Zero-copy TX: span_writer packer, static buffers, no vector returns

firmware/include/dispatch.h

@@ -2,10 +2,9 @@
 #include <cstdint>
 #include <functional>
 #include <span>
-#include <vector>
 #include "wire.h"
 
-using handler_fn = std::vector<std::vector<uint8_t>> (*)(uint32_t message_id, std::span<const uint8_t> payload);
+using handler_fn = size_t (*)(uint32_t message_id, std::span<const uint8_t> payload, span_writer &out);
 
 struct handler_entry {
     int8_t type_id;
@@ -13,16 +12,16 @@ struct handler_entry {
 };
 
 template <typename Req, auto Fn>
-std::vector<std::vector<uint8_t>> typed_handler(uint32_t message_id, std::span<const uint8_t> payload) {
+size_t typed_handler(uint32_t message_id, std::span<const uint8_t> payload, span_writer &out) {
     msgpack::parser p(payload.data(), static_cast<int>(payload.size()));
     Req req;
     auto tup = req.as_tuple();
     auto r = msgpack::unpack(p, tup);
     if (!r) {
-        return {encode_response(message_id, DeviceError{1, "decode request ext_id=" +
+        return encode_response_into(out, message_id, DeviceError{1, "decode request ext_id=" +
-            std::to_string(Req::ext_id) + ": msgpack error " + std::to_string(static_cast<int>(r.error()))})};
+            std::to_string(Req::ext_id) + ": msgpack error " + std::to_string(static_cast<int>(r.error()))});
     }
-    return Fn(message_id, req);
+    return Fn(message_id, req, out);
 }
 
 void dispatch_init();

firmware/include/handlers.h

@@ -2,11 +2,10 @@
 #include <cstdint>
 #include <span>
 #include <string_view>
-#include <vector>
 #include "wire.h"
 
 extern std::string_view firmware_name;
 
-std::vector<std::vector<uint8_t>> handle_picoboot(uint32_t message_id, std::span<const uint8_t> payload);
+size_t handle_picoboot(uint32_t message_id, std::span<const uint8_t> payload, span_writer &out);
-std::vector<std::vector<uint8_t>> handle_info(uint32_t message_id, std::span<const uint8_t> payload);
+size_t handle_info(uint32_t message_id, std::span<const uint8_t> payload, span_writer &out);
-std::vector<std::vector<uint8_t>> handle_log(uint32_t message_id, std::span<const uint8_t> payload);
+size_t handle_log(uint32_t message_id, std::span<const uint8_t> payload, span_writer &out);

firmware/include/msgpack.h

@@ -6,11 +6,11 @@
 #include <expected>
 #include <iterator>
 #include <limits>
-#include <memory>
 #include <string_view>
 #include <tuple>
 #include <type_traits>
 #include <vector>
+#include "span_writer.h"
 
 namespace msgpack {
 
@@ -163,26 +163,23 @@ inline result<body_info> get_body_info(const uint8_t *p, int size) {
 }
 
 class packer {
-public:
-  using buffer = std::vector<std::uint8_t>;
-
 private:
-  std::shared_ptr<buffer> m_buffer;
+  span_writer m_buf;
 
   template <typename T> void push_big_endian(T n) {
     auto p = reinterpret_cast<std::uint8_t *>(&n) + (sizeof(T) - 1);
     for (size_t i = 0; i < sizeof(T); ++i, --p) {
-      m_buffer->push_back(*p);
+      m_buf.push_back(*p);
     }
   }
 
   template <class Range> void push(const Range &r) {
-    m_buffer->insert(m_buffer->end(), std::begin(r), std::end(r));
+    m_buf.insert(m_buf.end(), std::begin(r), std::end(r));
   }
 
 public:
-  packer() : m_buffer(std::make_shared<buffer>()) {}
+  packer(uint8_t *data, size_t capacity) : m_buf(data, capacity) {}
-  packer(const std::shared_ptr<buffer> &buf) : m_buffer(buf) {}
+  packer(span_writer buf) : m_buf(buf) {}
 
