refactor: index, decouple 'Init' from 'Start'

So indexes can be initialized without spawning
the sync thread.

This makes asynchronous indexes startup
possible in the following commits.

src/index/base.cpp

@@ -81,6 +81,13 @@ BaseIndex::~BaseIndex()
 
 bool BaseIndex::Init()
 {
+    // m_chainstate member gives indexing code access to node internals. It is
+    // removed in followup https://github.com/bitcoin/bitcoin/pull/24230
+    m_chainstate = &m_chain->context()->chainman->ActiveChainstate();
+    // Register to validation interface before setting the 'm_synced' flag, so that
+    // callbacks are not missed once m_synced is true.
+    RegisterValidationInterface(this);
+
     CBlockLocator locator;
     if (!GetDB().ReadBestBlock(locator)) {
         locator.SetNull();
@@ -147,6 +154,7 @@ bool BaseIndex::Init()
     // datadir and an index enabled. If this is the case, indexation will happen solely
     // via `BlockConnected` signals until, possibly, the next restart.
     m_synced = synced;
+    m_init = true;
     return true;
 }
 
@@ -401,15 +409,9 @@ void BaseIndex::Interrupt()
     m_interrupt();
 }
 
-bool BaseIndex::Start()
+bool BaseIndex::StartBackgroundSync()
 {
-    // m_chainstate member gives indexing code access to node internals. It is
+    if (!m_init) throw std::logic_error("Error: Cannot start a non-initialized index");
-    // removed in followup https://github.com/bitcoin/bitcoin/pull/24230
-    m_chainstate = &m_chain->context()->chainman->ActiveChainstate();
-    // Need to register this ValidationInterface before running Init(), so that
-    // callbacks are not missed if Init sets m_synced to true.
-    RegisterValidationInterface(this);
-    if (!Init()) return false;
 
     m_thread_sync = std::thread(&util::TraceThread, GetName(), [this] { ThreadSync(); });
     return true;

src/index/base.h

@@ -54,6 +54,8 @@ protected:
     };
 
 private:
+    /// Whether the index has been initialized or not.
+    std::atomic<bool> m_init{false};
     /// Whether the index is in sync with the main chain. The flag is flipped
     /// from false to true once, after which point this starts processing
     /// ValidationInterface notifications to stay in sync.
@@ -69,9 +71,6 @@ private:
     std::thread m_thread_sync;
     CThreadInterrupt m_interrupt;
 
-    /// Read best block locator and check that data needed to sync has not been pruned.
-    bool Init();
-
     /// Sync the index with the block index starting from the current best block.
     /// Intended to be run in its own thread, m_thread_sync, and can be
     /// interrupted with m_interrupt. Once the index gets in sync, the m_synced
@@ -142,9 +141,12 @@ public:
 
     void Interrupt();
 
-    /// Start initializes the sync state and registers the instance as a
+    /// Initializes the sync state and registers the instance to the
-    /// ValidationInterface so that it stays in sync with blockchain updates.
+    /// validation interface so that it stays in sync with blockchain updates.
-    [[nodiscard]] bool Start();
+    [[nodiscard]] bool Init();
+
+    /// Starts the initial sync process.
+    [[nodiscard]] bool StartBackgroundSync();
 
     /// Stops the instance from staying in sync with blockchain updates.
     void Stop();

src/init.cpp

@@ -1567,8 +1567,11 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         node.indexes.emplace_back(g_coin_stats_index.get());
     }
 
+    // Init indexes
+    for (auto index : node.indexes) if (!index->Init()) return false;
+
     // Now that all indexes are loaded, start them
-    StartIndexes(node);
+    if (!StartIndexBackgroundSync(node)) return false;
 
     // ********************************************************* Step 9: load wallet
     for (const auto& client : node.chain_clients) {
@@ -1876,8 +1879,8 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     return true;
 }
 
-bool StartIndexes(NodeContext& node)
+bool StartIndexBackgroundSync(NodeContext& node)
 {
-    for (auto index : node.indexes) if (!index->Start()) return false;
+    for (auto index : node.indexes) if (!index->StartBackgroundSync()) return false;
     return true;
 }

src/init.h

@@ -74,6 +74,6 @@ bool AppInitMain(node::NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip
 void SetupServerArgs(ArgsManager& argsman);
 
