[validation] return correct result when already-in-mempool tx gets evicted

Bug fix: a transaction may be in the mempool when package evaluation
begins (so it is added to results_final with MEMPOOL_ENTRY or
DIFFERENT_WITNESS), but get evicted due to another transaction
submission.

Author: glozow

src/validation.cpp

@@ -1525,9 +1525,19 @@ PackageMempoolAcceptResult MemPoolAccept::AcceptPackage(const Package& package,
             Assume(results_final.count(wtxid) == 0);
             results_final.emplace(wtxid, multi_submission_result.m_tx_results.at(wtxid));
         } else if (const auto it{results_final.find(wtxid)}; it != results_final.end()) {
-            // Already-in-mempool transaction.
+            // Already-in-mempool transaction. Check to see if it's still there, as it could have
+            // been evicted when LimitMempoolSize() was called.
             Assume(it->second.m_result_type != MempoolAcceptResult::ResultType::INVALID);
             Assume(individual_results_nonfinal.count(wtxid) == 0);
+            // Query by txid to include the same-txid-different-witness ones.
+            if (!m_pool.exists(GenTxid::Txid(tx->GetHash()))) {
+                package_state_final.Invalid(PackageValidationResult::PCKG_TX, "transaction failed");
+                TxValidationState mempool_full_state;
+                mempool_full_state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "mempool full");
+                // Replace the previous result.
+                results_final.erase(wtxid);
+                results_final.emplace(wtxid, MempoolAcceptResult::Failure(mempool_full_state));
+            }
         } else if (const auto it{individual_results_nonfinal.find(wtxid)}; it != individual_results_nonfinal.end()) {
             Assume(it->second.m_result_type == MempoolAcceptResult::ResultType::INVALID);
             // Interesting result from previous processing.