index: verify blocks data existence only once

At present, during init, we traverse the chain (once per index)
to confirm that all necessary blocks to sync each index up to
the current tip are present.

To make the process more efficient, we can fetch the oldest block
from the indexers and perform the chain data existence check from
that point only once.

This also moves the pruning violation check to the end of the
'loadinit' thread, which is where the reindex, block loading and
chain activation processes happen.

Making the node's startup process faster, allowing us to remove
the global g_indexes_ready_to_sync flag, and enabling the
execution of the pruning violation verification even when the
reindex or reindex-chainstate flags are enabled (which has being
skipped so far).

src/index/base.cpp

@@ -23,8 +23,6 @@
 #include <string>
 #include <utility>
 
-using node::g_indexes_ready_to_sync;
-
 constexpr uint8_t DB_BEST_BLOCK{'B'};
 
 constexpr auto SYNC_LOG_INTERVAL{30s};
@@ -107,32 +105,8 @@ bool BaseIndex::Init()
         SetBestBlockIndex(locator_index);
     }
 
-    // Skip pruning check if indexes are not ready to sync (because reindex-chainstate has wiped the chain).
-    const CBlockIndex* start_block = m_best_block_index.load();
-    bool synced = start_block == active_chain.Tip();
-    if (!synced && g_indexes_ready_to_sync) {
-        const CBlockIndex* block_to_test = start_block ? start_block : active_chain.Genesis();
-
-        // Assert block_to_test is not null here. It can't be null because the
-        // genesis block can't be null here. The genesis block will be null
-        // during this BaseIndex::Init() call if the node is being started for
-        // the first time, or if -reindex is used. But in both of these cases
-        // m_best_block_index is also null so this branch is not reached.
-        assert(block_to_test);
-
-        if (!active_chain.Contains(block_to_test)) {
-            // if the bestblock is not part of the mainchain, find the fork
-            // so we can make sure we have all data down to the fork
-            block_to_test = active_chain.FindFork(block_to_test);
-        }
-
-        // make sure we have all block data back to the start block
-        if (!m_chainstate->m_blockman.CheckBlockDataAvailability(*active_chain.Tip(), *Assert(block_to_test))) {
-            return InitError(strprintf(Untranslated("%s best block of the index goes beyond pruned data. Please disable the index or reindex (which will download the whole blockchain again)"), GetName()));
-        }
-    }
-
     // Child init
+    const CBlockIndex* start_block = m_best_block_index.load();
     if (!CustomInit(start_block ? std::make_optional(interfaces::BlockKey{start_block->GetBlockHash(), start_block->nHeight}) : std::nullopt)) {
         return false;
     }
@@ -140,7 +114,7 @@ bool BaseIndex::Init()
     // Note: this will latch to true immediately if the user starts up with an empty
     // 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_synced = start_block == active_chain.Tip();
     m_init = true;
     return true;
 }
@@ -412,7 +386,13 @@ IndexSummary BaseIndex::GetSummary() const
     IndexSummary summary{};
     summary.name = GetName();
     summary.synced = m_synced;
-    summary.best_block_height = m_best_block_index ? m_best_block_index.load()->nHeight : 0;
+    if (const auto& pindex = m_best_block_index.load()) {
+        summary.best_block_height = pindex->nHeight;
+        summary.best_block_hash = pindex->GetBlockHash();
+    } else {
+        summary.best_block_height = 0;
+        summary.best_block_hash = m_chain->getBlockHash(0);
+    }
     return summary;
 }
 

src/index/base.h

@@ -23,6 +23,7 @@ struct IndexSummary {
     std::string name;
     bool synced{false};
     int best_block_height{0};
+    uint256 best_block_hash;
 };
 
 /**

src/init.cpp

@@ -125,7 +125,6 @@ using node::CalculateCacheSizes;
 using node::DEFAULT_PERSIST_MEMPOOL;
 using node::DEFAULT_PRINTPRIORITY;
 using node::fReindex;
-using node::g_indexes_ready_to_sync;
 using node::KernelNotifications;
 using node::LoadChainstate;
 using node::MempoolPath;
@@ -1545,8 +1544,6 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     // ********************************************************* Step 8: start indexers
 