   packer(const packer &) = delete;
   packer &operator=(const packer &) = delete;
@@ -190,12 +187,12 @@ public:
   using pack_result = result<std::reference_wrapper<packer>>;
 
   pack_result pack_nil() {
-    m_buffer->push_back(format::NIL);
+    m_buf.push_back(format::NIL);
     return *this;
   }
 
   pack_result pack_bool(bool v) {
-    m_buffer->push_back(v ? format::TRUE : format::FALSE);
+    m_buf.push_back(v ? format::TRUE : format::FALSE);
     return *this;
   }
 
@@ -203,36 +200,36 @@ public:
   pack_result pack_integer(T n) {
     if constexpr (std::is_signed_v<T>) {
       if (n >= 0 && n <= 0x7F) {
-        m_buffer->push_back(static_cast<uint8_t>(n));
+        m_buf.push_back(static_cast<uint8_t>(n));
       } else if (n >= -32 && n < 0) {
-        m_buffer->push_back(static_cast<uint8_t>(n));  // negative fixint
+        m_buf.push_back(static_cast<uint8_t>(n));  // negative fixint
       } else if (n >= std::numeric_limits<int8_t>::min() && n <= std::numeric_limits<int8_t>::max()) {
-        m_buffer->push_back(format::INT8);
+        m_buf.push_back(format::INT8);
-        m_buffer->push_back(static_cast<uint8_t>(n));
+        m_buf.push_back(static_cast<uint8_t>(n));
       } else if (n >= std::numeric_limits<int16_t>::min() && n <= std::numeric_limits<int16_t>::max()) {
-        m_buffer->push_back(format::INT16);
+        m_buf.push_back(format::INT16);
         push_big_endian(static_cast<int16_t>(n));
       } else if (n >= std::numeric_limits<int32_t>::min() && n <= std::numeric_limits<int32_t>::max()) {
-        m_buffer->push_back(format::INT32);
+        m_buf.push_back(format::INT32);
         push_big_endian(static_cast<int32_t>(n));
       } else {
-        m_buffer->push_back(format::INT64);
+        m_buf.push_back(format::INT64);
         push_big_endian(static_cast<int64_t>(n));
       }
     } else {
       if (n <= 0x7F) {
-        m_buffer->push_back(static_cast<uint8_t>(n));
+        m_buf.push_back(static_cast<uint8_t>(n));
       } else if (n <= std::numeric_limits<uint8_t>::max()) {
-        m_buffer->push_back(format::UINT8);
+        m_buf.push_back(format::UINT8);
-        m_buffer->push_back(static_cast<uint8_t>(n));
+        m_buf.push_back(static_cast<uint8_t>(n));
       } else if (n <= std::numeric_limits<uint16_t>::max()) {
-        m_buffer->push_back(format::UINT16);
+        m_buf.push_back(format::UINT16);
         push_big_endian(static_cast<uint16_t>(n));
       } else if (n <= std::numeric_limits<uint32_t>::max()) {
-        m_buffer->push_back(format::UINT32);
+        m_buf.push_back(format::UINT32);
         push_big_endian(static_cast<uint32_t>(n));
       } else {
-        m_buffer->push_back(format::UINT64);
+        m_buf.push_back(format::UINT64);
         push_big_endian(static_cast<uint64_t>(n));
       }
     }
@@ -240,13 +237,13 @@ public:
   }
 
   pack_result pack_float(float n) {
-    m_buffer->push_back(format::FLOAT32);
+    m_buf.push_back(format::FLOAT32);
     push_big_endian(n);
     return *this;
   }
 
