net: abstract sending side of transport serialization further

This makes the sending side of P2P transports mirror the receiver side: caller provides
message (consisting of type and payload) to be sent, and then asks what bytes must be
sent. Once the message has been fully sent, a new message can be provided.

This removes the assumption that P2P serialization of messages follows a strict structure
of header (a function of type and payload), followed by (unmodified) payload, and instead
lets transports decide the structure themselves.

It also removes the assumption that a message must always be sent at once, or that no
bytes are even sent on the wire when there is no message. This opens the door for
supporting traffic shaping mechanisms in the future.

4 files changed, 24 insertions, 12 deletions

diff --git a/src/test/fuzz/p2p_transport_serialization.cpp b/src/test/fuzz/p2p_transport_serialization.cpp
index dcf7529918..d96215e8e0 100644
--- a/src/test/fuzz/p2p_transport_serialization.cpp
+++ b/src/test/fuzz/p2p_transport_serialization.cpp
@@ -79,7 +79,16 @@ FUZZ_TARGET(p2p_transport_serialization, .init = initialize_p2p_transport_serial
 
             std::vector<unsigned char> header;
             auto msg2 = CNetMsgMaker{msg.m_recv.GetVersion()}.Make(msg.m_type, Span{msg.m_recv});
-            send_transport.prepareForTransport(msg2, header);
+            bool queued = send_transport.SetMessageToSend(msg2);
+            assert(queued);
+            std::optional<bool> known_more;
+            while (true) {
+                const auto& [to_send, more, _msg_type] = send_transport.GetBytesToSend();
+                if (known_more) assert(!to_send.empty() == *known_more);
+                if (to_send.empty()) break;
+                send_transport.MarkBytesSent(to_send.size());
+                known_more = more;
+            }
         }
     }
 }
diff --git a/src/test/fuzz/process_messages.cpp b/src/test/fuzz/process_messages.cpp
index 2617be3fa8..98962fceb5 100644
--- a/src/test/fuzz/process_messages.cpp
+++ b/src/test/fuzz/process_messages.cpp
@@ -67,7 +67,7 @@ FUZZ_TARGET(process_messages, .init = initialize_process_messages)
 
         CNode& random_node = *PickValue(fuzzed_data_provider, peers);
 
-        (void)connman.ReceiveMsgFrom(random_node, net_msg);
+        (void)connman.ReceiveMsgFrom(random_node, std::move(net_msg));
         random_node.fPauseSend = false;
 
         try {
diff --git a/src/test/util/net.cpp b/src/test/util/net.cpp
index 0031770028..c071355bc0 100644
--- a/src/test/util/net.cpp
+++ b/src/test/util/net.cpp
@@ -41,7 +41,7 @@ void ConnmanTestMsg::Handshake(CNode& node,
                 relay_txs),
     };
 
-    (void)connman.ReceiveMsgFrom(node, msg_version);
+    (void)connman.ReceiveMsgFrom(node, std::move(msg_version));
     node.fPauseSend = false;
     connman.ProcessMessagesOnce(node);
     peerman.SendMessages(&node);
@@ -54,7 +54,7 @@ void ConnmanTestMsg::Handshake(CNode& node,
     assert(statestats.their_services == remote_services);
     if (successfully_connected) {
         CSerializedNetMsg msg_verack{mm.Make(NetMsgType::VERACK)};
-        (void)connman.ReceiveMsgFrom(node, msg_verack);
+        (void)connman.ReceiveMsgFrom(node, std::move(msg_verack));
         node.fPauseSend = false;
         connman.ProcessMessagesOnce(node);
         peerman.SendMessages(&node);
@@ -70,14 +70,17 @@ void ConnmanTestMsg::NodeReceiveMsgBytes(CNode& node, Span<const uint8_t> msg_by
     }
 }
 
-bool ConnmanTestMsg::ReceiveMsgFrom(CNode& node, CSerializedNetMsg& ser_msg) const
+bool ConnmanTestMsg::ReceiveMsgFrom(CNode& node, CSerializedNetMsg&& ser_msg) const
 {
-    std::vector<uint8_t> ser_msg_header;
-    node.m_transport->prepareForTransport(ser_msg, ser_msg_header);
-
-    bool complete;
-    NodeReceiveMsgBytes(node, ser_msg_header, complete);
-    NodeReceiveMsgBytes(node, ser_msg.data, complete);
+    bool queued = node.m_transport->SetMessageToSend(ser_msg);
+    assert(queued);
+    bool complete{false};
+    while (true) {
+        const auto& [to_send, _more, _msg_type] = node.m_transport->GetBytesToSend();
+        if (to_send.empty()) break;
+        NodeReceiveMsgBytes(node, to_send, complete);
+        node.m_transport->MarkBytesSent(to_send.size());
+    }
     return complete;
 }
 
diff --git a/src/test/util/net.h b/src/test/util/net.h
index b2f6ebb163..687ce1e813 100644
--- a/src/test/util/net.h
+++ b/src/test/util/net.h
@@ -54,7 +54,7 @@ struct ConnmanTestMsg : public CConnman {
 
     void NodeReceiveMsgBytes(CNode& node, Span<const uint8_t> msg_bytes, bool& complete) const;
 
-    bool ReceiveMsgFrom(CNode& node, CSerializedNetMsg& ser_msg) const;
+    bool ReceiveMsgFrom(CNode& node, CSerializedNetMsg&& ser_msg) const;
 };
 
 constexpr ServiceFlags ALL_SERVICE_FLAGS[]{