Merge #10179: Give CValidationInterface Support for calling notifications on the CScheduler Thread

1f668b6 Expose if CScheduler is being serviced, assert its not in EmptyQueue (Matt Corallo)
3192975 Flush CValidationInterface callbacks prior to destruction (Matt Corallo)
08096bb Support more than one CScheduler thread for serial clients (Matt Corallo)
2fbf2db Add default arg to CScheduler to schedule() a callback now (Matt Corallo)
cda1429 Give CMainSignals a reference to the global scheduler (Matt Corallo)
3a19fed Make ValidationInterface signals-type-agnostic (Matt Corallo)
ff6a834 Use TestingSetup to DRY qt rpcnestedtests (Matt Corallo)

Tree-SHA512: fab91e34e30b080ed4d0a6d8c1214910e383c45440676e37be61d0bde6ae98d61e8903d22b846e95ba4e73a6ce788798350266feba246d8a2ab357e8523e4ac5

8 files changed, 274 insertions, 73 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 672ef77e80..12f29a4ca8 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -215,6 +215,19 @@ void Shutdown()
         fFeeEstimatesInitialized = false;
     }
 
+    // FlushStateToDisk generates a SetBestChain callback, which we should avoid missing
+    FlushStateToDisk();
+
+    // After there are no more peers/RPC left to give us new data which may generate
+    // CValidationInterface callbacks, flush them...
+    GetMainSignals().FlushBackgroundCallbacks();
+
+    // Any future callbacks will be dropped. This should absolutely be safe - if
+    // missing a callback results in an unrecoverable situation, unclean shutdown
+    // would too. The only reason to do the above flushes is to let the wallet catch
+    // up with our current chain to avoid any strange pruning edge cases and make
+    // next startup faster by avoiding rescan.
+
     {
         LOCK(cs_main);
         if (pcoinsTip != NULL) {
@@ -251,6 +264,7 @@ void Shutdown()
     }
 #endif
     UnregisterAllValidationInterfaces();
+    GetMainSignals().UnregisterBackgroundSignalScheduler();
 #ifdef ENABLE_WALLET
     for (CWalletRef pwallet : vpwallets) {
         delete pwallet;
@@ -1203,6 +1217,8 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
     CScheduler::Function serviceLoop = boost::bind(&CScheduler::serviceQueue, &scheduler);
     threadGroup.create_thread(boost::bind(&TraceThread<CScheduler::Function>, "scheduler", serviceLoop));
 
+    GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
+
     /* Start the RPC server already.  It will be started in "warmup" mode
      * and not really process calls already (but it will signify connections
      * that the server is there and will be ready later).  Warmup mode will
diff --git a/src/qt/test/rpcnestedtests.cpp b/src/qt/test/rpcnestedtests.cpp
index 26dec3c610..fbad9e544a 100644
--- a/src/qt/test/rpcnestedtests.cpp
+++ b/src/qt/test/rpcnestedtests.cpp
@@ -12,6 +12,7 @@
 #include "rpc/server.h"
 #include "rpcconsole.h"
 #include "test/testutil.h"
+#include "test/test_bitcoin.h"
 #include "univalue.h"
 #include "util.h"
 
@@ -35,8 +36,6 @@ void RPCNestedTests::rpcNestedTests()
 {
     // do some test setup
     // could be moved to a more generic place when we add more tests on QT level
-    const CChainParams& chainparams = Params();
-    RegisterAllCoreRPCCommands(tableRPC);
     tableRPC.appendCommand("rpcNestedTest", &vRPCCommands[0]);
     ClearDatadirCache();
     std::string path = QDir::tempPath().toStdString() + "/" + strprintf("test_bitcoin_qt_%lu_%i", (unsigned long)GetTime(), (int)(GetRand(100000)));
@@ -44,15 +43,8 @@ void RPCNestedTests::rpcNestedTests()
     dir.mkpath(".");
     ForceSetArg("-datadir", path);
     //mempool.setSanityCheck(1.0);
-    pblocktree = new CBlockTreeDB(1 << 20, true);
-    pcoinsdbview = new CCoinsViewDB(1 << 23, true);
-    pcoinsTip = new CCoinsViewCache(pcoinsdbview);
-    InitBlockIndex(chainparams);
-    {
-        CValidationState state;
-        bool ok = ActivateBestChain(state, chainparams);
-        QVERIFY(ok);
-    }
+
+    TestingSetup test;
 
