Sanity assert GetAncestor() != nullptr where appropriate

Add sanity asserts for return value of `CBlockIndex::GetAncestor()` where appropriate.

In validation.cpp `CheckSequenceLocks`, check the return value of `tip->GetAncestor(maxInputHeight)` stored into `lp->maxInputBlock`. If it ever returns `nullptr` because the ancestor isn't found, it's going to be a bad bug to keep going, since a `LockPoints` object with the `maxInputBlock` member set to `nullptr` signifies no relative lock time.

In the other places, the added asserts would prevent accidental dereferencing of a null pointer which is undefined behavior.

Co-Authored-By: Aurèle Oulès <aurele@oules.com>
Co-Authored-By: danra <danra@users.noreply.github.com>

5 files changed, 39 insertions, 28 deletions

diff --git a/src/consensus/tx_verify.cpp b/src/consensus/tx_verify.cpp
index 5738c333ce..154146f08d 100644
--- a/src/consensus/tx_verify.cpp
+++ b/src/consensus/tx_verify.cpp
@@ -11,6 +11,7 @@
 #include <consensus/validation.h>
 #include <primitives/transaction.h>
 #include <script/interpreter.h>
+#include <util/check.h>
 #include <util/moneystr.h>
 
 bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime)
@@ -74,7 +75,7 @@ std::pair<int, int64_t> CalculateSequenceLocks(const CTransaction &tx, int flags
         int nCoinHeight = prevHeights[txinIndex];
 
         if (txin.nSequence & CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG) {
-            int64_t nCoinTime = block.GetAncestor(std::max(nCoinHeight-1, 0))->GetMedianTimePast();
+            const int64_t nCoinTime{Assert(block.GetAncestor(std::max(nCoinHeight - 1, 0)))->GetMedianTimePast()};
             // NOTE: Subtract 1 to maintain nLockTime semantics
             // BIP 68 relative lock times have the semantics of calculating
             // the first block or time at which the transaction would be
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index d6a6bd5f31..e3e988d473 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -1618,9 +1618,9 @@ static RPCHelpMan getchaintxstats()
         }
     }
 
-    const CBlockIndex* pindexPast = pindex->GetAncestor(pindex->nHeight - blockcount);
-    int nTimeDiff = pindex->GetMedianTimePast() - pindexPast->GetMedianTimePast();
-    int nTxDiff = pindex->nChainTx - pindexPast->nChainTx;
+    const CBlockIndex& past_block{*CHECK_NONFATAL(pindex->GetAncestor(pindex->nHeight - blockcount))};
+    const int64_t nTimeDiff{pindex->GetMedianTimePast() - past_block.GetMedianTimePast()};
+    const int nTxDiff = pindex->nChainTx - past_block.nChainTx;
 
     UniValue ret(UniValue::VOBJ);
     ret.pushKV("time", (int64_t)pindex->nTime);
@@ -1761,8 +1761,7 @@ static RPCHelpMan getblockstats()
 {
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
     LOCK(cs_main);
-    const CBlockIndex* pindex{ParseHashOrHeight(request.params[0], chainman)};
-    CHECK_NONFATAL(pindex != nullptr);
+    const CBlockIndex& pindex{*CHECK_NONFATAL(ParseHashOrHeight(request.params[0], chainman))};
 
     std::set<std::string> stats;
     if (!request.params[1].isNull()) {
@@ -1773,8 +1772,8 @@ static RPCHelpMan getblockstats()
         }
     }
 
-    const CBlock block = GetBlockChecked(chainman.m_blockman, pindex);
-    const CBlockUndo blockUndo = GetUndoChecked(chainman.m_blockman, pindex);
+    const CBlock& block = GetBlockChecked(chainman.m_blockman, &pindex);
+    const CBlockUndo& blockUndo = GetUndoChecked(chainman.m_blockman, &pindex);
 
