refactor: Add CalculateLockPointsAtTip() function

src/validation.cpp

@@ -170,6 +170,87 @@ bool CheckFinalTxAtTip(const CBlockIndex& active_chain_tip, const CTransaction&
     return IsFinalTx(tx, nBlockHeight, nBlockTime);
 }
 
+namespace {
+/**
+ * A helper which calculates heights of inputs of a given transaction.
+ *
+ * @param[in] tip    The current chain tip. If an input belongs to a mempool
+ *                   transaction, we assume it will be confirmed in the next block.
+ * @param[in] coins  Any CCoinsView that provides access to the relevant coins.
+ * @param[in] tx     The transaction being evaluated.
+ *
+ * @returns A vector of input heights or nullopt, in case of an error.
+ */
+std::optional<std::vector<int>> CalculatePrevHeights(
+    const CBlockIndex& tip,
+    const CCoinsView& coins,
+    const CTransaction& tx)
+{
+    std::vector<int> prev_heights;
+    prev_heights.resize(tx.vin.size());
+    for (size_t i = 0; i < tx.vin.size(); ++i) {
+        const CTxIn& txin = tx.vin[i];
+        Coin coin;
+        if (!coins.GetCoin(txin.prevout, coin)) {
+            LogPrintf("ERROR: %s: Missing input %d in transaction \'%s\'\n", __func__, i, tx.GetHash().GetHex());
+            return std::nullopt;
+        }
+        if (coin.nHeight == MEMPOOL_HEIGHT) {
+            // Assume all mempool transaction confirm in the next block.
+            prev_heights[i] = tip.nHeight + 1;
+        } else {
+            prev_heights[i] = coin.nHeight;
+        }
+    }
+    return prev_heights;
+}
+} // namespace
+
+std::optional<LockPoints> CalculateLockPointsAtTip(
+    CBlockIndex* tip,
+    const CCoinsView& coins_view,
+    const CTransaction& tx)
+{
+    assert(tip);
+
+    auto prev_heights{CalculatePrevHeights(*tip, coins_view, tx)};
+    if (!prev_heights.has_value()) return std::nullopt;
+
+    CBlockIndex next_tip;
+    next_tip.pprev = tip;
+    // When SequenceLocks() is called within ConnectBlock(), the height
+    // of the block *being* evaluated is what is used.
+    // Thus if we want to know if a transaction can be part of the
+    // *next* block, we need to use one more than active_chainstate.m_chain.Height()
+    next_tip.nHeight = tip->nHeight + 1;
+    const auto [min_height, min_time] = CalculateSequenceLocks(tx, STANDARD_LOCKTIME_VERIFY_FLAGS, prev_heights.value(), next_tip);
+
+    // Also store the hash of the block with the highest height of
+    // all the blocks which have sequence locked prevouts.
+    // This hash needs to still be on the chain
+    // for these LockPoint calculations to be valid
+    // Note: It is impossible to correctly calculate a maxInputBlock
+    // if any of the sequence locked inputs depend on unconfirmed txs,
+    // except in the special case where the relative lock time/height
+    // is 0, which is equivalent to no sequence lock. Since we assume
+    // input height of tip+1 for mempool txs and test the resulting
+    // min_height and min_time from CalculateSequenceLocks against tip+1.
+    int max_input_height{0};
+    for (const int height : prev_heights.value()) {
+        // Can ignore mempool inputs since we'll fail if they had non-zero locks
+        if (height != next_tip.nHeight) {
+            max_input_height = std::max(max_input_height, height);
+        }
+    }
+
+    // tip->GetAncestor(max_input_height) should never return a nullptr
+    // because max_input_height 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.
+    return LockPoints{min_height, min_time, Assert(tip->GetAncestor(max_input_height))};
+}
+
 bool CheckSequenceLocksAtTip(CBlockIndex* tip,
                         const CCoinsView& coins_view,
                         const CTransaction& tx,

src/validation.h

@@ -244,6 +244,29 @@ PackageMempoolAcceptResult ProcessNewPackage(Chainstate& active_chainstate, CTxM
  */
 bool CheckFinalTxAtTip(const CBlockIndex& active_chain_tip, const CTransaction& tx) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+/**
+ * Calculate LockPoints required to check if transaction will be BIP68 final in the next block
+ * to be created on top of tip.
+ *
+ * @param[in]   tip             Chain tip for which tx sequence locks are calculated. For
+ *                              example, the tip of the current active chain.
+ * @param[in]   coins_view      Any CCoinsView that provides access to the relevant coins for
+ *                              checking sequence locks. For example, it can be a CCoinsViewCache
+ *                              that isn't connected to anything but contains all the relevant
+ *                              coins, or a CCoinsViewMemPool that is connected to the
+ *                              mempool and chainstate UTXO set. In the latter case, the caller
+ *                              is responsible for holding the appropriate locks to ensure that
+ *                              calls to GetCoin() return correct coins.
+ * @param[in]   tx              The transaction being evaluated.
+ *
+ * @returns The resulting height and time calculated and the hash of the block needed for
+ *          calculation, or std::nullopt if there is an error.
+ */
+std::optional<LockPoints> CalculateLockPointsAtTip(
+    CBlockIndex* tip,
+    const CCoinsView& coins_view,
+    const CTransaction& tx);
+
 /**
  * Check if transaction will be BIP68 final in the next block to be created on top of tip.
  * @param[in]   tip             Chain tip to check tx sequence locks against. For example,