Merge #18338: Fix wallet unload race condition

41b0baf43c243b64b394e774e336475a489cca2b gui: Handle WalletModel::unload asynchronous (João Barbosa)
ab31b9d6fe7b39713682e3f52d11238dbe042c16 Fix wallet unload race condition (Russell Yanofsky)

Pull request description:

  This PR consists in two fixes. The first fixes a concurrency issues with `boost::signals2`. The second fixes a wallet model destruction while it's being used.

  From boost signal documentation at https://www.boost.org/doc/libs/1_72_0/doc/html/signals2/thread-safety.html:

  > When a signal is invoked by calling signal::operator(), the invocation first acquires a lock on the signal's mutex. Then it obtains a handle to the signal's slot list and combiner. Next it releases the signal's mutex, before invoking the combiner to iterate through the slot list.

  This means that `UnregisterValidationInterface` doesn't prevent more calls to that interface. The fix consists in capturing the `shared_ptr<CValidationInterface>` in each internal slot.

  The GUI bug is fixed by using a `Qt::QueuedConnection` in the `WalletModel::unload` connection.

ACKs for top commit:
  ryanofsky:
    Code review ACK 41b0baf43c243b64b394e774e336475a489cca2b. Only change is moving assert as suggested
  hebasto:
    ACK 41b0baf43c243b64b394e774e336475a489cca2b, tested on Linux Mint 19.3.

Tree-SHA512: 4f712d8de65bc1214411831250de5dc0a9fd505fb84da5baf9f2cc4d551bc3abffc061616f00afe43dba7525af2cd96c9b54aeead9383145e3b8801f25d85f50

10 files changed, 64 insertions, 44 deletions

diff --git a/src/bench/wallet_balance.cpp b/src/bench/wallet_balance.cpp
index 62568a9da5..0381369218 100644
--- a/src/bench/wallet_balance.cpp
+++ b/src/bench/wallet_balance.cpp
@@ -23,9 +23,8 @@ static void WalletBalance(benchmark::State& state, const bool set_dirty, const b
         wallet.SetupLegacyScriptPubKeyMan();
         bool first_run;
         if (wallet.LoadWallet(first_run) != DBErrors::LOAD_OK) assert(false);
-        wallet.handleNotifications();
     }
-
+    auto handler = chain->handleNotifications({ &wallet, [](CWallet*) {} });
 
     const Optional<std::string> address_mine{add_mine ? Optional<std::string>{getnewaddress(wallet)} : nullopt};
     if (add_watchonly) importaddress(wallet, ADDRESS_WATCHONLY);
diff --git a/src/interfaces/chain.cpp b/src/interfaces/chain.cpp
index 775a89f4cf..0b3cd08e22 100644
--- a/src/interfaces/chain.cpp
+++ b/src/interfaces/chain.cpp
@@ -148,22 +148,12 @@ class LockImpl : public Chain::Lock, public UniqueLock<RecursiveMutex>
     using UniqueLock::UniqueLock;
 };
 
-class NotificationsHandlerImpl : public Handler, CValidationInterface
+class NotificationsProxy : public CValidationInterface
 {
 public:
-    explicit NotificationsHandlerImpl(Chain& chain, Chain::Notifications& notifications)
-        : m_chain(chain), m_notifications(&notifications)
-    {
-        RegisterValidationInterface(this);
-    }
-    ~NotificationsHandlerImpl() override { disconnect(); }
-    void disconnect() override
-    {
-        if (m_notifications) {
-            m_notifications = nullptr;
-            UnregisterValidationInterface(this);
-        }
-    }
+    explicit NotificationsProxy(std::shared_ptr<Chain::Notifications> notifications)
+        : m_notifications(std::move(notifications)) {}
+    virtual ~NotificationsProxy() = default;
     void TransactionAddedToMempool(const CTransactionRef& tx) override
     {
         m_notifications->transactionAddedToMempool(tx);
@@ -185,8 +175,26 @@ public:
         m_notifications->updatedBlockTip();
     }
     void ChainStateFlushed(const CBlockLocator& locator) override { m_notifications->chainStateFlushed(locator); }
-    Chain& m_chain;
-    Chain::Notifications* m_notifications;
+    std::shared_ptr<Chain::Notifications> m_notifications;
+};
+
+class NotificationsHandlerImpl : public Handler
+{
+public:
+    explicit NotificationsHandlerImpl(std::shared_ptr<Chain::Notifications> notifications)
+        : m_proxy(std::make_shared<NotificationsProxy>(std::move(notifications)))
+    {
+        RegisterSharedValidationInterface(m_proxy);
+    }
+    ~NotificationsHandlerImpl() override { disconnect(); }
+    void disconnect() override
+    {
+        if (m_proxy) {
+            UnregisterSharedValidationInterface(m_proxy);
+            m_proxy.reset();
+        }
+    }
+    std::shared_ptr<NotificationsProxy> m_proxy;
 };
 