     SetRPCWarmupFinished();
 
@@ -145,13 +137,5 @@ void RPCNestedTests::rpcNestedTests()
     QVERIFY_EXCEPTION_THROWN(RPCConsole::RPCExecuteCommandLine(result, "rpcNestedTest(abc,,)"), std::runtime_error); //don't tollerate empty arguments when using ,
 #endif
 
-    UnloadBlockIndex();
-    delete pcoinsTip;
-    pcoinsTip = nullptr;
-    delete pcoinsdbview;
-    pcoinsdbview = nullptr;
-    delete pblocktree;
-    pblocktree = nullptr;
-
     fs::remove_all(fs::path(path));
 }
diff --git a/src/scheduler.cpp b/src/scheduler.cpp
index 923ba2c231..36a6d5110d 100644
--- a/src/scheduler.cpp
+++ b/src/scheduler.cpp
@@ -139,3 +139,69 @@ size_t CScheduler::getQueueInfo(boost::chrono::system_clock::time_point &first,
     }
     return result;
 }
+
+bool CScheduler::AreThreadsServicingQueue() const {
+    return nThreadsServicingQueue;
+}
+
+
+void SingleThreadedSchedulerClient::MaybeScheduleProcessQueue() {
+    {
+        LOCK(m_cs_callbacks_pending);
+        // Try to avoid scheduling too many copies here, but if we
+        // accidentally have two ProcessQueue's scheduled at once its
+        // not a big deal.
+        if (m_are_callbacks_running) return;
+        if (m_callbacks_pending.empty()) return;
+    }
+    m_pscheduler->schedule(std::bind(&SingleThreadedSchedulerClient::ProcessQueue, this));
+}
+
+void SingleThreadedSchedulerClient::ProcessQueue() {
+    std::function<void (void)> callback;
+    {
+        LOCK(m_cs_callbacks_pending);
+        if (m_are_callbacks_running) return;
+        if (m_callbacks_pending.empty()) return;
+        m_are_callbacks_running = true;
+
+        callback = std::move(m_callbacks_pending.front());
+        m_callbacks_pending.pop_front();
+    }
+
+    // RAII the setting of fCallbacksRunning and calling MaybeScheduleProcessQueue
+    // to ensure both happen safely even if callback() throws.
+    struct RAIICallbacksRunning {
+        SingleThreadedSchedulerClient* instance;
+        RAIICallbacksRunning(SingleThreadedSchedulerClient* _instance) : instance(_instance) {}
+        ~RAIICallbacksRunning() {
+            {
+                LOCK(instance->m_cs_callbacks_pending);
+                instance->m_are_callbacks_running = false;
+            }
+            instance->MaybeScheduleProcessQueue();
+        }
+    } raiicallbacksrunning(this);
+
+    callback();
+}
+
+void SingleThreadedSchedulerClient::AddToProcessQueue(std::function<void (void)> func) {
+    assert(m_pscheduler);
+
+    {
+        LOCK(m_cs_callbacks_pending);
+        m_callbacks_pending.emplace_back(std::move(func));
+    }
+    MaybeScheduleProcessQueue();
+}
+
+void SingleThreadedSchedulerClient::EmptyQueue() {
+    assert(!m_pscheduler->AreThreadsServicingQueue());
+    bool should_continue = true;
+    while (should_continue) {
+        ProcessQueue();
+        LOCK(m_cs_callbacks_pending);
+        should_continue = !m_callbacks_pending.empty();
+    }
+}
diff --git a/src/scheduler.h b/src/scheduler.h
index 27412a15b4..0365d668b2 100644
--- a/src/scheduler.h
+++ b/src/scheduler.h
@@ -14,6 +14,8 @@
 #include <boost/thread.hpp>
 #include <map>
 
+#include "sync.h"
+
 //
 // Simple class for background tasks that should be run
 // periodically or once "after a while"
@@ -41,7 +43,7 @@ public:
     typedef std::function<void(void)> Function;
 
