wallet: Minimal fix to restore conflicted transaction notifications

This fix is a based on the fix by Antoine Riard <ariard@student.42.fr> in
https://github.com/bitcoin/bitcoin/pull/18600.

Unlike that PR, which implements some new behavior, this just restores previous
wallet notification and status behavior for transactions removed from the
mempool because they conflict with transactions in a block. The behavior was
accidentally changed in two `CWallet::BlockConnected` updates:
a31be09bfd77eed497a8e251d31358e16e2f2eb1 and
7e89994133725125dddbfa8d45484e3b9ed51c6e from
https://github.com/bitcoin/bitcoin/pull/16624, causing issue
https://github.com/bitcoin/bitcoin/issues/18325.

The change here could be improved and replaced with a more comprehensive
cleanup, so it includes a detailed comment explaining future considerations.

Fixes #18325

Co-authored-by: Antoine Riard <ariard@student.42.fr>

7 files changed, 45 insertions, 12 deletions

diff --git a/src/interfaces/chain.cpp b/src/interfaces/chain.cpp
index d8e459a8e8..d1e04b114d 100644
--- a/src/interfaces/chain.cpp
+++ b/src/interfaces/chain.cpp
@@ -63,9 +63,9 @@ public:
     {
         m_notifications->transactionAddedToMempool(tx);
     }
-    void TransactionRemovedFromMempool(const CTransactionRef& tx) override
+    void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) override
     {
-        m_notifications->transactionRemovedFromMempool(tx);
+        m_notifications->transactionRemovedFromMempool(tx, reason);
     }
     void BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* index) override
     {
diff --git a/src/interfaces/chain.h b/src/interfaces/chain.h
index 7dfc77db7b..61d7ddb934 100644
--- a/src/interfaces/chain.h
+++ b/src/interfaces/chain.h
@@ -20,6 +20,7 @@ class CRPCCommand;
 class CScheduler;
 class Coin;
 class uint256;
+enum class MemPoolRemovalReason;
 enum class RBFTransactionState;
 struct bilingual_str;
 struct CBlockLocator;
@@ -239,7 +240,7 @@ public:
     public:
         virtual ~Notifications() {}
         virtual void transactionAddedToMempool(const CTransactionRef& tx) {}
-        virtual void transactionRemovedFromMempool(const CTransactionRef& ptx) {}
+        virtual void transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {}
         virtual void blockConnected(const CBlock& block, int height) {}
         virtual void blockDisconnected(const CBlock& block, int height) {}
         virtual void updatedBlockTip() {}
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index c5c0208d8f..7d8eb8a323 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -410,7 +410,7 @@ void CTxMemPool::removeUnchecked(txiter it, MemPoolRemovalReason reason)
         // for any reason except being included in a block. Clients interested
         // in transactions included in blocks can subscribe to the BlockConnected
         // notification.
-        GetMainSignals().TransactionRemovedFromMempool(it->GetSharedTx());
+        GetMainSignals().TransactionRemovedFromMempool(it->GetSharedTx(), reason);
     }
 
     const uint256 hash = it->GetTx().GetHash();
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index 9437f9c817..22d13704fb 100644
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -208,9 +208,9 @@ void CMainSignals::TransactionAddedToMempool(const CTransactionRef &ptx) {
                           ptx->GetWitnessHash().ToString());
 }
 
-void CMainSignals::TransactionRemovedFromMempool(const CTransactionRef &ptx) {
-    auto event = [ptx, this] {
-        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionRemovedFromMempool(ptx); });
+void CMainSignals::TransactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) {
+    auto event = [ptx, reason, this] {
+        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionRemovedFromMempool(ptx, reason); });
     };
     ENQUEUE_AND_LOG_EVENT(event, "%s: txid=%s wtxid=%s", __func__,
                           ptx->GetHash().ToString(),
diff --git a/src/validationinterface.h b/src/validationinterface.h
index 9c23965bc1..b35f01c4bb 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -21,6 +21,7 @@ class CConnman;
 class CValidationInterface;
 class uint256;
 class CScheduler;
+enum class MemPoolRemovalReason;
 
 /** Register subscriber */
 void RegisterValidationInterface(CValidationInterface* callbacks);
@@ -129,7 +130,7 @@ protected:
      *
      * Called on a background thread.
      */
-    virtual void TransactionRemovedFromMempool(const CTransactionRef &ptx) {}
+    virtual void TransactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) {}
     /**
      * Notifies listeners of a block being connected.
      * Provides a vector of transactions evicted from the mempool as a result.
@@ -197,7 +198,7 @@ public:
 