     const bool do_all = stats.size() == 0; // Calculate everything if nothing selected (default)
     const bool do_mediantxsize = do_all || stats.count("mediantxsize") != 0;
@@ -1892,25 +1891,25 @@ static RPCHelpMan getblockstats()
     ret_all.pushKV("avgfee", (block.vtx.size() > 1) ? totalfee / (block.vtx.size() - 1) : 0);
     ret_all.pushKV("avgfeerate", total_weight ? (totalfee * WITNESS_SCALE_FACTOR) / total_weight : 0); // Unit: sat/vbyte
     ret_all.pushKV("avgtxsize", (block.vtx.size() > 1) ? total_size / (block.vtx.size() - 1) : 0);
-    ret_all.pushKV("blockhash", pindex->GetBlockHash().GetHex());
+    ret_all.pushKV("blockhash", pindex.GetBlockHash().GetHex());
     ret_all.pushKV("feerate_percentiles", feerates_res);
-    ret_all.pushKV("height", (int64_t)pindex->nHeight);
+    ret_all.pushKV("height", (int64_t)pindex.nHeight);
     ret_all.pushKV("ins", inputs);
     ret_all.pushKV("maxfee", maxfee);
     ret_all.pushKV("maxfeerate", maxfeerate);
     ret_all.pushKV("maxtxsize", maxtxsize);
     ret_all.pushKV("medianfee", CalculateTruncatedMedian(fee_array));
-    ret_all.pushKV("mediantime", pindex->GetMedianTimePast());
+    ret_all.pushKV("mediantime", pindex.GetMedianTimePast());
     ret_all.pushKV("mediantxsize", CalculateTruncatedMedian(txsize_array));
     ret_all.pushKV("minfee", (minfee == MAX_MONEY) ? 0 : minfee);
     ret_all.pushKV("minfeerate", (minfeerate == MAX_MONEY) ? 0 : minfeerate);
     ret_all.pushKV("mintxsize", mintxsize == MAX_BLOCK_SERIALIZED_SIZE ? 0 : mintxsize);
     ret_all.pushKV("outs", outputs);
-    ret_all.pushKV("subsidy", GetBlockSubsidy(pindex->nHeight, Params().GetConsensus()));
+    ret_all.pushKV("subsidy", GetBlockSubsidy(pindex.nHeight, Params().GetConsensus()));
     ret_all.pushKV("swtotal_size", swtotal_size);
     ret_all.pushKV("swtotal_weight", swtotal_weight);
     ret_all.pushKV("swtxs", swtxs);
-    ret_all.pushKV("time", pindex->GetBlockTime());
+    ret_all.pushKV("time", pindex.GetBlockTime());
     ret_all.pushKV("total_out", total_out);
     ret_all.pushKV("total_size", total_size);
     ret_all.pushKV("total_weight", total_weight);
diff --git a/src/test/miner_tests.cpp b/src/test/miner_tests.cpp
index f6c1d1efad..82a4257c76 100644
--- a/src/test/miner_tests.cpp
+++ b/src/test/miner_tests.cpp
@@ -446,16 +446,18 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
     BOOST_CHECK(CheckFinalTxAtTip(m_node.chainman->ActiveChain().Tip(), CTransaction{tx})); // Locktime passes
     BOOST_CHECK(!TestSequenceLocks(CTransaction{tx})); // Sequence locks fail
 
-    for (int i = 0; i < CBlockIndex::nMedianTimeSpan; i++)
-        m_node.chainman->ActiveChain().Tip()->GetAncestor(m_node.chainman->ActiveChain().Tip()->nHeight - i)->nTime += 512; //Trick the MedianTimePast
-
+    const int SEQUENCE_LOCK_TIME = 512; // Sequence locks pass 512 seconds later
+    for (int i = 0; i < CBlockIndex::nMedianTimeSpan; ++i)
+        m_node.chainman->ActiveChain().Tip()->GetAncestor(m_node.chainman->ActiveChain().Tip()->nHeight - i)->nTime += SEQUENCE_LOCK_TIME; // Trick the MedianTimePast
     {
         CBlockIndex* active_chain_tip = m_node.chainman->ActiveChain().Tip();
-        BOOST_CHECK(SequenceLocks(CTransaction(tx), flags, prevheights, CreateBlockIndex(active_chain_tip->nHeight + 1, active_chain_tip))); // Sequence locks pass 512 seconds later
+        BOOST_CHECK(SequenceLocks(CTransaction(tx), flags, prevheights, CreateBlockIndex(active_chain_tip->nHeight + 1, active_chain_tip)));
     }
 
-    for (int i = 0; i < CBlockIndex::nMedianTimeSpan; i++)
-        m_node.chainman->ActiveChain().Tip()->GetAncestor(m_node.chainman->ActiveChain().Tip()->nHeight - i)->nTime -= 512; //undo tricked MTP
+    for (int i = 0; i < CBlockIndex::nMedianTimeSpan; ++i) {
+        CBlockIndex* ancestor{Assert(m_node.chainman->ActiveChain().Tip()->GetAncestor(m_node.chainman->ActiveChain().Tip()->nHeight - i))};
+        ancestor->nTime -= SEQUENCE_LOCK_TIME; // undo tricked MTP
+    }
 