     // Call func at/after time t
-    void schedule(Function f, boost::chrono::system_clock::time_point t);
+    void schedule(Function f, boost::chrono::system_clock::time_point t=boost::chrono::system_clock::now());
 
     // Convenience method: call f once deltaSeconds from now
     void scheduleFromNow(Function f, int64_t deltaMilliSeconds);
@@ -69,6 +71,9 @@ public:
     size_t getQueueInfo(boost::chrono::system_clock::time_point &first,
                         boost::chrono::system_clock::time_point &last) const;
 
+    // Returns true if there are threads actively running in serviceQueue()
+    bool AreThreadsServicingQueue() const;
+
 private:
     std::multimap<boost::chrono::system_clock::time_point, Function> taskQueue;
     boost::condition_variable newTaskScheduled;
@@ -79,4 +84,30 @@ private:
     bool shouldStop() { return stopRequested || (stopWhenEmpty && taskQueue.empty()); }
 };
 
+/**
+ * Class used by CScheduler clients which may schedule multiple jobs
+ * which are required to be run serially. Does not require such jobs
+ * to be executed on the same thread, but no two jobs will be executed
+ * at the same time.
+ */
+class SingleThreadedSchedulerClient {
+private:
+    CScheduler *m_pscheduler;
+
+    CCriticalSection m_cs_callbacks_pending;
+    std::list<std::function<void (void)>> m_callbacks_pending;
+    bool m_are_callbacks_running = false;
+
+    void MaybeScheduleProcessQueue();
+    void ProcessQueue();
+
+public:
+    SingleThreadedSchedulerClient(CScheduler *pschedulerIn) : m_pscheduler(pschedulerIn) {}
+    void AddToProcessQueue(std::function<void (void)> func);
+
+    // Processes all remaining queue members on the calling thread, blocking until queue is empty
+    // Must be called after the CScheduler has no remaining processing threads!
+    void EmptyQueue();
+};
+
 #endif
diff --git a/src/test/test_bitcoin.cpp b/src/test/test_bitcoin.cpp
index 579e96524c..3ba81ed17b 100644
--- a/src/test/test_bitcoin.cpp
+++ b/src/test/test_bitcoin.cpp
@@ -62,6 +62,12 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
         pathTemp = GetTempPath() / strprintf("test_bitcoin_%lu_%i", (unsigned long)GetTime(), (int)(InsecureRandRange(100000)));
         fs::create_directories(pathTemp);
         ForceSetArg("-datadir", pathTemp.string());
+
+        // Note that because we don't bother running a scheduler thread here,
+        // callbacks via CValidationInterface are unreliable, but that's OK,
+        // our unit tests aren't testing multiple parts of the code at once.
+        GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
+
         mempool.setSanityCheck(1.0);
         pblocktree = new CBlockTreeDB(1 << 20, true);
         pcoinsdbview = new CCoinsViewDB(1 << 23, true);
@@ -88,6 +94,8 @@ TestingSetup::~TestingSetup()
         UnregisterNodeSignals(GetNodeSignals());
         threadGroup.interrupt_all();
         threadGroup.join_all();
+        GetMainSignals().FlushBackgroundCallbacks();
+        GetMainSignals().UnregisterBackgroundSignalScheduler();
         UnloadBlockIndex();
         delete pcoinsTip;
         delete pcoinsdbview;
diff --git a/src/test/test_bitcoin.h b/src/test/test_bitcoin.h
index c9e4a3427f..dd3b13c8c8 100644
--- a/src/test/test_bitcoin.h
+++ b/src/test/test_bitcoin.h
@@ -10,6 +10,7 @@
 #include "key.h"
 #include "pubkey.h"
 #include "random.h"
+#include "scheduler.h"
 #include "txdb.h"
 #include "txmempool.h"
 
@@ -53,6 +54,7 @@ struct TestingSetup: public BasicTestingSetup {
     fs::path pathTemp;
     boost::thread_group threadGroup;
     CConnman* connman;
+    CScheduler scheduler;
 
     TestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
     ~TestingSetup();
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index be2f20b863..bf20d606f8 100644
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -4,46 +4,123 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "validationinterface.h"
+#include "init.h"
+#include "scheduler.h"
+#include "sync.h"
+#include "util.h"
+
+#include <list>
+#include <atomic>
+
+#include <boost/signals2/signal.hpp>
+
+struct MainSignalsInstance {
+    boost::signals2::signal<void (const CBlockIndex *, const CBlockIndex *, bool fInitialDownload)> UpdatedBlockTip;
+    boost::signals2::signal<void (const CTransactionRef &)> TransactionAddedToMempool;
+    boost::signals2::signal<void (const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex, const std::vector<CTransactionRef>&)> BlockConnected;
+    boost::signals2::signal<void (const std::shared_ptr<const CBlock> &)> BlockDisconnected;
+    boost::signals2::signal<void (const CBlockLocator &)> SetBestChain;
+    boost::signals2::signal<void (const uint256 &)> Inventory;
+    boost::signals2::signal<void (int64_t nBestBlockTime, CConnman* connman)> Broadcast;
+    boost::signals2::signal<void (const CBlock&, const CValidationState&)> BlockChecked;
+    boost::signals2::signal<void (const CBlockIndex *, const std::shared_ptr<const CBlock>&)> NewPoWValidBlock;
+
+    // We are not allowed to assume the scheduler only runs in one thread,
+    // but must ensure all callbacks happen in-order, so we end up creating
+    // our own queue here :(
+    SingleThreadedSchedulerClient m_schedulerClient;
+
+    MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}
+};
 
 static CMainSignals g_signals;
 
+void CMainSignals::RegisterBackgroundSignalScheduler(CScheduler& scheduler) {
+    assert(!m_internals);
+    m_internals.reset(new MainSignalsInstance(&scheduler));
+}
+
+void CMainSignals::UnregisterBackgroundSignalScheduler() {
+    m_internals.reset(nullptr);
+}
+
+void CMainSignals::FlushBackgroundCallbacks() {
+    m_internals->m_schedulerClient.EmptyQueue();
+}
+
 CMainSignals& GetMainSignals()
 {
     return g_signals;
 }
 
 void RegisterValidationInterface(CValidationInterface* pwalletIn) {
-    g_signals.UpdatedBlockTip.connect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
-    g_signals.TransactionAddedToMempool.connect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));
-    g_signals.BlockConnected.connect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));
-    g_signals.BlockDisconnected.connect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));
-    g_signals.SetBestChain.connect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));
-    g_signals.Inventory.connect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));
-    g_signals.Broadcast.connect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));
-    g_signals.BlockChecked.connect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
-    g_signals.NewPoWValidBlock.connect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
+    g_signals.m_internals->UpdatedBlockTip.connect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
+    g_signals.m_internals->TransactionAddedToMempool.connect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));
+    g_signals.m_internals->BlockConnected.connect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));
+    g_signals.m_internals->BlockDisconnected.connect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));
+    g_signals.m_internals->SetBestChain.connect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));
+    g_signals.m_internals->Inventory.connect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));
+    g_signals.m_internals->Broadcast.connect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));
+    g_signals.m_internals->BlockChecked.connect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
+    g_signals.m_internals->NewPoWValidBlock.connect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
 }
 
 void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
-    g_signals.BlockChecked.disconnect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
-    g_signals.Broadcast.disconnect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));
-    g_signals.Inventory.disconnect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));
-    g_signals.SetBestChain.disconnect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));
-    g_signals.TransactionAddedToMempool.disconnect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));
-    g_signals.BlockConnected.disconnect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));
-    g_signals.BlockDisconnected.disconnect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));
-    g_signals.UpdatedBlockTip.disconnect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
-    g_signals.NewPoWValidBlock.disconnect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
+    g_signals.m_internals->BlockChecked.disconnect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
+    g_signals.m_internals->Broadcast.disconnect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));
+    g_signals.m_internals->Inventory.disconnect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));
+    g_signals.m_internals->SetBestChain.disconnect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));
+    g_signals.m_internals->TransactionAddedToMempool.disconnect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));
+    g_signals.m_internals->BlockConnected.disconnect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));
+    g_signals.m_internals->BlockDisconnected.disconnect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));
+    g_signals.m_internals->UpdatedBlockTip.disconnect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
+    g_signals.m_internals->NewPoWValidBlock.disconnect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
 }
 