   pack_result pack_double(double n) {
-    m_buffer->push_back(format::FLOAT64);
+    m_buf.push_back(format::FLOAT64);
     push_big_endian(n);
     return *this;
   }
@@ -255,15 +252,15 @@ public:
   pack_result pack_str(const Range &r) {
     auto sz = static_cast<size_t>(std::distance(std::begin(r), std::end(r)));
     if (sz < 32) {
-      m_buffer->push_back(format::FIXSTR_MIN | static_cast<uint8_t>(sz));
+      m_buf.push_back(format::FIXSTR_MIN | static_cast<uint8_t>(sz));
     } else if (sz <= std::numeric_limits<uint8_t>::max()) {
-      m_buffer->push_back(format::STR8);
+      m_buf.push_back(format::STR8);
-      m_buffer->push_back(static_cast<uint8_t>(sz));
+      m_buf.push_back(static_cast<uint8_t>(sz));
     } else if (sz <= std::numeric_limits<uint16_t>::max()) {
-      m_buffer->push_back(format::STR16);
+      m_buf.push_back(format::STR16);
       push_big_endian(static_cast<uint16_t>(sz));
     } else if (sz <= std::numeric_limits<uint32_t>::max()) {
-      m_buffer->push_back(format::STR32);
+      m_buf.push_back(format::STR32);
       push_big_endian(static_cast<uint32_t>(sz));
     } else {
       return std::unexpected(error_code::overflow);
@@ -280,13 +277,13 @@ public:
   pack_result pack_bin(const Range &r) {
     auto sz = static_cast<size_t>(std::distance(std::begin(r), std::end(r)));
     if (sz <= std::numeric_limits<uint8_t>::max()) {
-      m_buffer->push_back(format::BIN8);
+      m_buf.push_back(format::BIN8);
-      m_buffer->push_back(static_cast<uint8_t>(sz));
+      m_buf.push_back(static_cast<uint8_t>(sz));
     } else if (sz <= std::numeric_limits<uint16_t>::max()) {
-      m_buffer->push_back(format::BIN16);
+      m_buf.push_back(format::BIN16);
       push_big_endian(static_cast<uint16_t>(sz));
     } else if (sz <= std::numeric_limits<uint32_t>::max()) {
-      m_buffer->push_back(format::BIN32);
+      m_buf.push_back(format::BIN32);
       push_big_endian(static_cast<uint32_t>(sz));
     } else {
       return std::unexpected(error_code::overflow);
@@ -297,12 +294,12 @@ public:
 
   pack_result pack_array(size_t n) {
     if (n <= 15) {
-      m_buffer->push_back(format::FIXARRAY_MIN | static_cast<uint8_t>(n));
+      m_buf.push_back(format::FIXARRAY_MIN | static_cast<uint8_t>(n));
     } else if (n <= std::numeric_limits<uint16_t>::max()) {
-      m_buffer->push_back(format::ARRAY16);
+      m_buf.push_back(format::ARRAY16);
       push_big_endian(static_cast<uint16_t>(n));
     } else if (n <= std::numeric_limits<uint32_t>::max()) {
-      m_buffer->push_back(format::ARRAY32);
+      m_buf.push_back(format::ARRAY32);
       push_big_endian(static_cast<uint32_t>(n));
     } else {
       return std::unexpected(error_code::overflow);
@@ -312,12 +309,12 @@ public:
 
   pack_result pack_map(size_t n) {
     if (n <= 15) {
-      m_buffer->push_back(format::FIXMAP_MIN | static_cast<uint8_t>(n));
+      m_buf.push_back(format::FIXMAP_MIN | static_cast<uint8_t>(n));
     } else if (n <= std::numeric_limits<uint16_t>::max()) {
-      m_buffer->push_back(format::MAP16);
+      m_buf.push_back(format::MAP16);
       push_big_endian(static_cast<uint16_t>(n));
     } else if (n <= std::numeric_limits<uint32_t>::max()) {
-      m_buffer->push_back(format::MAP32);
+      m_buf.push_back(format::MAP32);
       push_big_endian(static_cast<uint32_t>(n));
     } else {
       return std::unexpected(error_code::overflow);
@@ -330,26 +327,26 @@ public:
     auto sz = static_cast<size_t>(std::distance(std::begin(r), std::end(r)));
 