     // absolute height locked
     tx.vin[0].prevout.hash = txFirst[2]->GetHash();
@@ -500,9 +502,11 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
     // but relative locked txs will if inconsistently added to mempool.
     // For now these will still generate a valid template until BIP68 soft fork
     BOOST_CHECK_EQUAL(pblocktemplate->block.vtx.size(), 3U);
-    // However if we advance height by 1 and time by 512, all of them should be mined
-    for (int i = 0; i < CBlockIndex::nMedianTimeSpan; i++)
-        m_node.chainman->ActiveChain().Tip()->GetAncestor(m_node.chainman->ActiveChain().Tip()->nHeight - i)->nTime += 512; //Trick the MedianTimePast
+    // However if we advance height by 1 and time by SEQUENCE_LOCK_TIME, all of them should be mined
+    for (int i = 0; i < CBlockIndex::nMedianTimeSpan; ++i) {
+        CBlockIndex* ancestor{Assert(m_node.chainman->ActiveChain().Tip()->GetAncestor(m_node.chainman->ActiveChain().Tip()->nHeight - i))};
+        ancestor->nTime += SEQUENCE_LOCK_TIME; // Trick the MedianTimePast
+    }
     m_node.chainman->ActiveChain().Tip()->nHeight++;
     SetMockTime(m_node.chainman->ActiveChain().Tip()->GetMedianTimePast() + 1);
 
diff --git a/src/validation.cpp b/src/validation.cpp
index 58686632f9..25c99159fd 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -246,7 +246,12 @@ bool CheckSequenceLocksAtTip(CBlockIndex* tip,
                     maxInputHeight = std::max(maxInputHeight, height);
                 }
             }
-            lp->maxInputBlock = tip->GetAncestor(maxInputHeight);
+            // tip->GetAncestor(maxInputHeight) should never return a nullptr
+            // because maxInputHeight is always less than the tip height.
+            // It would, however, be a bad bug to continue execution, since a
+            // LockPoints object with the maxInputBlock member set to nullptr
+            // signifies no relative lock time.
+            lp->maxInputBlock = Assert(tip->GetAncestor(maxInputHeight));
         }
     }
     return EvaluateSequenceLocks(index, lockPair);
@@ -4077,10 +4082,11 @@ bool CChainState::ReplayBlocks()
     // Roll forward from the forking point to the new tip.
     int nForkHeight = pindexFork ? pindexFork->nHeight : 0;
     for (int nHeight = nForkHeight + 1; nHeight <= pindexNew->nHeight; ++nHeight) {
-        const CBlockIndex* pindex = pindexNew->GetAncestor(nHeight);
-        LogPrintf("Rolling forward %s (%i)\n", pindex->GetBlockHash().ToString(), nHeight);
+        const CBlockIndex& pindex{*Assert(pindexNew->GetAncestor(nHeight))};
+
+        LogPrintf("Rolling forward %s (%i)\n", pindex.GetBlockHash().ToString(), nHeight);
         uiInterface.ShowProgress(_("Replaying blocks…").translated, (int) ((nHeight - nForkHeight) * 100.0 / (pindexNew->nHeight - nForkHeight)) , false);
-        if (!RollforwardBlock(pindex, cache)) return false;
+        if (!RollforwardBlock(&pindex, cache)) return false;
     }
 
     cache.SetBestBlock(pindexNew->GetBlockHash());
diff --git a/src/versionbits.cpp b/src/versionbits.cpp
index 7a297c2bbb..dc85655028 100644
--- a/src/versionbits.cpp
+++ b/src/versionbits.cpp
@@ -2,8 +2,9 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <versionbits.h>
 #include <consensus/params.h>
+#include <util/check.h>
+#include <versionbits.h>
 
 ThresholdState AbstractThresholdConditionChecker::GetStateFor(const CBlockIndex* pindexPrev, const Consensus::Params& params, ThresholdConditionCache& cache) const
 {
@@ -158,7 +159,7 @@ int AbstractThresholdConditionChecker::GetStateSinceHeightFor(const CBlockIndex*
     // if we are computing for the last block of a period, then pindexPrev points to the second to last block of the period, and
     // if we are computing for the first block of a period, then pindexPrev points to the last block of the previous period.
     // The parent of the genesis block is represented by nullptr.
-    pindexPrev = pindexPrev->GetAncestor(pindexPrev->nHeight - ((pindexPrev->nHeight + 1) % nPeriod));
+    pindexPrev = Assert(pindexPrev->GetAncestor(pindexPrev->nHeight - ((pindexPrev->nHeight + 1) % nPeriod)));
 
     const CBlockIndex* previousPeriodParent = pindexPrev->GetAncestor(pindexPrev->nHeight - nPeriod);