Replace relevant services logic with a function suite.

Adds HasAllRelevantServices and GetRelevantServices, which check
for NETWORK|WITNESS.

This changes the following:
 * Removes nRelevantServices from CConnman, disconnecting it a bit
   more from protocol-level logic.
 * Replaces our sometimes-connect-to-!WITNESS-nodes logic with
   simply always requiring WITNESS|NETWORK for outbound non-feeler
   connections (feelers still only require NETWORK).
 * This has the added benefit of removing nServicesExpected from
   CNode - instead letting net_processing's VERSION message
   handling simply check HasAllRelevantServices.
 * This implies we believe WITNESS nodes to continue to be a
   significant majority of nodes on the network, but also because
   we cannot sync properly from !WITNESS nodes, it is strange to
   continue using our valuable outbound slots on them.
 * In order to prevent this change from preventing connection to
   -connect= nodes which have !WITNESS, -connect nodes are now
   given the "addnode" flag. This also allows outbound connections
   to !NODE_NETWORK nodes for -connect nodes (which was already true
   of addnodes).
 * Has the (somewhat unintended) consequence of changing one of the
   eviction metrics from the same
   sometimes-connect-to-!WITNESS-nodes metric to requiring
   HasRelevantServices.

This should make NODE_NETWORK_LIMITED much simpler to implement.

6 files changed, 55 insertions, 48 deletions

diff --git a/src/init.cpp b/src/init.cpp
index b46b53ac13..951800f6c2 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -815,7 +815,6 @@ void InitLogging()
 
 namespace { // Variables internal to initialization process only
 
-ServiceFlags nRelevantServices = NODE_NETWORK;
 int nMaxConnections;
 int nUserMaxConnections;
 int nFD;
@@ -1604,9 +1603,6 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
         // Note that setting NODE_WITNESS is never required: the only downside from not
         // doing so is that after activation, no upgraded nodes will fetch from you.
         nLocalServices = ServiceFlags(nLocalServices | NODE_WITNESS);
-        // Only care about others providing witness capabilities if there is a softfork
-        // defined.
-        nRelevantServices = ServiceFlags(nRelevantServices | NODE_WITNESS);
     }
 
     // ********************************************************* Step 10: import blocks
@@ -1656,7 +1652,6 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     CConnman::Options connOptions;
     connOptions.nLocalServices = nLocalServices;
-    connOptions.nRelevantServices = nRelevantServices;
     connOptions.nMaxConnections = nMaxConnections;
     connOptions.nMaxOutbound = std::min(MAX_OUTBOUND_CONNECTIONS, connOptions.nMaxConnections);
     connOptions.nMaxAddnode = MAX_ADDNODE_CONNECTIONS;
diff --git a/src/net.cpp b/src/net.cpp
index ea3840a708..581151b4bd 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -444,7 +444,6 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
         uint64_t nonce = GetDeterministicRandomizer(RANDOMIZER_ID_LOCALHOSTNONCE).Write(id).Finalize();
         CAddress addr_bind = GetBindAddress(hSocket);
         CNode* pnode = new CNode(id, nLocalServices, GetBestHeight(), hSocket, addrConnect, CalculateKeyedNetGroup(addrConnect), nonce, addr_bind, pszDest ? pszDest : "", false);
-        pnode->nServicesExpected = ServiceFlags(addrConnect.nServices & nRelevantServices);
         pnode->AddRef();
 