     switch (sz) {
-    case 1:  m_buffer->push_back(format::FIXEXT1);  break;
+    case 1:  m_buf.push_back(format::FIXEXT1);  break;
-    case 2:  m_buffer->push_back(format::FIXEXT2);  break;
+    case 2:  m_buf.push_back(format::FIXEXT2);  break;
-    case 4:  m_buffer->push_back(format::FIXEXT4);  break;
+    case 4:  m_buf.push_back(format::FIXEXT4);  break;
-    case 8:  m_buffer->push_back(format::FIXEXT8);  break;
+    case 8:  m_buf.push_back(format::FIXEXT8);  break;
-    case 16: m_buffer->push_back(format::FIXEXT16); break;
+    case 16: m_buf.push_back(format::FIXEXT16); break;
     default:
       if (sz <= std::numeric_limits<uint8_t>::max()) {
-        m_buffer->push_back(format::EXT8);
+        m_buf.push_back(format::EXT8);
-        m_buffer->push_back(static_cast<uint8_t>(sz));
+        m_buf.push_back(static_cast<uint8_t>(sz));
       } else if (sz <= std::numeric_limits<uint16_t>::max()) {
-        m_buffer->push_back(format::EXT16);
+        m_buf.push_back(format::EXT16);
         push_big_endian(static_cast<uint16_t>(sz));
       } else if (sz <= std::numeric_limits<uint32_t>::max()) {
-        m_buffer->push_back(format::EXT32);
+        m_buf.push_back(format::EXT32);
         push_big_endian(static_cast<uint32_t>(sz));
       } else {
         return std::unexpected(error_code::overflow);
       }
     }
-    m_buffer->push_back(static_cast<uint8_t>(type));
+    m_buf.push_back(static_cast<uint8_t>(type));
     push(r);
     return *this;
   }
@@ -392,7 +389,8 @@ public:
   template <typename T>
     requires requires(const T &v) { { T::ext_id } -> std::convertible_to<int8_t>; v.as_tuple(); }
   pack_result pack(const T &v) {
-    packer inner;
+    uint8_t ext_buf[256];
+    packer inner(ext_buf, sizeof(ext_buf));
     auto r = inner.pack(v.as_tuple());
     if (!r) return r;
     return pack_ext(T::ext_id, inner.get_payload());
@@ -413,7 +411,7 @@ private:
   }
 
 public:
-  const buffer &get_payload() const { return *m_buffer; }
+  const span_writer &get_payload() const { return m_buf; }
 };
 
 class parser {

firmware/include/net.h

@@ -3,14 +3,14 @@
 #include <cstdint>
 #include <functional>
 #include <span>
-#include <vector>
+#include "span_writer.h"
 
 struct net_state {
     std::array<uint8_t, 6> mac;
     std::array<uint8_t, 4> ip;
 };
 
-using net_handler = std::function<std::vector<std::vector<uint8_t>>(std::span<const uint8_t> payload)>;
+using net_handler = std::function<size_t(std::span<const uint8_t> payload, span_writer &out)>;
 
 bool net_init();
 const net_state& net_get_state();

