Fix wallet unload race condition

Currently it's possible for ReleaseWallet to delete the CWallet pointer while
it is processing BlockConnected, etc chain notifications.

To fix this, unregister from notifications earlier in UnloadWallet instead of
ReleaseWallet, and use a new RegisterSharedValidationInterface function to
prevent the CValidationInterface shared_ptr from being deleted until the last
notification is actually finished.

src/validationinterface.cpp

@@ -75,8 +75,10 @@ CMainSignals& GetMainSignals()
     return g_signals;
 }
 
-void RegisterValidationInterface(CValidationInterface* pwalletIn) {
-    ValidationInterfaceConnections& conns = g_signals.m_internals->m_connMainSignals[pwalletIn];
+void RegisterSharedValidationInterface(std::shared_ptr<CValidationInterface> pwalletIn) {
+    // Each connection captures pwalletIn to ensure that each callback is
+    // executed before pwalletIn is destroyed. For more details see #18338.
+    ValidationInterfaceConnections& conns = g_signals.m_internals->m_connMainSignals[pwalletIn.get()];
     conns.UpdatedBlockTip = g_signals.m_internals->UpdatedBlockTip.connect(std::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3));
     conns.TransactionAddedToMempool = g_signals.m_internals->TransactionAddedToMempool.connect(std::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, std::placeholders::_1));
     conns.BlockConnected = g_signals.m_internals->BlockConnected.connect(std::bind(&CValidationInterface::BlockConnected, pwalletIn, std::placeholders::_1, std::placeholders::_2));
@@ -87,6 +89,18 @@ void RegisterValidationInterface(CValidationInterface* pwalletIn) {
     conns.NewPoWValidBlock = g_signals.m_internals->NewPoWValidBlock.connect(std::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, std::placeholders::_1, std::placeholders::_2));
 }
 
+void RegisterValidationInterface(CValidationInterface* callbacks)
+{
+    // Create a shared_ptr with a no-op deleter - CValidationInterface lifecycle
+    // is managed by the caller.
+    RegisterSharedValidationInterface({callbacks, [](CValidationInterface*){}});
+}
+
+void UnregisterSharedValidationInterface(std::shared_ptr<CValidationInterface> callbacks)
+{
+    UnregisterValidationInterface(callbacks.get());
+}
+
 void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
     if (g_signals.m_internals) {
         g_signals.m_internals->m_connMainSignals.erase(pwalletIn);