         return pnode;
@@ -985,7 +984,7 @@ bool CConnman::AttemptToEvictConnection()
                 continue;
             NodeEvictionCandidate candidate = {node->GetId(), node->nTimeConnected, node->nMinPingUsecTime,
                                                node->nLastBlockTime, node->nLastTXTime,
-                                               (node->nServices & nRelevantServices) == nRelevantServices,
+                                               HasAllDesirableServiceFlags(node->nServices),
                                                node->fRelayTxes, node->pfilter != nullptr, node->addr, node->nKeyedNetGroup};
             vEvictionCandidates.push_back(candidate);
         }
@@ -1602,7 +1601,7 @@ void CConnman::ThreadDNSAddressSeed()
         LOCK(cs_vNodes);
         int nRelevant = 0;
         for (auto pnode : vNodes) {
-            nRelevant += pnode->fSuccessfullyConnected && ((pnode->nServices & nRelevantServices) == nRelevantServices);
+            nRelevant += pnode->fSuccessfullyConnected && HasAllDesirableServiceFlags(pnode->nServices);
         }
         if (nRelevant >= 2) {
             LogPrintf("P2P peers available. Skipped DNS seeding.\n");
@@ -1624,7 +1623,7 @@ void CConnman::ThreadDNSAddressSeed()
         } else {
             std::vector<CNetAddr> vIPs;
             std::vector<CAddress> vAdd;
-            ServiceFlags requiredServiceBits = nRelevantServices;
+            ServiceFlags requiredServiceBits = GetDesirableServiceFlags(NODE_NONE);
             std::string host = GetDNSHost(seed, &requiredServiceBits);
             CNetAddr resolveSource;
             if (!resolveSource.SetInternal(host)) {
@@ -1705,7 +1704,7 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             for (const std::string& strAddr : connect)
             {
                 CAddress addr(CService(), NODE_NONE);
-                OpenNetworkConnection(addr, false, nullptr, strAddr.c_str());
+                OpenNetworkConnection(addr, false, nullptr, strAddr.c_str(), false, false, true);
                 for (int i = 0; i < 10 && i < nLoop; i++)
                 {
                     if (!interruptNet.sleep_for(std::chrono::milliseconds(500)))
@@ -1753,17 +1752,11 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
         // Only connect out to one peer per network group (/16 for IPv4).
         // Do this here so we don't have to critsect vNodes inside mapAddresses critsect.
         int nOutbound = 0;
-        int nOutboundRelevant = 0;
         std::set<std::vector<unsigned char> > setConnected;
         {
             LOCK(cs_vNodes);
             for (CNode* pnode : vNodes) {
                 if (!pnode->fInbound && !pnode->fAddnode) {
-
-                    // Count the peers that have all relevant services
-                    if (pnode->fSuccessfullyConnected && !pnode->fFeeler && ((pnode->nServices & nRelevantServices) == nRelevantServices)) {
-                        nOutboundRelevant++;
-                    }
                     // Netgroups for inbound and addnode peers are not excluded because our goal here
                     // is to not use multiple of our limited outbound slots on a single netgroup
                     // but inbound and addnode peers do not use our outbound slots.  Inbound peers
@@ -1818,21 +1811,16 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             if (IsLimited(addr))
                 continue;
 
-            // only connect to full nodes
-            if ((addr.nServices & REQUIRED_SERVICES) != REQUIRED_SERVICES)
-                continue;
-
             // only consider very recently tried nodes after 30 failed attempts
             if (nANow - addr.nLastTry < 600 && nTries < 30)
                 continue;
 
-            // only consider nodes missing relevant services after 40 failed attempts and only if less than half the outbound are up.
-            ServiceFlags nRequiredServices = nRelevantServices;
-            if (nTries >= 40 && nOutbound < (nMaxOutbound >> 1)) {
-                nRequiredServices = REQUIRED_SERVICES;
-            }
-
-            if ((addr.nServices & nRequiredServices) != nRequiredServices) {
+            // for non-feelers, require all the services we'll want,
+            // for feelers, only require they be a full node (only because most
+            // SPV clients don't have a good address DB available)
+            if (!fFeeler && !HasAllDesirableServiceFlags(addr.nServices)) {
+                continue;
+            } else if (fFeeler && !MayHaveUsefulAddressDB(addr.nServices)) {
                 continue;
             }
 
@@ -1841,13 +1829,6 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
                 continue;
 
             addrConnect = addr;
-
-            // regardless of the services assumed to be available, only require the minimum if half or more outbound have relevant services
-            if (nOutboundRelevant >= (nMaxOutbound >> 1)) {
-                addrConnect.nServices = REQUIRED_SERVICES;
-            } else {
-                addrConnect.nServices = nRequiredServices;
-            }
             break;
         }
 
@@ -2712,7 +2693,6 @@ CNode::CNode(NodeId idIn, ServiceFlags nLocalServicesIn, int nMyStartingHeightIn
     nSendVersion(0)
 {
     nServices = NODE_NONE;
-    nServicesExpected = NODE_NONE;
     hSocket = hSocketIn;
     nRecvVersion = INIT_PROTO_VERSION;
     nLastSend = 0;
diff --git a/src/net.h b/src/net.h
index 905d6eb956..b4873b1e4d 100644
--- a/src/net.h
+++ b/src/net.h
@@ -84,8 +84,6 @@ static const bool DEFAULT_FORCEDNSSEED = false;
 static const size_t DEFAULT_MAXRECEIVEBUFFER = 5 * 1000;
 static const size_t DEFAULT_MAXSENDBUFFER    = 1 * 1000;
 
-static const ServiceFlags REQUIRED_SERVICES = NODE_NETWORK;
-
 // NOTE: When adjusting this, update rpcnet:setban's help ("24h")
 static const unsigned int DEFAULT_MISBEHAVING_BANTIME = 60 * 60 * 24;  // Default 24-hour ban
 
@@ -130,7 +128,6 @@ public:
     struct Options
     {
         ServiceFlags nLocalServices = NODE_NONE;
-        ServiceFlags nRelevantServices = NODE_NONE;
         int nMaxConnections = 0;
         int nMaxOutbound = 0;
         int nMaxAddnode = 0;
@@ -152,7 +149,6 @@ public:
 
     void Init(const Options& connOptions) {
         nLocalServices = connOptions.nLocalServices;
-        nRelevantServices = connOptions.nRelevantServices;
         nMaxConnections = connOptions.nMaxConnections;
         nMaxOutbound = std::min(connOptions.nMaxOutbound, connOptions.nMaxConnections);
         nMaxAddnode = connOptions.nMaxAddnode;
@@ -390,9 +386,6 @@ private:
     /** Services this instance offers */
     ServiceFlags nLocalServices;
 
