wallet: introduce active db txn listeners

Useful to ensure that the in-memory state is updated only
after successfully committing the data to disk.

src/wallet/bdb.h

@@ -208,6 +208,7 @@ public:
     bool TxnBegin() override;
     bool TxnCommit() override;
     bool TxnAbort() override;
+    bool HasActiveTxn() override { return activeTxn != nullptr; }
     DbTxn* txn() const { return activeTxn; }
 };
 

src/wallet/db.h

@@ -122,6 +122,7 @@ public:
     virtual bool TxnBegin() = 0;
     virtual bool TxnCommit() = 0;
     virtual bool TxnAbort() = 0;
+    virtual bool HasActiveTxn() = 0;
 };
 
 /** An instance of this class represents one database.

src/wallet/migrate.h

@@ -115,6 +115,7 @@ public:
     bool TxnBegin() override { return false; }
     bool TxnCommit() override { return false; }
     bool TxnAbort() override { return false; }
+    bool HasActiveTxn() override { return false; }
 };
 
 //! Return object giving access to Berkeley Read Only database at specified path.

src/wallet/salvage.cpp

@@ -44,6 +44,7 @@ public:
     bool TxnBegin() override { return true; }
     bool TxnCommit() override { return true; }
     bool TxnAbort() override { return true; }
+    bool HasActiveTxn() override { return false; }
 };
 
 /** A dummy WalletDatabase that does nothing and never fails. Only used by salvage.

src/wallet/sqlite.h

@@ -95,6 +95,7 @@ public:
     bool TxnBegin() override;
     bool TxnCommit() override;
     bool TxnAbort() override;
+    bool HasActiveTxn() override { return m_txn; }
 };
 
 /** An instance of this class represents one SQLite3 database.

src/wallet/test/util.h

@@ -95,6 +95,7 @@ public:
     bool TxnBegin() override { return m_pass; }
     bool TxnCommit() override { return m_pass; }
     bool TxnAbort() override { return m_pass; }
+    bool HasActiveTxn() override { return false; }
 };
 
 /** A WalletDatabase whose contents and return values can be modified as needed for testing

src/wallet/walletdb.cpp

@@ -1347,12 +1347,34 @@ bool WalletBatch::TxnBegin()
 
 bool WalletBatch::TxnCommit()
 {
-    return m_batch->TxnCommit();
+    bool res = m_batch->TxnCommit();
+    if (res) {
+        for (const auto& listener : m_txn_listeners) {
+            listener.on_commit();
+        }
+        // txn finished, clear listeners
+        m_txn_listeners.clear();
+    }
+    return res;
 }
 
 bool WalletBatch::TxnAbort()
 {
-    return m_batch->TxnAbort();
+    bool res = m_batch->TxnAbort();
+    if (res) {
+        for (const auto& listener : m_txn_listeners) {
+            listener.on_abort();
+        }
+        // txn finished, clear listeners
+        m_txn_listeners.clear();
+    }
+    return res;
+}
+
+void WalletBatch::RegisterTxnListener(const DbTxnListener& l)
+{
+    assert(m_batch->HasActiveTxn());
+    m_txn_listeners.emplace_back(l);
 }
 
 std::unique_ptr<WalletDatabase> MakeDatabase(const fs::path& path, const DatabaseOptions& options, DatabaseStatus& status, bilingual_str& error)

src/wallet/walletdb.h

@@ -180,6 +180,11 @@ public:
     }
 };
 
+struct DbTxnListener
+{
+    std::function<void()> on_commit, on_abort;
+};
+
 /** Access to the wallet database.
  * Opens the database and provides read and write access to it. Each read and write is its own transaction.
  * Multiple operation transactions can be started using TxnBegin() and committed using TxnCommit()
@@ -292,9 +297,18 @@ public:
     bool TxnCommit();
     //! Abort current transaction
     bool TxnAbort();
+    bool HasActiveTxn() { return m_batch->HasActiveTxn(); }
+
+    //! Registers db txn callback functions
+    void RegisterTxnListener(const DbTxnListener& l);
+
 private:
     std::unique_ptr<DatabaseBatch> m_batch;
     WalletDatabase& m_database;
+
+    // External functions listening to the current db txn outcome.
+    // Listeners are cleared at the end of the transaction.
+    std::vector<DbTxnListener> m_txn_listeners;
 };
 
 /**