Merge bitcoin/bitcoin#22777: net processing: don't request tx relay on feeler connections

eaf6be0114a6d7763767da9496907fe8a670ff9e [net processing] Do not request transaction relay from feeler connections (John Newbery)
0220b834b175dc8c45a2c60213474a72c0ef8193 [test] Add testing for outbound feeler connections (John Newbery)

Pull request description:

  Feelers are short-lived connections used to test the viability of peers. The bitcoind node will periodically open feeler connections to addresses in its addrman, wait for a `version` message from the peer, and then close the connection.

  Currently, we set `fRelay` to `1` in the `version` message for feeler connections, indicating that we want the peer to relay transactions to us. However, we close the connection immediately on receipt of the `version` message, and so never process any incoming transaction announcements. This PR changes that behaviour to instead set `fRelay` to `0` indicating that we do not wish to receive transaction announcements from the peer.

  This PR also extends the `addconnection` RPC to allow creating outbound feeler connections from the node to the test framework, and a test to verify that the node sets `fRelay` to `0` in the `version` message to feeler connections.

ACKs for top commit:
  naumenkogs:
    ACK eaf6be0114a6d7763767da9496907fe8a670ff9e
  MarcoFalke:
    review ACK eaf6be0114a6d7763767da9496907fe8a670ff9e 🏃

Tree-SHA512: 1c56837dbd0a396fe404a5e39f7459864d15f666664d6b35ad109628b13158e077e417e586bf48946a23bd5cbe63716cb4bf22cdf8781b74dfce6047b87b465a

4 files changed, 9 insertions, 5 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 2e2aa9dcef..fcebdf35b3 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -1225,7 +1225,6 @@ bool CConnman::AddConnection(const std::string& address, ConnectionType conn_typ
     switch (conn_type) {
     case ConnectionType::INBOUND:
     case ConnectionType::MANUAL:
-    case ConnectionType::FEELER:
         return false;
     case ConnectionType::OUTBOUND_FULL_RELAY:
         max_connections = m_max_outbound_full_relay;
@@ -1236,6 +1235,9 @@ bool CConnman::AddConnection(const std::string& address, ConnectionType conn_typ
     // no limit for ADDR_FETCH because -seednode has no limit either
     case ConnectionType::ADDR_FETCH:
         break;
+    // no limit for FEELER connections since they're short-lived
+    case ConnectionType::FEELER:
+        break;
     } // no default case, so the compiler can warn about missing cases
 
     // Count existing connections
diff --git a/src/net.h b/src/net.h
index 3c03f7f9da..a8f8232f8f 100644
--- a/src/net.h
+++ b/src/net.h
@@ -884,8 +884,8 @@ public:
      * Attempts to open a connection. Currently only used from tests.
      *
      * @param[in]   address     Address of node to try connecting to
-     * @param[in]   conn_type   ConnectionType::OUTBOUND or ConnectionType::BLOCK_RELAY
-     *                          or ConnectionType::ADDR_FETCH
+     * @param[in]   conn_type   ConnectionType::OUTBOUND, ConnectionType::BLOCK_RELAY,
+     *                          ConnectionType::ADDR_FETCH or ConnectionType::FEELER
      * @return      bool        Returns false if there are no available
      *                          slots for this connection:
      *                          - conn_type not a supported ConnectionType
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 809fb8efde..8c897b9a0b 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -1104,7 +1104,7 @@ void PeerManagerImpl::PushNodeVersion(CNode& pnode, int64_t nTime)
     CService addr_you = addr.IsRoutable() && !IsProxy(addr) && addr.IsAddrV1Compatible() ? addr : CService();
     uint64_t your_services{addr.nServices};
 
-    const bool tx_relay = !m_ignore_incoming_txs && pnode.m_tx_relay != nullptr;
+    const bool tx_relay = !m_ignore_incoming_txs && pnode.m_tx_relay != nullptr && !pnode.IsFeelerConn();
     m_connman.PushMessage(&pnode, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::VERSION, PROTOCOL_VERSION, my_services, nTime,
             your_services, addr_you, // Together the pre-version-31402 serialization of CAddress "addrYou" (without nTime)
             my_services, CService(), // Together the pre-version-31402 serialization of CAddress "addrMe" (without nTime)
diff --git a/src/rpc/net.cpp b/src/rpc/net.cpp
index 756f6860f6..eea854c375 100644
--- a/src/rpc/net.cpp
+++ b/src/rpc/net.cpp
@@ -328,7 +328,7 @@ static RPCHelpMan addconnection()
         "\nOpen an outbound connection to a specified node. This RPC is for testing only.\n",
         {
             {"address", RPCArg::Type::STR, RPCArg::Optional::NO, "The IP address and port to attempt connecting to."},
-            {"connection_type", RPCArg::Type::STR, RPCArg::Optional::NO, "Type of connection to open (\"outbound-full-relay\", \"block-relay-only\" or \"addr-fetch\")."},
+            {"connection_type", RPCArg::Type::STR, RPCArg::Optional::NO, "Type of connection to open (\"outbound-full-relay\", \"block-relay-only\", \"addr-fetch\" or \"feeler\")."},
         },
         RPCResult{
             RPCResult::Type::OBJ, "", "",
@@ -356,6 +356,8 @@ static RPCHelpMan addconnection()
         conn_type = ConnectionType::BLOCK_RELAY;
     } else if (conn_type_in == "addr-fetch") {
         conn_type = ConnectionType::ADDR_FETCH;
+    } else if (conn_type_in == "feeler") {
+        conn_type = ConnectionType::FEELER;
     } else {
         throw JSONRPCError(RPC_INVALID_PARAMETER, self.ToString());
     }