-    /** Services this instance cares about */
-    ServiceFlags nRelevantServices;
-
     CSemaphore *semOutbound;
     CSemaphore *semAddnode;
     int nMaxConnections;
@@ -585,7 +578,6 @@ class CNode
 public:
     // socket
     std::atomic<ServiceFlags> nServices;
-    ServiceFlags nServicesExpected;
     SOCKET hSocket;
     size_t nSendSize; // total size of all vSendMsg entries
     size_t nSendOffset; // offset inside the first vSendMsg already sent
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 7fced41d4f..39cf989eea 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -1232,11 +1232,11 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         {
             connman->SetServices(pfrom->addr, nServices);
         }
-        if (pfrom->nServicesExpected & ~nServices)
+        if (!pfrom->fInbound && !pfrom->fFeeler && !pfrom->fAddnode && !HasAllDesirableServiceFlags(nServices))
         {
-            LogPrint(BCLog::NET, "peer=%d does not offer the expected services (%08x offered, %08x expected); disconnecting\n", pfrom->GetId(), nServices, pfrom->nServicesExpected);
+            LogPrint(BCLog::NET, "peer=%d does not offer the expected services (%08x offered, %08x expected); disconnecting\n", pfrom->GetId(), nServices, GetDesirableServiceFlags(nServices));
             connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_NONSTANDARD,
-                               strprintf("Expected to offer services %08x", pfrom->nServicesExpected)));
+                               strprintf("Expected to offer services %08x", GetDesirableServiceFlags(nServices))));
             pfrom->fDisconnect = true;
             return false;
         }
@@ -1455,7 +1455,10 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
             if (interruptMsgProc)
                 return true;
 
-            if ((addr.nServices & REQUIRED_SERVICES) != REQUIRED_SERVICES)
+            // We only bother storing full nodes, though this may include
+            // things which we would not make an outbound connection to, in
+            // part because we may make feeler connections to them.
+            if (!MayHaveUsefulAddressDB(addr.nServices))
                 continue;
 
             if (addr.nTime <= 100000000 || addr.nTime > nNow + 10 * 60)
diff --git a/src/protocol.h b/src/protocol.h
index 67e01d9606..56b59aed3f 100644
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -277,6 +277,43 @@ enum ServiceFlags : uint64_t {
     // BIP process.
 };
 
+/**
+ * Gets the set of service flags which are "desirable" for a given peer.
+ *
+ * These are the flags which are required for a peer to support for them
+ * to be "interesting" to us, ie for us to wish to use one of our few
+ * outbound connection slots for or for us to wish to prioritize keeping
+ * their connection around.
+ *
+ * Relevant service flags may be peer- and state-specific in that the
+ * version of the peer may determine which flags are required (eg in the
+ * case of NODE_NETWORK_LIMITED where we seek out NODE_NETWORK peers
+ * unless they set NODE_NETWORK_LIMITED and we are out of IBD, in which
+ * case NODE_NETWORK_LIMITED suffices).
+ *
+ * Thus, generally, avoid calling with peerServices == NODE_NONE.
+ */
+static ServiceFlags GetDesirableServiceFlags(ServiceFlags services) {
+    return ServiceFlags(NODE_NETWORK | NODE_WITNESS);
+}
+
+/**
+ * A shortcut for (services & GetDesirableServiceFlags(services))
+ * == GetDesirableServiceFlags(services), ie determines whether the given
+ * set of service flags are sufficient for a peer to be "relevant".
+ */
+static inline bool HasAllDesirableServiceFlags(ServiceFlags services) {
+    return !(GetDesirableServiceFlags(services) & (~services));
+}
+
+/**
+ * Checks if a peer with the given service flags may be capable of having a
+ * robust address-storage DB. Currently an alias for checking NODE_NETWORK.
+ */
+static inline bool MayHaveUsefulAddressDB(ServiceFlags services) {
+    return services & NODE_NETWORK;
+}
+
 /** A CService with information about it as peer */
 class CAddress : public CService
 {
diff --git a/src/rpc/net.cpp b/src/rpc/net.cpp
index a3d3df26a3..fa0d15e0cd 100644
--- a/src/rpc/net.cpp
+++ b/src/rpc/net.cpp
@@ -217,7 +217,7 @@ UniValue addnode(const JSONRPCRequest& request)
     if (strCommand == "onetry")
     {
         CAddress addr;
-        g_connman->OpenNetworkConnection(addr, false, nullptr, strNode.c_str());
+        g_connman->OpenNetworkConnection(addr, false, nullptr, strNode.c_str(), false, false, true);
         return NullUniValue;
     }