d012803a10793d74f1ebb88da9"));
-  tx.vin.emplace_back(uint256(), 0, script_sig);
+  tx.vin.emplace_back(COutPoint(), script_sig);
   // stake
   tx.vin.emplace_back(uint256(), 1);
   tx.vin[1].scriptWitness.stack.emplace_back();  // signature, not checked
@@ -108,7 +108,7 @@ BOOST_AUTO_TEST_CASE(check_empty_block) {
   const staking::BlockValidationResult validation_result = block_validator->CheckBlock(block, nullptr);
 
   BOOST_CHECK(!validation_result);
-  BOOST_CHECK(validation_result.Is(Error::NO_TRANSACTIONS));
+  BOOST_CHECK_MESSAGE(validation_result.Is(Error::NO_TRANSACTIONS), validation_result.GetRejectionMessage());
 }
 
 BOOST_AUTO_TEST_CASE(check_first_transaction_not_a_coinbase_transaction) {
@@ -124,7 +124,24 @@ BOOST_AUTO_TEST_CASE(check_first_transaction_not_a_coinbase_transaction) {
   const staking::BlockValidationResult validation_result = block_validator->CheckBlock(block, nullptr);
 
   BOOST_CHECK(!validation_result);
-  BOOST_CHECK(validation_result.Is(Error::FIRST_TRANSACTION_NOT_A_COINBASE_TRANSACTION));
+  BOOST_CHECK_MESSAGE(validation_result.Is(Error::FIRST_TRANSACTION_NOT_A_COINBASE_TRANSACTION), validation_result.GetRejectionMessage());
+}
+
+BOOST_AUTO_TEST_CASE(check_coinbase_meta_input_malformed) {
+  const auto block_validator = staking::BlockValidator::New(b.get());
+
+  CBlock block;
+  {
+    CMutableTransaction tx;
+    tx.SetType(TxType::COINBASE);
+    tx.vin.emplace_back(uint256::zero, 0);
+    block.vtx.push_back(MakeTransactionRef(CTransaction(tx)));
+  }
+
+  const auto validation_result = block_validator->CheckBlock(block, nullptr);
+
+  BOOST_CHECK(!validation_result);
+  BOOST_CHECK_MESSAGE(validation_result.Is(Error::INVALID_META_INPUT_PREVOUT), validation_result.GetRejectionMessage());
 }
 
 BOOST_AUTO_TEST_CASE(check_coinbase_other_than_first) {