firmware/include/span_writer.h

@@ -0,0 +1,35 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+class span_writer {
+    uint8_t *m_data;
+    size_t m_capacity;
+    size_t m_size = 0;
+
+public:
+    span_writer(uint8_t *data, size_t capacity) : m_data(data), m_capacity(capacity) {}
+
+    void push_back(uint8_t v) {
+        if (m_size < m_capacity) m_data[m_size++] = v;
+    }
+
+    template <class It>
+    void insert(uint8_t *, It first, It last) {
+        while (first != last && m_size < m_capacity)
+            m_data[m_size++] = *first++;
+    }
+
+    size_t size() const { return m_size; }
+    size_t capacity() const { return m_capacity; }
+    bool full() const { return m_size >= m_capacity; }
+
+    uint8_t *data() { return m_data; }
+    const uint8_t *data() const { return m_data; }
+
+    uint8_t *begin() { return m_data; }
+    uint8_t *end() { return m_data + m_size; }
+    const uint8_t *begin() const { return m_data; }
+    const uint8_t *end() const { return m_data + m_size; }
+};

firmware/include/wire.h

@@ -1,6 +1,7 @@
 #pragma once
 #include <array>
 #include <cstdint>
+#include <span>
 #include <string>
 #include <tuple>
 #include <vector>
@@ -96,18 +97,27 @@ struct DecodedMessage {
     std::vector<uint8_t> payload;
 };
 
-inline std::vector<uint8_t> pack_envelope(uint32_t message_id, const std::vector<uint8_t> &payload) {
+inline size_t pack_envelope_into(span_writer &out, uint32_t message_id, const uint8_t *payload, size_t payload_len) {
-    uint32_t checksum = halfsiphash::hash32(payload.data(), payload.size(), hash_key);
+    uint32_t checksum = halfsiphash::hash32(payload, payload_len, hash_key);
-    msgpack::packer p;
+    uint8_t env_buf[512];
-    p.pack(Envelope{message_id, checksum, payload});
+    span_writer env_body(env_buf, sizeof(env_buf));
-    return p.get_payload();
+    msgpack::packer env_p(env_body);
+    env_p.pack_array(3);
+    env_p.pack(message_id);
+    env_p.pack(checksum);
+    env_p.pack_bin(std::span<const uint8_t>{payload, payload_len});
+    msgpack::packer outer(out);
+    outer.pack_ext(Envelope::ext_id, env_body);
+    return out.size();
 }
 
 template <typename T>
-inline std::vector<uint8_t> encode_response(uint32_t message_id, const T &msg) {
+inline size_t encode_response_into(span_writer &out, uint32_t message_id, const T &msg) {
-    msgpack::packer inner;
+    uint8_t inner_buf[256];
+    msgpack::packer inner(inner_buf, sizeof(inner_buf));
     inner.pack(msg);
-    return pack_envelope(message_id, inner.get_payload());
+    auto &pl = inner.get_payload();
+    return pack_envelope_into(out, message_id, pl.data(), pl.size());
 }
 
 inline msgpack::result<DecodedMessage> try_decode(const uint8_t *data, size_t len) {
@@ -154,8 +164,6 @@ inline msgpack::result<T> decode_response(const uint8_t *data, size_t len) {
     return out;
 }
 
-inline std::vector<uint8_t> encode_request(uint32_t message_id, const auto &msg) {
+inline size_t encode_request_into(span_writer &out, uint32_t message_id, const auto &msg) {
-    msgpack::packer inner;
+    return encode_response_into(out, message_id, msg);
-    inner.pack(msg);
-    return pack_envelope(message_id, inner.get_payload());
 }

firmware/lib/dispatch.cpp

@@ -22,20 +22,21 @@ void dispatch_schedule_ms(uint32_t ms, std::function<void()> fn) {
 }
 
 [[noreturn]] void dispatch_run(std::span<const handler_entry> handlers) {
-    std::unordered_map<int8_t, std::vector<std::vector<uint8_t>> (*)(uint32_t, std::span<const uint8_t>)> handler_map;
+    std::unordered_map<int8_t, handler_fn> handler_map;
     for (auto& entry : handlers) {
         handler_map[entry.type_id] = entry.handle;
     }
 
     static usb_cdc usb;
     static static_vector<uint8_t, 256> usb_rx_buf;
+    static uint8_t tx_buf[1514];
 
