From 4afbde6028708541c4da8732a1bd12fb8735fdae Mon Sep 17 00:00:00 2001
From: Alex Morcos <morcos@chaincode.com>
Date: Fri, 16 Dec 2016 11:34:39 -0500
Subject: [PATCH] Introduce MemPoolConflictRemovalTracker

Analogue to ConnectTrace that tracks transactions that have been removed from the mempool due to conflicts and then passes them through SyncTransaction at the end of its scope.
---
 src/validation.cpp | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/src/validation.cpp b/src/validation.cpp
index d382e121bc..b882416961 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -157,6 +157,39 @@ namespace {
     set<int> setDirtyFileInfo;
 } // anon namespace
 
+/* Use this class to start tracking transactions that are removed from the
+ * mempool and pass all those transactions through SyncTransaction when the
+ * object goes out of scope. This is currently only used to call SyncTransaction
+ * on conflicts removed from the mempool during block connection.  Applied in
+ * ActivateBestChain around ActivateBestStep which in turn calls:
+ * ConnectTip->removeForBlock->removeConflicts
+ */
+class MemPoolConflictRemovalTracker
+{
+private:
+    std::vector<CTransactionRef> conflictedTxs;
+    CTxMemPool &pool;
+
+public:
+    MemPoolConflictRemovalTracker(CTxMemPool &_pool) : pool(_pool) {
+        pool.NotifyEntryRemoved.connect(boost::bind(&MemPoolConflictRemovalTracker::NotifyEntryRemoved, this, _1, _2));
+    }
+
+    void NotifyEntryRemoved(CTransactionRef txRemoved, MemPoolRemovalReason reason) {
+        if (reason == MemPoolRemovalReason::CONFLICT) {
+            conflictedTxs.push_back(txRemoved);
+        }
+    }
+
+    ~MemPoolConflictRemovalTracker() {
+        pool.NotifyEntryRemoved.disconnect(boost::bind(&MemPoolConflictRemovalTracker::NotifyEntryRemoved, this, _1, _2));
+        for (const auto& tx : conflictedTxs) {
+            GetMainSignals().SyncTransaction(*tx, NULL, CMainSignals::SYNC_TRANSACTION_NOT_IN_BLOCK);
+        }
+        conflictedTxs.clear();
+    }
+};
+
 CBlockIndex* FindForkInGlobalIndex(const CChain& chain, const CBlockLocator& locator)
 {
     // Find the first block the caller has in the main chain
@@ -2453,6 +2486,14 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
         bool fInitialDownload;
         {
             LOCK(cs_main);
+            { // TODO: Tempoarily ensure that mempool removals are notified before
+              // connected transactions.  This shouldn't matter, but the abandoned
+              // state of transactions in our wallet is currently cleared when we
+              // receive another notification and there is a race condition where
+              // notification of a connected conflict might cause an outside process
+              // to abandon a transaction and then have it inadvertantly cleared by
+              // the notification that the conflicted transaction was evicted.
+            MemPoolConflictRemovalTracker mrt(mempool);
             CBlockIndex *pindexOldTip = chainActive.Tip();
             if (pindexMostWork == NULL) {
                 pindexMostWork = FindMostWorkChain();
@@ -2476,6 +2517,10 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
             fInitialDownload = IsInitialBlockDownload();
 
             // throw all transactions though the signal-interface
+
+            } // MemPoolConflictRemovalTracker destroyed and conflict evictions are notified
+
+            // Transactions in the connnected block are notified
             for (const auto& pair : connectTrace.blocksConnected) {
                 assert(pair.second);
                 const CBlock& block = *(pair.second);