[net] Signal NODE_COMPACT_FILTERS if we're serving compact filters.

if -peercfilters is configured, signal the NODE_COMPACT_FILTERS service
bit to indicate that we are able to serve compact block filters, headers
and checkpoints.

src/index/base.h

@@ -91,6 +91,10 @@ class BaseIndex : public CValidationInterface
     /// Destructor interrupts sync thread if running and blocks until it exits.
     virtual ~BaseIndex();
 
+    /// Returns whether index has completed the initial sync with the active chain.
+    /// After returning true once, this function will return true on all subsequent calls.
+    bool IsSynced() const { return m_synced; }
+
     /// Blocks the current thread until the index is caught up to the current
     /// state of the block chain. This only blocks if the index has gotten in
     /// sync once and only needs to process blocks in the ValidationInterface

src/init.cpp

@@ -994,11 +994,13 @@ bool AppInitParameterInteraction()
         }
     }
 
-    // Basic filters are the only supported filters. The basic filters index must be enabled
-    // to serve compact filters
-    if (gArgs.GetBoolArg("-peerblockfilters", DEFAULT_PEERBLOCKFILTERS) &&
-        g_enabled_filter_types.count(BlockFilterType::BASIC) != 1) {
-        return InitError(_("Cannot set -peerblockfilters without -blockfilterindex."));
+    // Signal NODE_COMPACT_FILTERS if peercfilters and basic filters index are both enabled.
+    if (gArgs.GetBoolArg("-peerblockfilters", DEFAULT_PEERBLOCKFILTERS)) {
+        if (g_enabled_filter_types.count(BlockFilterType::BASIC) != 1) {
+            return InitError(_("Cannot set -peerblockfilters without -blockfilterindex."));
+        }
+
+        nLocalServices = ServiceFlags(nLocalServices | NODE_COMPACT_FILTERS);
     }
 
     // if using block pruning, then disallow txindex
@@ -1782,6 +1784,19 @@ bool AppInitMain(NodeContext& node)
         GetBlockFilterIndex(filter_type)->Start();
     }
 
+    if (nLocalServices & NODE_COMPACT_FILTERS) {
+        const BlockFilterIndex* const basic_filter_index =
+            GetBlockFilterIndex(BlockFilterType::BASIC);
+        if (!basic_filter_index) {
+            error("NODE_COMPACT_FILTERS is signaled, but filter index is not available");
+            return false;
+        }
+        if (!basic_filter_index->IsSynced()) {
+            InitError(strprintf(_("Cannot enable -peercfilters until basic block filter index is in sync. Please disable and reenable once filters have been indexed.")));
+            return false;
+        }
+    }
+
     // ********************************************************* Step 9: load wallet
     for (const auto& client : node.chain_clients) {
         if (!client->load()) {

src/net_processing.cpp

@@ -1994,7 +1994,7 @@ static bool PrepareBlockFilterRequest(CNode* pfrom, const CChainParams& chain_pa
 {
     const bool supported_filter_type =
         (filter_type == BlockFilterType::BASIC &&
-         gArgs.GetBoolArg("-peerblockfilters", DEFAULT_PEERBLOCKFILTERS));
+         (pfrom->GetLocalServices() & NODE_COMPACT_FILTERS));
     if (!supported_filter_type) {
         LogPrint(BCLog::NET, "peer %d requested unsupported block filter type: %d\n",
                  pfrom->GetId(), static_cast<uint8_t>(filter_type));

src/protocol.h

@@ -295,6 +295,9 @@ enum ServiceFlags : uint64_t {
     // NODE_WITNESS indicates that a node can be asked for blocks and transactions including
     // witness data.
     NODE_WITNESS = (1 << 3),
+    // NODE_COMPACT_FILTERS means the node will service basic block filter requests.
+    // See BIP157 and BIP158 for details on how this is implemented.
+    NODE_COMPACT_FILTERS = (1 << 6),
     // NODE_NETWORK_LIMITED means the same as NODE_NETWORK with the limitation of only
     // serving the last 288 (2 day) blocks
     // See BIP159 for details on how this is implemented.

src/qt/guiutil.cpp

@@ -751,6 +751,7 @@ QString serviceFlagToStr(const quint64 mask, const int bit)
     case NODE_GETUTXO:         return "GETUTXO";
     case NODE_BLOOM:           return "BLOOM";
     case NODE_WITNESS:         return "WITNESS";
+    case NODE_COMPACT_FILTERS: return "COMPACT_FILTERS";
     case NODE_NETWORK_LIMITED: return "NETWORK_LIMITED";
     // Not using default, so we get warned when a case is missing
     }

src/rpc/util.cpp

@@ -851,6 +851,8 @@ UniValue GetServicesNames(ServiceFlags services)
         servicesNames.push_back("BLOOM");
     if (services & NODE_WITNESS)
         servicesNames.push_back("WITNESS");
+    if (services & NODE_COMPACT_FILTERS)
+        servicesNames.push_back("COMPACT_FILTERS");
     if (services & NODE_NETWORK_LIMITED)
         servicesNames.push_back("NETWORK_LIMITED");