rpc: Return accurate results for scanblocks

This makes use of undo data to accurately verify results
from blockfilters.

src/rpc/blockchain.cpp

@@ -25,6 +25,7 @@
 #include <net_processing.h>
 #include <node/blockstorage.h>
 #include <node/context.h>
+#include <node/transaction.h>
 #include <node/utxo_snapshot.h>
 #include <primitives/transaction.h>
 #include <rpc/server.h>
@@ -2268,17 +2269,47 @@ public:
     }
 };
 
+static bool CheckBlockFilterMatches(BlockManager& blockman, const CBlockIndex& blockindex, const GCSFilter::ElementSet& needles)
+{
+    const CBlock block{GetBlockChecked(blockman, &blockindex)};
+    const CBlockUndo block_undo{GetUndoChecked(blockman, &blockindex)};
+
+    // Check if any of the outputs match the scriptPubKey
+    for (const auto& tx : block.vtx) {
+        if (std::any_of(tx->vout.cbegin(), tx->vout.cend(), [&](const auto& txout) {
+                return needles.count(std::vector<unsigned char>(txout.scriptPubKey.begin(), txout.scriptPubKey.end())) != 0;
+            })) {
+            return true;
+        }
+    }
+    // Check if any of the inputs match the scriptPubKey
+    for (const auto& txundo : block_undo.vtxundo) {
+        if (std::any_of(txundo.vprevout.cbegin(), txundo.vprevout.cend(), [&](const auto& coin) {
+                return needles.count(std::vector<unsigned char>(coin.out.scriptPubKey.begin(), coin.out.scriptPubKey.end())) != 0;
+            })) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 static RPCHelpMan scanblocks()
 {
     return RPCHelpMan{"scanblocks",
-        "\nReturn relevant blockhashes for given descriptors.\n"
+        "\nReturn relevant blockhashes for given descriptors (requires blockfilterindex).\n"
         "This call may take several minutes. Make sure to use no RPC timeout (bitcoin-cli -rpcclienttimeout=0)",
         {
             scan_action_arg_desc,
             scan_objects_arg_desc,
             RPCArg{"start_height", RPCArg::Type::NUM, RPCArg::Default{0}, "Height to start to scan from"},
             RPCArg{"stop_height", RPCArg::Type::NUM, RPCArg::DefaultHint{"chain tip"}, "Height to stop to scan"},
-            RPCArg{"filtertype", RPCArg::Type::STR, RPCArg::Default{BlockFilterTypeName(BlockFilterType::BASIC)}, "The type name of the filter"}
+            RPCArg{"filtertype", RPCArg::Type::STR, RPCArg::Default{BlockFilterTypeName(BlockFilterType::BASIC)}, "The type name of the filter"},
+            RPCArg{"options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "",
+                {
+                    {"filter_false_positives", RPCArg::Type::BOOL, RPCArg::Default{false}, "Filter false positives (slower and may fail on pruned nodes). Otherwise they may occur at a rate of 1/M"},
+                },
+                RPCArgOptions{.oneline_description="\"options\""}},
         },
         {
             scan_result_status_none,
@@ -2338,6 +2369,9 @@ static RPCHelpMan scanblocks()
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Unknown filtertype");
         }
 
+        UniValue options{request.params[5].isNull() ? UniValue::VOBJ : request.params[5]};
+        bool filter_false_positives{options.exists("filter_false_positives") ? options["filter_false_positives"].get_bool() : false};
+
         BlockFilterIndex* index = GetBlockFilterIndex(filtertype);
         if (!index) {
             throw JSONRPCError(RPC_MISC_ERROR, "Index is not enabled for filtertype " + filtertype_name);
@@ -2408,6 +2442,15 @@ static RPCHelpMan scanblocks()
                     for (const BlockFilter& filter : filters) {
                         // compare the elements-set with each filter
                         if (filter.GetFilter().MatchAny(needle_set)) {
+                            if (filter_false_positives) {
+                                // Double check the filter matches by scanning the block
+                                const CBlockIndex& blockindex = *CHECK_NONFATAL(WITH_LOCK(cs_main, return chainman.m_blockman.LookupBlockIndex(filter.GetBlockHash())));
+
+                                if (!CheckBlockFilterMatches(chainman.m_blockman, blockindex, needle_set)) {
+                                    continue;
+                                }
+                            }
+
                             blocks.push_back(filter.GetBlockHash().GetHex());
                             LogPrint(BCLog::RPC, "scanblocks: found match in %s\n", filter.GetBlockHash().GetHex());
                         }

src/rpc/client.cpp

@@ -86,6 +86,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "scanblocks", 1, "scanobjects" },
     { "scanblocks", 2, "start_height" },
     { "scanblocks", 3, "stop_height" },
+    { "scanblocks", 5, "options" },
     { "scantxoutset", 1, "scanobjects" },
     { "addmultisigaddress", 0, "nrequired" },
     { "addmultisigaddress", 1, "keys" },