 void UnregisterAllValidationInterfaces() {
-    g_signals.BlockChecked.disconnect_all_slots();
-    g_signals.Broadcast.disconnect_all_slots();
-    g_signals.Inventory.disconnect_all_slots();
-    g_signals.SetBestChain.disconnect_all_slots();
-    g_signals.TransactionAddedToMempool.disconnect_all_slots();
-    g_signals.BlockConnected.disconnect_all_slots();
-    g_signals.BlockDisconnected.disconnect_all_slots();
-    g_signals.UpdatedBlockTip.disconnect_all_slots();
-    g_signals.NewPoWValidBlock.disconnect_all_slots();
+    g_signals.m_internals->BlockChecked.disconnect_all_slots();
+    g_signals.m_internals->Broadcast.disconnect_all_slots();
+    g_signals.m_internals->Inventory.disconnect_all_slots();
+    g_signals.m_internals->SetBestChain.disconnect_all_slots();
+    g_signals.m_internals->TransactionAddedToMempool.disconnect_all_slots();
+    g_signals.m_internals->BlockConnected.disconnect_all_slots();
+    g_signals.m_internals->BlockDisconnected.disconnect_all_slots();
+    g_signals.m_internals->UpdatedBlockTip.disconnect_all_slots();
+    g_signals.m_internals->NewPoWValidBlock.disconnect_all_slots();
+}
+
+void CMainSignals::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {
+    m_internals->UpdatedBlockTip(pindexNew, pindexFork, fInitialDownload);
+}
+
+void CMainSignals::TransactionAddedToMempool(const CTransactionRef &ptx) {
+    m_internals->TransactionAddedToMempool(ptx);
+}
+
+void CMainSignals::BlockConnected(const std::shared_ptr<const CBlock> &pblock, const CBlockIndex *pindex, const std::vector<CTransactionRef>& vtxConflicted) {
+    m_internals->BlockConnected(pblock, pindex, vtxConflicted);
+}
+
+void CMainSignals::BlockDisconnected(const std::shared_ptr<const CBlock> &pblock) {
+    m_internals->BlockDisconnected(pblock);
+}
+
+void CMainSignals::SetBestChain(const CBlockLocator &locator) {
+    m_internals->SetBestChain(locator);
+}
+
+void CMainSignals::Inventory(const uint256 &hash) {
+    m_internals->Inventory(hash);
+}
+
+void CMainSignals::Broadcast(int64_t nBestBlockTime, CConnman* connman) {
+    m_internals->Broadcast(nBestBlockTime, connman);
+}
+
+void CMainSignals::BlockChecked(const CBlock& block, const CValidationState& state) {
+    m_internals->BlockChecked(block, state);
+}
+
+void CMainSignals::NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock> &block) {
+    m_internals->NewPoWValidBlock(pindex, block);
 }
diff --git a/src/validationinterface.h b/src/validationinterface.h
index 17545018df..568da66df2 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -6,7 +6,6 @@
 #ifndef BITCOIN_VALIDATIONINTERFACE_H
 #define BITCOIN_VALIDATIONINTERFACE_H
 
-#include <boost/signals2/signal.hpp>
 #include <memory>
 
 #include "primitives/transaction.h" // CTransaction(Ref)
@@ -20,6 +19,7 @@ class CReserveScript;
 class CValidationInterface;
 class CValidationState;
 class uint256;
+class CScheduler;
 
 // These functions dispatch to one or all registered wallets
 
@@ -32,49 +32,66 @@ void UnregisterAllValidationInterfaces();
 