-    net_set_handler([&](std::span<const uint8_t> payload) -> std::vector<std::vector<uint8_t>> {
+    net_set_handler([&](std::span<const uint8_t> payload, span_writer &out) -> size_t {
         auto msg = try_decode(payload.data(), payload.size());
-        if (!msg) return {};
+        if (!msg) return 0;
         auto it = handler_map.find(msg->type_id);
-        if (it == handler_map.end()) return {};
+        if (it == handler_map.end()) return 0;
-        return it->second(msg->message_id, msg->payload);
+        return it->second(msg->message_id, msg->payload, out);
     });
 
     while (true) {
@@ -62,13 +63,16 @@ void dispatch_schedule_ms(uint32_t ms, std::function<void()> fn) {
 
             auto it = handler_map.find(msg->type_id);
             if (it != handler_map.end()) {
-                for (auto& response : it->second(msg->message_id, msg->payload)) {
+                span_writer out(tx_buf, sizeof(tx_buf));
-                    if (response.size() > usb.tx.free()) {
+                size_t resp_len = it->second(msg->message_id, msg->payload, out);
-                        auto err = encode_response(msg->message_id,
+                if (resp_len > 0) {
-                            DeviceError{2, "response too large: " + std::to_string(response.size())});
+                    if (resp_len > usb.tx.free()) {
-                        usb.send(err);
+                        span_writer err_out(tx_buf, sizeof(tx_buf));
+                        size_t err_len = encode_response_into(err_out, msg->message_id,
+                            DeviceError{2, "response too large: " + std::to_string(resp_len)});
+                        usb.send(std::span<const uint8_t>{tx_buf, err_len});
                     } else {
-                        usb.send(response);
+                        usb.send(std::span<const uint8_t>{tx_buf, resp_len});
                     }
                 }
             }

firmware/lib/handlers.cpp

@@ -5,12 +5,12 @@
 #include "net.h"
 #include "debug_log.h"
 
-std::vector<std::vector<uint8_t>> handle_picoboot(uint32_t message_id, std::span<const uint8_t>) {
+size_t handle_picoboot(uint32_t message_id, std::span<const uint8_t>, span_writer &out) {
     dispatch_schedule_ms(100, []{ reset_usb_boot(0, 1); });
-    return {encode_response(message_id, ResponsePICOBOOT{})};
+    return encode_response_into(out, message_id, ResponsePICOBOOT{});
 }
 
-std::vector<std::vector<uint8_t>> handle_info(uint32_t message_id, std::span<const uint8_t>) {
+size_t handle_info(uint32_t message_id, std::span<const uint8_t>, span_writer &out) {
     ResponseInfo resp;
     pico_unique_board_id_t uid;
     pico_get_unique_board_id(&uid);
@@ -19,12 +19,12 @@ std::vector<std::vector<uint8_t>> handle_info(uint32_t message_id, std::span<con
     resp.mac = ns.mac;
     resp.ip = ns.ip;
     resp.firmware_name = firmware_name;
-    return {encode_response(message_id, resp)};
+    return encode_response_into(out, message_id, resp);
 }
 
-std::vector<std::vector<uint8_t>> handle_log(uint32_t message_id, std::span<const uint8_t>) {
+size_t handle_log(uint32_t message_id, std::span<const uint8_t>, span_writer &out) {
     ResponseLog resp;
     for (auto& e : dlog_drain())
         resp.entries.push_back(LogEntry{e.timestamp_us, std::move(e.message)});
-    return {encode_response(message_id, resp)};
+    return encode_response_into(out, message_id, resp);
 }

firmware/lib/net.cpp

@@ -139,6 +139,8 @@ static void handle_arp(const uint8_t* frame, size_t len) {
     send_raw(&reply, sizeof(reply));
 }
 