 class RpcHandlerImpl : public Handler
@@ -342,9 +350,9 @@ public:
     {
         ::uiInterface.ShowProgress(title, progress, resume_possible);
     }
-    std::unique_ptr<Handler> handleNotifications(Notifications& notifications) override
+    std::unique_ptr<Handler> handleNotifications(std::shared_ptr<Notifications> notifications) override
     {
-        return MakeUnique<NotificationsHandlerImpl>(*this, notifications);
+        return MakeUnique<NotificationsHandlerImpl>(std::move(notifications));
     }
     void waitForNotificationsIfTipChanged(const uint256& old_tip) override
     {
diff --git a/src/interfaces/chain.h b/src/interfaces/chain.h
index caefa87e11..e1bc9bbbf3 100644
--- a/src/interfaces/chain.h
+++ b/src/interfaces/chain.h
@@ -229,7 +229,7 @@ public:
     };
 
     //! Register handler for notifications.
-    virtual std::unique_ptr<Handler> handleNotifications(Notifications& notifications) = 0;
+    virtual std::unique_ptr<Handler> handleNotifications(std::shared_ptr<Notifications> notifications) = 0;
 
     //! Wait for pending notifications to be processed unless block hash points to the current
     //! chain tip.
diff --git a/src/qt/walletcontroller.cpp b/src/qt/walletcontroller.cpp
index 233c0ab6be..88c694567e 100644
--- a/src/qt/walletcontroller.cpp
+++ b/src/qt/walletcontroller.cpp
@@ -116,7 +116,7 @@ WalletModel* WalletController::getOrCreateWallet(std::unique_ptr<interfaces::Wal
     const bool called = QMetaObject::invokeMethod(wallet_model, "startPollBalance");
     assert(called);
 
-    connect(wallet_model, &WalletModel::unload, [this, wallet_model] {
+    connect(wallet_model, &WalletModel::unload, this, [this, wallet_model] {
         // Defer removeAndDeleteWallet when no modal widget is active.
         // TODO: remove this workaround by removing usage of QDiallog::exec.
         if (QApplication::activeModalWidget()) {
@@ -128,7 +128,7 @@ WalletModel* WalletController::getOrCreateWallet(std::unique_ptr<interfaces::Wal
         } else {
             removeAndDeleteWallet(wallet_model);
         }
-    });
+    }, Qt::QueuedConnection);
 
     // Re-emit coinsSent signal from wallet model.
     connect(wallet_model, &WalletModel::coinsSent, this, &WalletController::coinsSent);
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index c895904b19..f9f61e8a09 100644
--- a/src/validationinterface.cpp
+++ b/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);
diff --git a/src/validationinterface.h b/src/validationinterface.h
index 5707422635..f9a359b7ad 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -30,6 +30,14 @@ void RegisterValidationInterface(CValidationInterface* pwalletIn);
 void UnregisterValidationInterface(CValidationInterface* pwalletIn);
 /** Unregister all wallets from core */
 void UnregisterAllValidationInterfaces();
+
+// Alternate registration functions that release a shared_ptr after the last
+// notification is sent. These are useful for race-free cleanup, since
+// unregistration is nonblocking and can return before the last notification is
+// processed.
+void RegisterSharedValidationInterface(std::shared_ptr<CValidationInterface> callbacks);
+void UnregisterSharedValidationInterface(std::shared_ptr<CValidationInterface> callbacks);
+
 /**
  * Pushes a function to callback onto the notification queue, guaranteeing any
  * callbacks generated prior to now are finished when the function is called.
@@ -163,7 +171,7 @@ protected:
      * Notifies listeners that a block which builds directly on our current tip
      * has been received and connected to the headers tree, though not validated yet */
     virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& block) {};
-    friend void ::RegisterValidationInterface(CValidationInterface*);
+    friend void ::RegisterSharedValidationInterface(std::shared_ptr<CValidationInterface>);
     friend void ::UnregisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterAllValidationInterfaces();
 };
@@ -173,7 +181,7 @@ class CMainSignals {
 private:
     std::unique_ptr<MainSignalsInstance> m_internals;
 