-    // If reindex-chainstate was specified, delay syncing indexes until ImportBlocks has reindexed the chain
-    if (!fReindexChainState) g_indexes_ready_to_sync = true;
     if (args.GetBoolArg("-txindex", DEFAULT_TXINDEX)) {
         auto result{WITH_LOCK(cs_main, return CheckLegacyTxindex(*Assert(chainman.m_blockman.m_block_tree_db)))};
         if (!result) {
@@ -1884,6 +1881,44 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
 bool StartIndexBackgroundSync(NodeContext& node)
 {
+    // Find the oldest block among all indexes.
+    // This block is used to verify that we have the required blocks' data stored on disk,
+    // starting from that point up to the current tip.
+    // indexes_start_block='nullptr' means "start from height 0".
+    std::optional<const CBlockIndex*> indexes_start_block;
+    std::string older_index_name;
+
+    ChainstateManager& chainman = *Assert(node.chainman);
+    for (auto index : node.indexes) {
+        const IndexSummary& summary = index->GetSummary();
+        if (summary.synced) continue;
+
+        // Get the last common block between the index best block and the active chain
+        LOCK(::cs_main);
+        const CChain& active_chain = chainman.ActiveChain();
+        const CBlockIndex* pindex = chainman.m_blockman.LookupBlockIndex(summary.best_block_hash);
+        if (!active_chain.Contains(pindex)) {
+            pindex = active_chain.FindFork(pindex);
+        }
+
+        if (!indexes_start_block || !pindex || pindex->nHeight < indexes_start_block.value()->nHeight) {
+            indexes_start_block = pindex;
+            older_index_name = summary.name;
+            if (!pindex) break; // Starting from genesis so no need to look for earlier block.
+        }
+    };
+
+    // Verify all blocks needed to sync to current tip are present.
+    if (indexes_start_block) {
+        LOCK(::cs_main);
+        const CBlockIndex* start_block = *indexes_start_block;
+        if (!start_block) start_block = chainman.ActiveChain().Genesis();
+        if (!chainman.m_blockman.CheckBlockDataAvailability(*chainman.ActiveChain().Tip(), *Assert(start_block))) {
+            return InitError(strprintf(Untranslated("%s best block of the index goes beyond pruned data. Please disable the index or reindex (which will download the whole blockchain again)"), older_index_name));
+        }
+    }
+
+    // Start threads
     for (auto index : node.indexes) if (!index->StartBackgroundSync()) return false;
     return true;
 }

src/node/blockstorage.cpp

@@ -26,7 +26,6 @@
 
 namespace node {
 std::atomic_bool fReindex(false);
-std::atomic_bool g_indexes_ready_to_sync{false};
 
 bool CBlockIndexWorkComparator::operator()(const CBlockIndex* pa, const CBlockIndex* pb) const
 {
@@ -954,7 +953,5 @@ void ImportBlocks(ChainstateManager& chainman, std::vector<fs::path> vImportFile
             return;
         }
     } // End scope of ImportingNow
-
-    g_indexes_ready_to_sync = true;
 }
 } // namespace node

src/node/blockstorage.h

@@ -50,7 +50,6 @@ static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; // 128 MiB
 static constexpr size_t BLOCK_SERIALIZATION_HEADER_SIZE = CMessageHeader::MESSAGE_START_SIZE + sizeof(unsigned int);
 
 extern std::atomic_bool fReindex;
-extern std::atomic_bool g_indexes_ready_to_sync;
 
 // Because validation code takes pointers to the map's CBlockIndex objects, if
 // we ever switch to another associative container, we need to either use a

src/test/util/setup_common.cpp

@@ -157,7 +157,6 @@ BasicTestingSetup::BasicTestingSetup(const ChainType chainType, const std::vecto
         noui_connect();
         noui_connected = true;
     }
-    node::g_indexes_ready_to_sync = true;
 }
 
 BasicTestingSetup::~BasicTestingSetup()