Merge #18982: wallet: Minimal fix to restore conflicted transaction notifications

7eaf86d3bfc83f2beb3ef449707d5156853126fb trivial: Suggested cleanups to surrounding code (Russell Yanofsky)
b604c5c8b5892842f13dee89ae31812a28ab25d1 wallet: Minimal fix to restore conflicted transaction notifications (Russell Yanofsky)

Pull request description:

  This fix is a based on the fix by Antoine Riard (ariard) 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)

ACKs for top commit:
  jonatack:
    Re-ACK 7eaf86d3bfc83f
  ariard:
    ACK 7eaf86d, reviewed, built and ran tests.
  MarcoFalke:
    ACK 7eaf86d3bfc83f2beb3ef449707d5156853126fb 🍡

Tree-SHA512: 9a1efe975969bb522a9dd73c41064a9348887cb67883cd92c6571fd2df4321b9f4568363891abdaae14a3b9b168ef8142e95c373fc04677e46289b251fb84689

doc/release-notes-18982.md

@@ -0,0 +1,7 @@
+Notification changes
+--------------------
+
+`-walletnotify` notifications are now sent for wallet transactions that are
+removed from the mempool because they conflict with a new block. These
+notifications were sent previously before the v18.1 release, but had been
+broken since that release.

src/interfaces/chain.h

@@ -27,6 +27,7 @@ class CFeeRate;
 class CBlockIndex;
 class Coin;
 class uint256;
+enum class MemPoolRemovalReason;
 struct bilingual_str;
 struct CBlockLocator;
 struct FeeCalculation;
@@ -258,7 +259,7 @@ public:
     public:
         virtual ~Notifications() {}
         virtual void transactionAddedToMempool(const CTransactionRef& tx, int64_t nAcceptTime) {}
-        virtual void transactionRemovedFromMempool(const CTransactionRef& ptx, MemPoolRemovalReason reason) {}
+        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() {}

src/validationinterface.cpp

@@ -208,20 +208,20 @@ void CMainSignals::SynchronousUpdatedBlockTip(const CBlockIndex *pindexNew, cons
     m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.SynchronousUpdatedBlockTip(pindexNew, pindexFork, fInitialDownload); });
 }
 
-void CMainSignals::TransactionAddedToMempool(const CTransactionRef &ptx, int64_t nAcceptTime) {
+void CMainSignals::TransactionAddedToMempool(const CTransactionRef& tx, int64_t nAcceptTime) {
-    auto event = [ptx, nAcceptTime, this] {
+    auto event = [tx, nAcceptTime, this] {
-        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionAddedToMempool(ptx, nAcceptTime); });
+        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionAddedToMempool(tx, nAcceptTime); });
     };
     ENQUEUE_AND_LOG_EVENT(event, "%s: txid=%s", __func__,
-                          ptx->GetHash().ToString());
+                          tx->GetHash().ToString());
 }
 
-void CMainSignals::TransactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) {
+void CMainSignals::TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {
-    auto event = [ptx, reason, this] {
+    auto event = [tx, reason, this] {
-        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionRemovedFromMempool(ptx, reason); });
+        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionRemovedFromMempool(tx, reason); });
     };
     ENQUEUE_AND_LOG_EVENT(event, "%s: txid=%s", __func__,
-                          ptx->GetHash().ToString());
+                          tx->GetHash().ToString());
 }
 
 void CMainSignals::BlockConnected(const std::shared_ptr<const CBlock> &pblock, const CBlockIndex *pindex) {

src/validationinterface.h

@@ -117,7 +117,7 @@ protected:
      *
      * Called on a background thread.
      */
-    virtual void TransactionAddedToMempool(const CTransactionRef &ptxn, int64_t nAcceptTime) {}
+    virtual void TransactionAddedToMempool(const CTransactionRef &xn, int64_t nAcceptTime) {}
     /**
      * Notifies listeners of a transaction leaving mempool.
      *
@@ -149,7 +149,7 @@ protected:
      *
      * Called on a background thread.
      */
-    virtual void TransactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) {}
+    virtual void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {}
     /**
      * Notifies listeners of a block being connected.
      * Provides a vector of transactions evicted from the mempool as a result.

src/wallet/wallet.cpp

@@ -1210,26 +1210,58 @@ void CWallet::SyncTransaction(const CTransactionRef& ptx, CWalletTx::Confirmatio
     fAnonymizableTallyCachedNonDenom = false;
 }
 