-    friend void ::RegisterValidationInterface(CValidationInterface*);
+    friend void ::RegisterSharedValidationInterface(std::shared_ptr<CValidationInterface>);
     friend void ::UnregisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterAllValidationInterfaces();
     friend void ::CallFunctionInValidationInterfaceQueue(std::function<void ()> func);
diff --git a/src/wallet/test/wallet_test_fixture.cpp b/src/wallet/test/wallet_test_fixture.cpp
index ba0843f352..b9e714946d 100644
--- a/src/wallet/test/wallet_test_fixture.cpp
+++ b/src/wallet/test/wallet_test_fixture.cpp
@@ -10,7 +10,6 @@ WalletTestingSetup::WalletTestingSetup(const std::string& chainName)
 {
     bool fFirstRun;
     m_wallet.LoadWallet(fFirstRun);
-    m_wallet.handleNotifications();
-
+    m_chain_notifications_handler = m_chain->handleNotifications({ &m_wallet, [](CWallet*) {} });
     m_chain_client->registerRpcs();
 }
diff --git a/src/wallet/test/wallet_test_fixture.h b/src/wallet/test/wallet_test_fixture.h
index 4e4129fb2c..81d8a60b8a 100644
--- a/src/wallet/test/wallet_test_fixture.h
+++ b/src/wallet/test/wallet_test_fixture.h
@@ -23,6 +23,7 @@ struct WalletTestingSetup: public TestingSetup {
     std::unique_ptr<interfaces::Chain> m_chain = interfaces::MakeChain(m_node);
     std::unique_ptr<interfaces::ChainClient> m_chain_client = interfaces::MakeWalletClient(*m_chain, {});
     CWallet m_wallet;
+    std::unique_ptr<interfaces::Handler> m_chain_notifications_handler;
 };
 
 #endif // BITCOIN_WALLET_TEST_WALLET_TEST_FIXTURE_H
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 9a972febab..98f308f927 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -62,8 +62,10 @@ bool AddWallet(const std::shared_ptr<CWallet>& wallet)
 
 bool RemoveWallet(const std::shared_ptr<CWallet>& wallet)
 {
-    LOCK(cs_wallets);
     assert(wallet);
+    // Unregister with the validation interface which also drops shared ponters.
+    wallet->m_chain_notifications_handler.reset();
+    LOCK(cs_wallets);
     std::vector<std::shared_ptr<CWallet>>::iterator i = std::find(vpwallets.begin(), vpwallets.end(), wallet);
     if (i == vpwallets.end()) return false;
     vpwallets.erase(i);
@@ -105,13 +107,9 @@ static std::set<std::string> g_unloading_wallet_set;
 // Custom deleter for shared_ptr<CWallet>.
 static void ReleaseWallet(CWallet* wallet)
 {
-    // Unregister and delete the wallet right after BlockUntilSyncedToCurrentChain
-    // so that it's in sync with the current chainstate.
     const std::string name = wallet->GetName();
     wallet->WalletLogPrintf("Releasing wallet\n");
-    wallet->BlockUntilSyncedToCurrentChain();
     wallet->Flush();
-    wallet->m_chain_notifications_handler.reset();
     delete wallet;
     // Wallet is now released, notify UnloadWallet, if any.
     {
@@ -137,6 +135,7 @@ void UnloadWallet(std::shared_ptr<CWallet>&& wallet)
     // Notify the unload intent so that all remaining shared pointers are
     // released.
     wallet->NotifyUnload();
+
     // Time to ditch our shared_ptr and wait for ReleaseWallet call.
     wallet.reset();
     {
@@ -4092,7 +4091,7 @@ std::shared_ptr<CWallet> CWallet::CreateWalletFromFile(interfaces::Chain& chain,
     }
 
     // Register with the validation interface. It's ok to do this after rescan since we're still holding locked_chain.
-    walletInstance->handleNotifications();
+    walletInstance->m_chain_notifications_handler = walletInstance->chain().handleNotifications(walletInstance);
 
     walletInstance->SetBroadcastTransactions(gArgs.GetBoolArg("-walletbroadcast", DEFAULT_WALLETBROADCAST));
 
@@ -4105,11 +4104,6 @@ std::shared_ptr<CWallet> CWallet::CreateWalletFromFile(interfaces::Chain& chain,
     return walletInstance;
 }
 
-void CWallet::handleNotifications()
-{
-    m_chain_notifications_handler = m_chain->handleNotifications(*this);
-}
-
 void CWallet::postInitProcess()
 {
     auto locked_chain = chain().lock();
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 9a83c84fc4..fe59773488 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -606,7 +606,7 @@ class WalletRescanReserver; //forward declarations for ScanForWalletTransactions
 /**
  * A CWallet maintains a set of transactions and balances, and provides the ability to create new transactions.
  */
-class CWallet final : public WalletStorage, private interfaces::Chain::Notifications
+class CWallet final : public WalletStorage, public interfaces::Chain::Notifications
 {
 private:
     CKeyingMaterial vMasterKey GUARDED_BY(cs_wallet);
@@ -782,9 +782,6 @@ public:
     /** Registered interfaces::Chain::Notifications handler. */
     std::unique_ptr<interfaces::Handler> m_chain_notifications_handler;
 
-    /** Register the wallet for chain notifications */
-    void handleNotifications();
-
     /** Interface for accessing chain state. */
     interfaces::Chain& chain() const { assert(m_chain); return *m_chain; }