+static uint8_t tx_buf[1514];
+
 static void handle_udp(const uint8_t* frame, size_t len) {
     if (len < sizeof(udp_header)) return;
     auto& pkt = *reinterpret_cast<const udp_header*>(frame);
@@ -154,16 +156,14 @@ static void handle_udp(const uint8_t* frame, size_t len) {
     auto* payload = frame + sizeof(udp_header);
     size_t payload_len = udp_len - 8;
 
-    auto responses = msg_handler(std::span<const uint8_t>{payload, payload_len});
+    span_writer resp(tx_buf + sizeof(udp_header), sizeof(tx_buf) - sizeof(udp_header));
+    size_t resp_len = msg_handler(std::span<const uint8_t>{payload, payload_len}, resp);
+    if (resp_len == 0) return;
 
-    for (auto& resp : responses) {
+    size_t ip_total = 20 + 8 + resp_len;
-        uint8_t reply_buf[1514];
-        size_t udp_data_len = resp.size();
-        size_t ip_total = 20 + 8 + udp_data_len;
     size_t reply_len = sizeof(eth_header) + ip_total;
-        if (reply_len > sizeof(reply_buf)) continue;
 
-        auto& rip = *reinterpret_cast<ipv4_header*>(reply_buf);
+    auto& rip = *reinterpret_cast<ipv4_header*>(tx_buf);
     rip.eth.dst = pkt.ip.eth.src;
     rip.eth.src = state.mac;
     rip.eth.ethertype = ETH_IPV4;
@@ -179,16 +179,13 @@ static void handle_udp(const uint8_t* frame, size_t len) {
     rip.dst = pkt.ip.src;
     rip.checksum = ip_checksum(rip.ip_start(), 20);
 
-        auto& rudp = *reinterpret_cast<udp_header*>(reply_buf);
+    auto& rudp = *reinterpret_cast<udp_header*>(tx_buf);
     rudp.src_port = PICOMAP_PORT;
     rudp.dst_port = pkt.src_port;
-        rudp.length = __builtin_bswap16(8 + udp_data_len);
+    rudp.length = __builtin_bswap16(8 + resp_len);
     rudp.checksum = 0;
 
-        memcpy(reply_buf + sizeof(udp_header), resp.data(), udp_data_len);
+    send_raw(tx_buf, reply_len);
-
-        send_raw(reply_buf, reply_len);
-    }
 }
 
 static void handle_icmp(const uint8_t* frame, size_t len) {

firmware/test.cpp

@@ -29,8 +29,10 @@ static ResponseTest test_discovery() {
     ResponseTest resp;
     resp.pass = true;
 
-    auto req = encode_request(0, RequestInfo{});
+    uint8_t req_buf[256];
-    auto send_result = w6300::send(test_socket, std::span<const uint8_t>{req});
+    span_writer req_out(req_buf, sizeof(req_buf));
+    size_t req_len = encode_request_into(req_out, 0, RequestInfo{});
+    auto send_result = w6300::send(test_socket, std::span<const uint8_t>{req_buf, req_len});
     if (!send_result) {
         resp.pass = false;
         resp.messages.push_back("send: error " + std::to_string(static_cast<int>(send_result.error())));
@@ -95,13 +97,11 @@ static const std::unordered_map<std::string_view, test_fn> tests = {
     {"discovery", test_discovery},
 };
 
-static std::vector<std::vector<uint8_t>> handle_test(uint32_t message_id, const RequestTest& req) {
+static size_t handle_test(uint32_t message_id, const RequestTest& req, span_writer &out) {
     auto it = tests.find(req.name);
-    if (it == tests.end()) {
+    if (it == tests.end())
-        return {encode_response(message_id, ResponseTest{false, {"unknown test: " + req.name}})};
+        return encode_response_into(out, message_id, ResponseTest{false, {"unknown test: " + req.name}});
-    }
+    return encode_response_into(out, message_id, it->second());
-
-    return {encode_response(message_id, it->second())};
 }
 
 static constexpr handler_entry handlers[] = {