add ValidationInterface::ActiveTipChange

This is a synchronous callback notifying clients of all tip changes.

It allows clients to respond to a new block immediately after it is
connected. The synchronicity is important for things like
m_recent_rejects, in which a transaction's validity can change (rejected
vs accepted) when this event is processed. For example, the transaction
might have a timelock condition that has just been met. This is distinct
from something like m_recent_confirmed_transactions, in which the
validation outcome is the same (valid vs already-have), so it does not
need to be reset immediately.

src/validation.cpp

@@ -3488,6 +3488,7 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
 
         {
             LOCK(cs_main);
+            {
             // Lock transaction pool for at least as long as it takes for connectTrace to be consumed
             LOCK(MempoolMutex());
             const bool was_in_ibd = m_chainman.IsInitialBlockDownload();
@@ -3564,7 +3565,12 @@ bool Chainstate::ActivateBestChain(BlockValidationState& state, std::shared_ptr<
                     break;
                 }
             }
+            } // release MempoolMutex
+            // Notify external listeners about the new tip, even if pindexFork == pindexNewTip.
+            if (m_chainman.m_options.signals && this == &m_chainman.ActiveChainstate()) {
+                m_chainman.m_options.signals->ActiveTipChange(pindexNewTip, m_chainman.IsInitialBlockDownload());
             }
+        } // release cs_main
         // When we reach this point, we switched to a new tip (stored in pindexNewTip).
 
         if (exited_ibd) {
@@ -3783,6 +3789,12 @@ bool Chainstate::InvalidateBlock(BlockValidationState& state, CBlockIndex* pinde
         // distinguish user-initiated invalidateblock changes from other
         // changes.
         (void)m_chainman.GetNotifications().blockTip(GetSynchronizationState(m_chainman.IsInitialBlockDownload(), m_chainman.m_blockman.m_blockfiles_indexed), *to_mark_failed->pprev);
+
+        // Fire ActiveTipChange now for the current chain tip to make sure clients are notified.
+        // ActivateBestChain may call this as well, but not necessarily.
+        if (m_chainman.m_options.signals) {
+            m_chainman.m_options.signals->ActiveTipChange(m_chain.Tip(), m_chainman.IsInitialBlockDownload());
+        }
     }
     return true;
 }

src/validationinterface.cpp

@@ -183,6 +183,12 @@ void ValidationSignals::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlo
                           fInitialDownload);
 }
 
+void ValidationSignals::ActiveTipChange(const CBlockIndex *new_tip, bool is_ibd)
+{
+    LOG_EVENT("%s: new block hash=%s block height=%d", __func__, new_tip->GetBlockHash().ToString(), new_tip->nHeight);
+    m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.ActiveTipChange(new_tip, is_ibd); });
+}
+
 void ValidationSignals::TransactionAddedToMempool(const NewMempoolTransactionInfo& tx, uint64_t mempool_sequence)
 {
     auto event = [tx, mempool_sequence, this] {

src/validationinterface.h

@@ -61,6 +61,10 @@ protected:
      * Called on a background thread. Only called for the active chainstate.
      */
     virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
+    /**
+     * Notifies listeners any time the block chain tip changes, synchronously.
+     */
+    virtual void ActiveTipChange(const CBlockIndex* new_tip, bool is_ibd) {};
     /**
      * Notifies listeners of a transaction having been added to mempool.
      *
@@ -214,6 +218,7 @@ public:
     void SyncWithValidationInterfaceQueue() LOCKS_EXCLUDED(cs_main);
 
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
+    void ActiveTipChange(const CBlockIndex*, bool);
     void TransactionAddedToMempool(const NewMempoolTransactionInfo&, uint64_t mempool_sequence);
     void TransactionRemovedFromMempool(const CTransactionRef&, MemPoolRemovalReason, uint64_t mempool_sequence);
     void MempoolTransactionsRemovedForBlock(const std::vector<RemovedMempoolTransactionInfo>&, unsigned int nBlockHeight);