[validation] don't package validate if not policy or missing inputs

Package validation policy only differs from individual policy in its
evaluation of feerate. Minimize DoS surface; don't validate all over
again if we know the result will be the same.

Author: glozow

src/validation.cpp

@@ -1342,6 +1342,7 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
     // the new transactions. This ensures we don't double-count transaction counts and sizes when
     // checking ancestor/descendant limits, or double-count transaction fees for fee-related policy.
     ATMPArgs single_args = ATMPArgs::SingleInPackageAccept(args);
+    bool quit_early{false};
     std::vector<CTransactionRef> txns_new;
     for (const auto& tx : package) {
         const auto& wtxid = tx->GetWitnessHash();
@@ -1375,14 +1376,26 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
                 // in package validation, because its fees should only be "used" once.
                 assert(m_pool.exists(GenTxid::Wtxid(wtxid)));
                 results.emplace(wtxid, single_res);
+            } else if (single_res.m_state.GetResult() != TxValidationResult::TX_MEMPOOL_POLICY &&
+                       single_res.m_state.GetResult() != TxValidationResult::TX_MISSING_INPUTS) {
+                // Package validation policy only differs from individual policy in its evaluation
+                // of feerate. For example, if a transaction fails here due to violation of a
+                // consensus rule, the result will not change when it is submitted as part of a
+                // package. To minimize the amount of repeated work, unless the transaction fails
+                // due to feerate or missing inputs (its parent is a previous transaction in the
+                // package that failed due to feerate), don't run package validation. Note that this
+                // decision might not make sense if different types of packages are allowed in the
+                // future.  Continue individually validating the rest of the transactions, because
+                // some of them may still be valid.
+                quit_early = true;
             } else {
                 txns_new.push_back(tx);
             }
         }
     }
 
     // Nothing to do if the entire package has already been submitted.
-    if (txns_new.empty()) {
+    if (quit_early || txns_new.empty()) {
         // No package feerate when no package validation was done.
         return PackageMempoolAcceptResult(package_state, std::move(results));
     }