-void CWallet::transactionAddedToMempool(const CTransactionRef& ptx, int64_t nAcceptTime) {
+void CWallet::transactionAddedToMempool(const CTransactionRef& tx, int64_t nAcceptTime) {
     LOCK(cs_wallet);
     CWalletTx::Confirmation confirm(CWalletTx::Status::UNCONFIRMED, /* block_height */ 0, {}, /* nIndex */ 0);
     WalletBatch batch(GetDatabase());
-    SyncTransaction(ptx, confirm, batch);
+    SyncTransaction(tx, confirm, batch);
 
-    auto it = mapWallet.find(ptx->GetHash());
+    auto it = mapWallet.find(tx->GetHash());
     if (it != mapWallet.end()) {
         it->second.fInMempool = true;
     }
 }
 
-void CWallet::transactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) {
+void CWallet::transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {
     if (reason != MemPoolRemovalReason::CONFLICT) {
         LOCK(cs_wallet);
-        auto it = mapWallet.find(ptx->GetHash());
+        auto it = mapWallet.find(tx->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
+        LOCK(cs_wallet);
+        WalletBatch batch(GetDatabase());
+        SyncTransaction(tx, {CWalletTx::Status::UNCONFIRMED, /* block height */ 0, /* block hash */ {}, /* index */ 0}, batch);
+    }
 }
 
 void CWallet::blockConnected(const CBlock& block, int height)
@@ -1241,9 +1273,8 @@ void CWallet::blockConnected(const CBlock& block, int height)
     m_last_block_processed = block_hash;
     WalletBatch batch(GetDatabase());
     for (size_t index = 0; index < block.vtx.size(); index++) {
-        CWalletTx::Confirmation confirm(CWalletTx::Status::CONFIRMED, height, block_hash, index);
+        SyncTransaction(block.vtx[index], {CWalletTx::Status::CONFIRMED, height, block_hash, (int)index}, batch);
-        SyncTransaction(block.vtx[index], confirm, batch);
+        transactionRemovedFromMempool(block.vtx[index], MemPoolRemovalReason::BLOCK);
-        transactionRemovedFromMempool(block.vtx[index], MemPoolRemovalReason::MANUAL);
     }
 
     // reset cache to make sure no longer immature coins are included
@@ -1263,8 +1294,7 @@ void CWallet::blockDisconnected(const CBlock& block, int height)
     m_last_block_processed = block.hashPrevBlock;
     WalletBatch batch(GetDatabase());
     for (const CTransactionRef& ptx : block.vtx) {
-        CWalletTx::Confirmation confirm(CWalletTx::Status::UNCONFIRMED, /* block_height */ 0, {}, /* nIndex */ 0);
+        SyncTransaction(ptx, {CWalletTx::Status::UNCONFIRMED, /* block height */ 0, /* block hash */ {}, /* index */ 0}, batch);
-        SyncTransaction(ptx, confirm, batch);
     }
 
     // reset cache to make sure no longer mature coins are excluded
@@ -1957,8 +1987,7 @@ CWallet::ScanResult CWallet::ScanForWalletTransactions(const uint256& start_bloc
                 break;
             }
             for (size_t posInBlock = 0; posInBlock < block.vtx.size(); ++posInBlock) {
-                CWalletTx::Confirmation confirm(CWalletTx::Status::CONFIRMED, block_height, block_hash, posInBlock);
+                SyncTransaction(block.vtx[posInBlock], {CWalletTx::Status::CONFIRMED, block_height, block_hash, (int)posInBlock}, batch, fUpdate);
-                SyncTransaction(block.vtx[posInBlock], confirm, batch, fUpdate);
             }
             // scan succeeded, record block as most recent successfully scanned
             result.last_scanned_block = block_hash;

src/wallet/wallet.h

@@ -1026,7 +1026,7 @@ public:
         uint256 last_failed_block;
     };
     ScanResult ScanForWalletTransactions(const uint256& start_block, int start_height, std::optional<int> max_height, const WalletRescanReserver& reserver, bool fUpdate);
-    void transactionRemovedFromMempool(const CTransactionRef &ptx, MemPoolRemovalReason reason) override;
+    void transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) override;
     void ReacceptWalletTransactions() EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     void ResendWalletTransactions();
     struct Balance {