 /** Validates requirements to run the indexes and spawns each index initial sync thread */
-bool StartIndexes(node::NodeContext& node);
+bool StartIndexBackgroundSync(node::NodeContext& node);
 
 #endif // BITCOIN_INIT_H

src/test/blockfilter_index_tests.cpp

@@ -113,6 +113,7 @@ bool BuildChainTestingSetup::BuildChain(const CBlockIndex* pindex,
 BOOST_FIXTURE_TEST_CASE(blockfilter_index_initial_sync, BuildChainTestingSetup)
 {
     BlockFilterIndex filter_index(interfaces::MakeChain(m_node), BlockFilterType::BASIC, 1 << 20, true);
+    BOOST_REQUIRE(filter_index.Init());
 
     uint256 last_header;
 
@@ -139,7 +140,7 @@ BOOST_FIXTURE_TEST_CASE(blockfilter_index_initial_sync, BuildChainTestingSetup)
     // BlockUntilSyncedToCurrentChain should return false before index is started.
     BOOST_CHECK(!filter_index.BlockUntilSyncedToCurrentChain());
 
-    BOOST_REQUIRE(filter_index.Start());
+    BOOST_REQUIRE(filter_index.StartBackgroundSync());
 
     // Allow filter index to catch up with the block index.
     IndexWaitSynced(filter_index);

src/test/coinstatsindex_tests.cpp

@@ -18,6 +18,7 @@ BOOST_AUTO_TEST_SUITE(coinstatsindex_tests)
 BOOST_FIXTURE_TEST_CASE(coinstatsindex_initial_sync, TestChain100Setup)
 {
     CoinStatsIndex coin_stats_index{interfaces::MakeChain(m_node), 1 << 20, true};
+    BOOST_REQUIRE(coin_stats_index.Init());
 
     const CBlockIndex* block_index;
     {
@@ -32,7 +33,7 @@ BOOST_FIXTURE_TEST_CASE(coinstatsindex_initial_sync, TestChain100Setup)
     // is started.
     BOOST_CHECK(!coin_stats_index.BlockUntilSyncedToCurrentChain());
 
-    BOOST_REQUIRE(coin_stats_index.Start());
+    BOOST_REQUIRE(coin_stats_index.StartBackgroundSync());
 
     IndexWaitSynced(coin_stats_index);
 
@@ -83,7 +84,8 @@ BOOST_FIXTURE_TEST_CASE(coinstatsindex_unclean_shutdown, TestChain100Setup)
     const CChainParams& params = Params();
     {
         CoinStatsIndex index{interfaces::MakeChain(m_node), 1 << 20};
-        BOOST_REQUIRE(index.Start());
+        BOOST_REQUIRE(index.Init());
+        BOOST_REQUIRE(index.StartBackgroundSync());
         IndexWaitSynced(index);
         std::shared_ptr<const CBlock> new_block;
         CBlockIndex* new_block_index = nullptr;
@@ -109,8 +111,9 @@ BOOST_FIXTURE_TEST_CASE(coinstatsindex_unclean_shutdown, TestChain100Setup)
 
     {
         CoinStatsIndex index{interfaces::MakeChain(m_node), 1 << 20};
+        BOOST_REQUIRE(index.Init());
         // Make sure the index can be loaded.
-        BOOST_REQUIRE(index.Start());
+        BOOST_REQUIRE(index.StartBackgroundSync());
         index.Stop();
     }
 }

src/test/txindex_tests.cpp

@@ -17,6 +17,7 @@ BOOST_AUTO_TEST_SUITE(txindex_tests)
 BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
 {
     TxIndex txindex(interfaces::MakeChain(m_node), 1 << 20, true);
+    BOOST_REQUIRE(txindex.Init());
 
     CTransactionRef tx_disk;
     uint256 block_hash;
@@ -29,7 +30,7 @@ BOOST_FIXTURE_TEST_CASE(txindex_initial_sync, TestChain100Setup)
     // BlockUntilSyncedToCurrentChain should return false before txindex is started.
     BOOST_CHECK(!txindex.BlockUntilSyncedToCurrentChain());
 
-    BOOST_REQUIRE(txindex.Start());
+    BOOST_REQUIRE(txindex.StartBackgroundSync());
 
     // Allow tx index to catch up with the block index.
     IndexWaitSynced(txindex);