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

1 files changed, 16 insertions, 2 deletions

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);