From 8c99d1b525562ab3b733e9d7ef770882646bad5c Mon Sep 17 00:00:00 2001
From: Gregory Maxwell <greg@xiph.org>
Date: Mon, 13 Jun 2016 17:27:44 +0000
Subject: [PATCH] Treat orphans as implicit inv for parents, discard when
 parents rejected.

An orphan whos parents were rejected is never going to connect, so there
 is little utility in keeping it.

Orphans also helpfully tell us what we're missing, so go ahead and treat
 it as INVed.
---
 src/main.cpp | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 44d96d614e..6a2290bc05 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -5156,13 +5156,29 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         }
         else if (fMissingInputs)
         {
-            AddOrphanTx(tx, pfrom->GetId());
+            bool fRejectedParents = false; // It may be the case that the orphans parents have all been rejected
+            BOOST_FOREACH(const CTxIn& txin, tx.vin) {
+                if (recentRejects->contains(txin.prevout.hash)) {
+                    fRejectedParents = true;
+                    break;
+                }
+            }
+            if (!fRejectedParents) {
+                BOOST_FOREACH(const CTxIn& txin, tx.vin) {
+                    CInv inv(MSG_TX, txin.prevout.hash);
+                    pfrom->AddInventoryKnown(inv);
+                    if (!AlreadyHave(inv)) pfrom->AskFor(inv);
+                }
+                AddOrphanTx(tx, pfrom->GetId());
 
-            // DoS prevention: do not allow mapOrphanTransactions to grow unbounded
-            unsigned int nMaxOrphanTx = (unsigned int)std::max((int64_t)0, GetArg("-maxorphantx", DEFAULT_MAX_ORPHAN_TRANSACTIONS));
-            unsigned int nEvicted = LimitOrphanTxSize(nMaxOrphanTx);
-            if (nEvicted > 0)
-                LogPrint("mempool", "mapOrphan overflow, removed %u tx\n", nEvicted);
+                // DoS prevention: do not allow mapOrphanTransactions to grow unbounded
+                unsigned int nMaxOrphanTx = (unsigned int)std::max((int64_t)0, GetArg("-maxorphantx", DEFAULT_MAX_ORPHAN_TRANSACTIONS));
+                unsigned int nEvicted = LimitOrphanTxSize(nMaxOrphanTx);
+                if (nEvicted > 0)
+                    LogPrint("mempool", "mapOrphan overflow, removed %u tx\n", nEvicted);
+            } else {
+                LogPrint("mempool", "not keeping orphan with rejected parents %s\n",tx.GetHash().ToString());
+            }
         } else {
             assert(recentRejects);
             recentRejects->insert(tx.GetHash());