 class CValidationInterface {
 protected:
-    virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
-    virtual void TransactionAddedToMempool(const CTransactionRef &ptxn) {}
-    virtual void BlockConnected(const std::shared_ptr<const CBlock> &block, const CBlockIndex *pindex, const std::vector<CTransactionRef> &txnConflicted) {}
-    virtual void BlockDisconnected(const std::shared_ptr<const CBlock> &block) {}
-    virtual void SetBestChain(const CBlockLocator &locator) {}
-    virtual void Inventory(const uint256 &hash) {}
-    virtual void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) {}
-    virtual void BlockChecked(const CBlock&, const CValidationState&) {}
-    virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& block) {};
-    friend void ::RegisterValidationInterface(CValidationInterface*);
-    friend void ::UnregisterValidationInterface(CValidationInterface*);
-    friend void ::UnregisterAllValidationInterfaces();
-};
-
-struct CMainSignals {
     /** Notifies listeners of updated block chain tip */
-    boost::signals2::signal<void (const CBlockIndex *, const CBlockIndex *, bool fInitialDownload)> UpdatedBlockTip;
+    virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
     /** Notifies listeners of a transaction having been added to mempool. */
-    boost::signals2::signal<void (const CTransactionRef &)> TransactionAddedToMempool;
+    virtual void TransactionAddedToMempool(const CTransactionRef &ptxn) {}
     /**
      * Notifies listeners of a block being connected.
      * Provides a vector of transactions evicted from the mempool as a result.
      */
-    boost::signals2::signal<void (const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex, const std::vector<CTransactionRef> &)> BlockConnected;
+    virtual void BlockConnected(const std::shared_ptr<const CBlock> &block, const CBlockIndex *pindex, const std::vector<CTransactionRef> &txnConflicted) {}
     /** Notifies listeners of a block being disconnected */
-    boost::signals2::signal<void (const std::shared_ptr<const CBlock> &)> BlockDisconnected;
-    /** Notifies listeners of a new active block chain. */
-    boost::signals2::signal<void (const CBlockLocator &)> SetBestChain;
+    virtual void BlockDisconnected(const std::shared_ptr<const CBlock> &block) {}
+    /** Notifies listeners of the new active block chain on-disk. */
+    virtual void SetBestChain(const CBlockLocator &locator) {}
     /** Notifies listeners about an inventory item being seen on the network. */
-    boost::signals2::signal<void (const uint256 &)> Inventory;
+    virtual void Inventory(const uint256 &hash) {}
     /** Tells listeners to broadcast their data. */
-    boost::signals2::signal<void (int64_t nBestBlockTime, CConnman* connman)> Broadcast;
+    virtual void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) {}
     /**
      * Notifies listeners of a block validation result.
      * If the provided CValidationState IsValid, the provided block
      * is guaranteed to be the current best block at the time the
      * callback was generated (not necessarily now)
      */
-    boost::signals2::signal<void (const CBlock&, const CValidationState&)> BlockChecked;
+    virtual void BlockChecked(const CBlock&, const CValidationState&) {}
     /**
      * 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 */
-    boost::signals2::signal<void (const CBlockIndex *, const std::shared_ptr<const CBlock>&)> NewPoWValidBlock;
+    virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& block) {};
+    friend void ::RegisterValidationInterface(CValidationInterface*);
+    friend void ::UnregisterValidationInterface(CValidationInterface*);
+    friend void ::UnregisterAllValidationInterfaces();
+};
+
+struct MainSignalsInstance;
+class CMainSignals {
+private:
+    std::unique_ptr<MainSignalsInstance> m_internals;
+
+    friend void ::RegisterValidationInterface(CValidationInterface*);
+    friend void ::UnregisterValidationInterface(CValidationInterface*);
+    friend void ::UnregisterAllValidationInterfaces();
+
+public:
+    /** Register a CScheduler to give callbacks which should run in the background (may only be called once) */
+    void RegisterBackgroundSignalScheduler(CScheduler& scheduler);
+    /** Unregister a CScheduler to give callbacks which should run in the background - these callbacks will now be dropped! */
+    void UnregisterBackgroundSignalScheduler();
+    /** Call any remaining callbacks on the calling thread */
+    void FlushBackgroundCallbacks();
+
+    void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
+    void TransactionAddedToMempool(const CTransactionRef &);
+    void BlockConnected(const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex, const std::vector<CTransactionRef> &);
+    void BlockDisconnected(const std::shared_ptr<const CBlock> &);
+    void UpdatedTransaction(const uint256 &);
+    void SetBestChain(const CBlockLocator &);
+    void Inventory(const uint256 &);
+    void Broadcast(int64_t nBestBlockTime, CConnman* connman);
+    void BlockChecked(const CBlock&, const CValidationState&);
+    void NewPoWValidBlock(const CBlockIndex *, const std::shared_ptr<const CBlock>&);
 };
 
 CMainSignals& GetMainSignals();