Add CallFunctionInQueue to wait on validation interface queue drain

src/validationinterface.cpp

@@ -104,6 +104,10 @@ void UnregisterAllValidationInterfaces() {
     g_signals.m_internals->NewPoWValidBlock.disconnect_all_slots();
 }
 
+void CallFunctionInValidationInterfaceQueue(std::function<void ()> func) {
+    g_signals.m_internals->m_schedulerClient.AddToProcessQueue(std::move(func));
+}
+
 void CMainSignals::MempoolEntryRemoved(CTransactionRef ptx, MemPoolRemovalReason reason) {
     if (reason != MemPoolRemovalReason::BLOCK && reason != MemPoolRemovalReason::CONFLICT) {
         m_internals->m_schedulerClient.AddToProcessQueue([ptx, this] {

src/validationinterface.h

@@ -6,10 +6,11 @@
 #ifndef BITCOIN_VALIDATIONINTERFACE_H
 #define BITCOIN_VALIDATIONINTERFACE_H
 
-#include <memory>
-
 #include "primitives/transaction.h" // CTransaction(Ref)
 
+#include <functional>
+#include <memory>
+
 class CBlock;
 class CBlockIndex;
 struct CBlockLocator;
@@ -31,6 +32,16 @@ void RegisterValidationInterface(CValidationInterface* pwalletIn);
 void UnregisterValidationInterface(CValidationInterface* pwalletIn);
 /** Unregister all wallets from core */
 void UnregisterAllValidationInterfaces();
+/**
+ * Pushes a function to callback onto the notification queue, guaranteeing any
+ * callbacks generated prior to now are finished when the function is called.
+ *
+ * Be very careful blocking on func to be called if any locks are held -
+ * validation interface clients may not be able to make progress as they often
+ * wait for things like cs_main, so blocking until func is called with cs_main
+ * will result in a deadlock (that DEBUG_LOCKORDER will miss).
+ */
+void CallFunctionInValidationInterfaceQueue(std::function<void ()> func);
 
 class CValidationInterface {
 protected:
@@ -86,6 +97,7 @@ private:
     friend void ::RegisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterAllValidationInterfaces();
+    friend void ::CallFunctionInValidationInterfaceQueue(std::function<void ()> func);
 
     void MempoolEntryRemoved(CTransactionRef tx, MemPoolRemovalReason reason);