rpc: scanblocks, do not traverse the whole chain block by block

The current flow walks-through every block in the active chain until
hits the chain tip or processes 10k blocks, then calls
`lookupFilterRange()` to obtain all the filters from that
particular range.

This is only done to obtain the heights range to look up the block
filters. Which is unneeded.

As `scanblocks` only lookup block filters in the active chain, we can
directly calculate the lookup range heights, by using the chain tip,
without requiring to traverse the chain block by block.

src/rpc/blockchain.cpp

@@ -2383,27 +2383,28 @@ static RPCHelpMan scanblocks()
         ChainstateManager& chainman = EnsureChainman(node);
 
         // set the start-height
-        const CBlockIndex* block = nullptr;
+        const CBlockIndex* start_index = nullptr;
         const CBlockIndex* stop_block = nullptr;
         {
             LOCK(cs_main);
             CChain& active_chain = chainman.ActiveChain();
-            block = active_chain.Genesis();
-            stop_block = active_chain.Tip();
+            start_index = active_chain.Genesis();
+            stop_block = active_chain.Tip(); // If no stop block is provided, stop at the chain tip.
             if (!request.params[2].isNull()) {
-                block = active_chain[request.params[2].getInt<int>()];
-                if (!block) {
+                start_index = active_chain[request.params[2].getInt<int>()];
+                if (!start_index) {
                     throw JSONRPCError(RPC_MISC_ERROR, "Invalid start_height");
                 }
             }
             if (!request.params[3].isNull()) {
                 stop_block = active_chain[request.params[3].getInt<int>()];
-                if (!stop_block || stop_block->nHeight < block->nHeight) {
+                if (!stop_block || stop_block->nHeight < start_index->nHeight) {
                     throw JSONRPCError(RPC_MISC_ERROR, "Invalid stop_height");
                 }
             }
         }
-        CHECK_NONFATAL(block);
+        CHECK_NONFATAL(start_index);
+        CHECK_NONFATAL(stop_block);
 
         // loop through the scan objects, add scripts to the needle_set
         GCSFilter::ElementSet needle_set;
@@ -2416,31 +2417,29 @@ static RPCHelpMan scanblocks()
         }
         UniValue blocks(UniValue::VARR);
         const int amount_per_chunk = 10000;
-        const CBlockIndex* start_index = block; // for remembering the start of a blockfilter range
         std::vector<BlockFilter> filters;
-        const CBlockIndex* start_block = block; // for progress reporting
-        const int total_blocks_to_process = stop_block->nHeight - start_block->nHeight;
+        int start_block_height = start_index->nHeight; // for progress reporting
+        const int total_blocks_to_process = stop_block->nHeight - start_block_height;
 
         g_scanfilter_should_abort_scan = false;
         g_scanfilter_progress = 0;
-        g_scanfilter_progress_height = start_block->nHeight;
+        g_scanfilter_progress_height = start_block_height;
 
-        while (block) {
+        const CBlockIndex* end_range = nullptr;
+        do {
             node.rpc_interruption_point(); // allow a clean shutdown
             if (g_scanfilter_should_abort_scan) {
-                LogPrintf("scanblocks RPC aborted at height %d.\n", block->nHeight);
+                LogPrintf("scanblocks RPC aborted at height %d.\n", end_range->nHeight);
                 break;
             }
-            const CBlockIndex* next = nullptr;
-            {
-                LOCK(cs_main);
-                CChain& active_chain = chainman.ActiveChain();
-                next = active_chain.Next(block);
-                if (block == stop_block) next = nullptr;
-            }
-            if (start_index->nHeight + amount_per_chunk == block->nHeight || next == nullptr) {
-                LogPrint(BCLog::RPC, "Fetching blockfilters from height %d to height %d.\n", start_index->nHeight, block->nHeight);
-                if (index->LookupFilterRange(start_index->nHeight, block, filters)) {
+
+            // split the lookup range in chunks if we are deeper than 'amount_per_chunk' blocks from the stopping block
+            int start_block = !end_range ? start_index->nHeight : start_index->nHeight + 1; // to not include the previous round 'end_range' block
+            end_range = (start_block + amount_per_chunk < stop_block->nHeight) ?
+                    WITH_LOCK(::cs_main, return chainman.ActiveChain()[start_block + amount_per_chunk]) :
+                    stop_block;
+
+            if (index->LookupFilterRange(start_block, end_range, filters)) {
                 for (const BlockFilter& filter : filters) {
                     // compare the elements-set with each filter
                     if (filter.GetFilter().MatchAny(needle_set)) {
@@ -2458,21 +2457,22 @@ static RPCHelpMan scanblocks()
                     }
                 }
             }
-                start_index = block;
+            start_index = end_range;
 
             // update progress
-                int blocks_processed = block->nHeight - start_block->nHeight;
+            int blocks_processed = end_range->nHeight - start_block_height;
             if (total_blocks_to_process > 0) { // avoid division by zero
                 g_scanfilter_progress = (int)(100.0 / total_blocks_to_process * blocks_processed);
             } else {
                 g_scanfilter_progress = 100;
             }
-                g_scanfilter_progress_height = block->nHeight;
-            }
-            block = next;
-        }
-        ret.pushKV("from_height", start_block->nHeight);
-        ret.pushKV("to_height", g_scanfilter_progress_height.load());
+            g_scanfilter_progress_height = end_range->nHeight;
+
+        // Finish if we reached the stop block
+        } while (start_index != stop_block);
+
+        ret.pushKV("from_height", start_block_height);
+        ret.pushKV("to_height", stop_block->nHeight);
         ret.pushKV("relevant_blocks", blocks);
     }
     else {