PackageV3Checks: Relax assumptions

Relax assumptions about in-mempool children of in-mempool
parents. With package RBF, we will allow a package of size
2 with conflicts on its parent and reconsider the parent
if its fee is insufficient on its own.

Consider:

TxA (in mempool) <- TxB (in mempool)

TxA (in mempool) <- TxB' (in package, conflicts with TxB) <-
TxC (in package)

If TxB' fails to RBF TxB due to insufficient feerate, the
package TxB' + TxC will be considered. PackageV3Checks
called on TxB' will see an in-mempool parent TxA, and
see the in-mempool child TxB. We cannot assume there is
no in-mempool sibling, rather detect it and fail normally.

Prior to package RBF, this would have failed on the first
conflict in package.

Author: instagibbs

src/policy/v3_policy.cpp

@@ -91,7 +91,6 @@ std::optional<std::string> PackageV3Checks(const CTransactionRef& ptx, int64_t v
             const auto parent_info = [&] {
                 if (mempool_ancestors.size() > 0) {
                     auto& mempool_parent = *mempool_ancestors.begin();
-                    Assume(mempool_parent->GetCountWithDescendants() == 1);
                     return ParentInfo{mempool_parent->GetTx().GetHash(),
                                       mempool_parent->GetTx().GetWitnessHash(),
                                       mempool_parent->GetTx().nVersion,
@@ -135,10 +134,7 @@ std::optional<std::string> PackageV3Checks(const CTransactionRef& ptx, int64_t v
                 }
             }
 
-            // It shouldn't be possible to have any mempool siblings at this point. SingleV3Checks
-            // catches mempool siblings and sibling eviction is not extended to packages. Also, if the package consists of connected transactions,
-            // any tx having a mempool ancestor would mean the package exceeds ancestor limits.
-            if (!Assume(!parent_info.m_has_mempool_descendant)) {
+            if (parent_info.m_has_mempool_descendant) {
                 return strprintf("tx %s (wtxid=%s) would exceed descendant count limit",
                                 parent_info.m_txid.ToString(), parent_info.m_wtxid.ToString());
             }