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).

2 files changed, 10 insertions, 29 deletions

diff --git a/src/index/base.cpp b/src/index/base.cpp
index 1babfacb15..55fb154d99 100644
--- a/src/index/base.cpp
+++ b/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;
 }
 
diff --git a/src/index/base.h b/src/index/base.h
index 8b986fe8c4..9b2a41dc92 100644
--- a/src/index/base.h
+++ b/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;
 };
 
 /**