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
     void TransactionAddedToMempool(const CTransactionRef &);
-    void TransactionRemovedFromMempool(const CTransactionRef &);
+    void TransactionRemovedFromMempool(const CTransactionRef &, MemPoolRemovalReason);
     void BlockConnected(const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex);
     void BlockDisconnected(const std::shared_ptr<const CBlock> &, const CBlockIndex* pindex);
     void ChainStateFlushed(const CBlockLocator &);
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 2b45c6a536..b95d8ea752 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -21,6 +21,7 @@
 #include <script/descriptor.h>
 #include <script/script.h>
 #include <script/signingprovider.h>
+#include <txmempool.h>
 #include <util/bip32.h>
 #include <util/check.h>
 #include <util/error.h>
@@ -1111,12 +1112,42 @@ void CWallet::transactionAddedToMempool(const CTransactionRef& ptx) {
     }
 }
 
-void CWallet::transactionRemovedFromMempool(const CTransactionRef &ptx) {
+void CWallet::transactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) {
     LOCK(cs_wallet);
     auto it = mapWallet.find(ptx->GetHash());
     if (it != mapWallet.end()) {
         it->second.fInMempool = false;
     }
+    // Handle transactions that were removed from the mempool because they
+    // conflict with transactions in a newly connected block.
+    if (reason == MemPoolRemovalReason::CONFLICT) {
+        // Call SyncNotifications, so external -walletnotify notifications will
+        // be triggered for these transactions. Set Status::UNCONFIRMED instead
+        // of Status::CONFLICTED for a few reasons:
+        //
+        // 1. The transactionRemovedFromMempool callback does not currently
+        //    provide the conflicting block's hash and height, and for backwards
+        //    compatibility reasons it may not be not safe to store conflicted
+        //    wallet transactions with a null block hash. See
+        //    https://github.com/bitcoin/bitcoin/pull/18600#discussion_r420195993.
+        // 2. For most of these transactions, the wallet's internal conflict
+        //    detection in the blockConnected handler will subsequently call
+        //    MarkConflicted and update them with CONFLICTED status anyway. This
+        //    applies to any wallet transaction that has inputs spent in the
+        //    block, or that has ancestors in the wallet with inputs spent by
+        //    the block.
+        // 3. Longstanding behavior since the sync implementation in
+        //    https://github.com/bitcoin/bitcoin/pull/9371 and the prior sync
+        //    implementation before that was to mark these transactions
+        //    unconfirmed rather than conflicted.
+        //
+        // Nothing described above should be seen as an unchangeable requirement
+        // when improving this code in the future. The wallet's heuristics for
+        // distinguishing between conflicted and unconfirmed transactions are
+        // imperfect, and could be improved in general, see
+        // https://github.com/bitcoin-core/bitcoin-devwiki/wiki/Wallet-Transaction-Conflict-Tracking
+        SyncTransaction(ptx, {CWalletTx::Status::UNCONFIRMED, /* block height  */ 0, /* block hash */ {}, /* index */ 0});
+    }
 }
 
 void CWallet::blockConnected(const CBlock& block, int height)
@@ -1129,7 +1160,7 @@ void CWallet::blockConnected(const CBlock& block, int height)
     for (size_t index = 0; index < block.vtx.size(); index++) {
         CWalletTx::Confirmation confirm(CWalletTx::Status::CONFIRMED, height, block_hash, index);
         SyncTransaction(block.vtx[index], confirm);
-        transactionRemovedFromMempool(block.vtx[index]);
+        transactionRemovedFromMempool(block.vtx[index], MemPoolRemovalReason::BLOCK);
     }
 }
 
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index a29fa22207..7d605beb78 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -923,7 +923,7 @@ public:
         uint256 last_failed_block;
     };
     ScanResult ScanForWalletTransactions(const uint256& start_block, int start_height, Optional<int> max_height, const WalletRescanReserver& reserver, bool fUpdate);
-    void transactionRemovedFromMempool(const CTransactionRef &ptx) override;
+    void transactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) override;
     void ReacceptWalletTransactions() EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     void ResendWalletTransactions();
     struct Balance {