validation: track last flushed block

This will be used to prevent the indexes from flushing their state ahead of
the chainstate.

Co-authored-by: Lőrinc <pap.lorinc@gmail.com>

src/test/chainstate_write_tests.cpp

@@ -85,6 +85,7 @@ BOOST_FIXTURE_TEST_CASE(write_during_multiblock_activation, TestChain100Setup)
 
     // Set m_next_write to current time
     chainstate.FlushStateToDisk(state_dummy, FlushStateMode::FORCE_FLUSH);
+    BOOST_CHECK_EQUAL(WITH_LOCK(::cs_main, return chainstate.GetLastFlushedBlock()), second_from_tip->pprev);
     m_node.validation_signals->SyncWithValidationInterfaceQueue();
     // The periodic flush interval is between 50 and 70 minutes (inclusive)
     // The next call to a PERIODIC write will flush
@@ -101,6 +102,7 @@ BOOST_FIXTURE_TEST_CASE(write_during_multiblock_activation, TestChain100Setup)
     // inside the outer loop.
     m_node.validation_signals->SyncWithValidationInterfaceQueue();
     BOOST_CHECK_EQUAL(sub->m_flushed_at_block, second_from_tip);
+    BOOST_CHECK_EQUAL(WITH_LOCK(::cs_main, return chainstate.GetLastFlushedBlock()), second_from_tip);
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/validation.cpp

@@ -2823,6 +2823,7 @@ bool Chainstate::FlushStateToDisk(
                 }
                 // Flush the chainstate (which may refer to block index entries).
                 empty_cache ? CoinsTip().Flush() : CoinsTip().Sync();
+                m_last_flushed_block = m_blockman.LookupBlockIndex(CoinsTip().GetBestBlock());
                 full_flush_completed = true;
                 TRACEPOINT(utxocache, flush,
                     int64_t{Ticks<std::chrono::microseconds>(NodeClock::now() - nNow)},
@@ -4584,6 +4585,7 @@ bool Chainstate::LoadChainTip()
     }
     m_chain.SetTip(*pindex);
     m_chainman.UpdateIBDStatus();
+    m_last_flushed_block = pindex;
     tip = m_chain.Tip();
 
     // nSequenceId is one of the keys used to sort setBlockIndexCandidates. Ensure all

src/validation.h

@@ -840,6 +840,9 @@ public:
 
     std::string ToString() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Get the last block that was flushed to disk.
+    const CBlockIndex* GetLastFlushedBlock() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main) { return m_last_flushed_block; }
+
     //! Indirection necessary to make lock annotations work with an optional mempool.
     RecursiveMutex* MempoolMutex() const LOCK_RETURNED(m_mempool->cs)
     {
@@ -890,6 +893,7 @@ protected:
         EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     NodeClock::time_point m_next_write{NodeClock::time_point::max()};
+    const CBlockIndex* m_last_flushed_block GUARDED_BY(::cs_main){nullptr};
 
     /**
      * In case of an invalid snapshot, rename the coins leveldb directory so