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>

1 files changed, 10 insertions, 4 deletions

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());