[udp] Eliminate per-loop heap allocation by using std::span for packet callbacks

Replace std::vector<uint8_t> with std::array on the stack for the receive
buffer and std::span<const uint8_t> for packet listener callbacks. This
eliminates a 508-byte heap allocation on every loop() iteration, reducing
heap fragmentation on long-running devices.

The callback signature changes from std::vector<uint8_t>& to
std::span<const uint8_t>, which preserves API compatibility for user
lambdas using data.size(), data.data(), data[i], and range-for.

The trigger/automation type remains std::vector<uint8_t> so that deferred
actions (e.g. delay) safely own their data. The listener lambda converts
span to vector at the trigger boundary.

PacketTransport::process_() now takes std::span<const uint8_t> directly,
with implicit conversion from std::vector for other transports.

esphome/codegen.py

@@ -87,6 +87,7 @@ from esphome.cpp_types import (  # noqa: F401
     size_t,
     std_ns,
     std_shared_ptr,
+    std_span,
     std_string,
     std_string_ref,
     std_vector,

esphome/components/packet_transport/packet_transport.cpp

@@ -396,9 +396,9 @@ static bool process_rolling_code(Provider &provider, PacketDecoder &decoder) {
 /**
  * Process a received packet
  */
-void PacketTransport::process_(const std::vector<uint8_t> &data) {
+void PacketTransport::process_(std::span<const uint8_t> data) {
   auto ping_key_seen = !this->ping_pong_enable_;
-  PacketDecoder decoder((data.data()), data.size());
+  PacketDecoder decoder(data.data(), data.size());
   char namebuf[256]{};
   uint8_t byte;
   FuData rdata{};

esphome/components/packet_transport/packet_transport.h

@@ -9,8 +9,9 @@
 #include "esphome/components/binary_sensor/binary_sensor.h"
 #endif
 
-#include <vector>
 #include <map>
+#include <span>
+#include <vector>
 
 /**
  * Providing packet encoding functions for exchanging data with a remote host.
@@ -113,7 +114,7 @@ class PacketTransport : public PollingComponent {
   virtual bool should_send() { return true; }
 
   // to be called by child classes when a data packet is received.
-  void process_(const std::vector<uint8_t> &data);
+  void process_(std::span<const uint8_t> data);
   void send_data_(bool all);
   void flush_();
   void add_data_(uint8_t key, const char *id, float data);

esphome/components/udp/__init__.py

@@ -23,8 +23,12 @@ MULTI_CONF = True
 udp_ns = cg.esphome_ns.namespace("udp")
 UDPComponent = udp_ns.class_("UDPComponent", cg.Component)
 UDPWriteAction = udp_ns.class_("UDPWriteAction", automation.Action)
-trigger_args = cg.std_vector.template(cg.uint8)
 trigger_argname = "data"
+# Listener callback type (non-owning span from UDP component)
+listener_args = cg.std_span.template(cg.uint8.operator("const"))
+listener_argtype = [(listener_args, trigger_argname)]
+# Automation/trigger type (owned vector, safe for deferred actions like delay)
+trigger_args = cg.std_vector.template(cg.uint8)
 trigger_argtype = [(trigger_args, trigger_argname)]
 
 CONF_ADDRESSES = "addresses"
@@ -118,7 +122,13 @@ async def to_code(config):
             trigger_id, trigger_argtype, on_receive
         )
         trigger_lambda = await cg.process_lambda(
-            trigger.trigger(literal(trigger_argname)), trigger_argtype
+            trigger.trigger(
+                cg.std_vector.template(cg.uint8)(
+                    literal(f"{trigger_argname}.begin()"),
+                    literal(f"{trigger_argname}.end()"),
+                )
+            ),
+            listener_argtype,
         )
         cg.add(var.add_listener(trigger_lambda))
         cg.add(var.set_should_listen())

esphome/components/udp/packet_transport/udp_transport.cpp

@@ -12,7 +12,7 @@ bool UDPTransport::should_send() { return network::is_connected(); }
 void UDPTransport::setup() {
   PacketTransport::setup();
   if (!this->providers_.empty() || this->is_encrypted_()) {
-    this->parent_->add_listener([this](std::vector<uint8_t> &buf) { this->process_(buf); });
+    this->parent_->add_listener([this](std::span<const uint8_t> data) { this->process_(data); });
   }
 }
 

esphome/components/udp/udp_component.cpp

@@ -103,8 +103,8 @@ void UDPComponent::setup() {
 }
 
 void UDPComponent::loop() {
-  auto buf = std::vector<uint8_t>(MAX_PACKET_SIZE);
   if (this->should_listen_) {
+    std::array<uint8_t, MAX_PACKET_SIZE> buf;
     for (;;) {
 #if defined(USE_SOCKET_IMPL_BSD_SOCKETS) || defined(USE_SOCKET_IMPL_LWIP_SOCKETS)
       auto len = this->listen_socket_->read(buf.data(), buf.size());
@@ -116,9 +116,9 @@ void UDPComponent::loop() {
 #endif
       if (len <= 0)
         break;
-      buf.resize(len);
-      ESP_LOGV(TAG, "Received packet of length %zu", len);
-      this->packet_listeners_.call(buf);
+      size_t packet_len = static_cast<size_t>(len);
+      ESP_LOGV(TAG, "Received packet of length %zu", packet_len);
+      this->packet_listeners_.call(std::span<const uint8_t>(buf.data(), packet_len));
     }
   }
 }

esphome/components/udp/udp_component.h

@@ -10,7 +10,9 @@
 #ifdef USE_SOCKET_IMPL_LWIP_TCP
 #include <WiFiUdp.h>
 #endif
+#include <array>
 #include <initializer_list>
+#include <span>
 #include <vector>
 
 namespace esphome::udp {
@@ -26,7 +28,7 @@ class UDPComponent : public Component {
   void set_broadcast_port(uint16_t port) { this->broadcast_port_ = port; }
   void set_should_broadcast() { this->should_broadcast_ = true; }
   void set_should_listen() { this->should_listen_ = true; }
-  void add_listener(std::function<void(std::vector<uint8_t> &)> &&listener) {
+  void add_listener(std::function<void(std::span<const uint8_t>)> &&listener) {
     this->packet_listeners_.add(std::move(listener));
   }
   void setup() override;
@@ -41,7 +43,7 @@ class UDPComponent : public Component {
   uint16_t broadcast_port_{};
   bool should_broadcast_{};
   bool should_listen_{};
-  CallbackManager<void(std::vector<uint8_t> &)> packet_listeners_{};
+  CallbackManager<void(std::span<const uint8_t>)> packet_listeners_{};
 
 #if defined(USE_SOCKET_IMPL_BSD_SOCKETS) || defined(USE_SOCKET_IMPL_LWIP_SOCKETS)
   std::unique_ptr<socket::Socket> broadcast_socket_ = nullptr;

esphome/cpp_types.py

@@ -12,6 +12,7 @@ std_shared_ptr = std_ns.class_("shared_ptr")
 std_string = std_ns.class_("string")
 std_string_ref = std_ns.namespace("string &")
 std_vector = std_ns.class_("vector")
+std_span = std_ns.class_("span")
 uint8 = global_ns.namespace("uint8_t")
 uint16 = global_ns.namespace("uint16_t")
 uint32 = global_ns.namespace("uint32_t")

tests/integration/test_udp.py

@@ -93,23 +93,34 @@ async def udp_listener(port: int = 0) -> AsyncGenerator[tuple[int, UDPReceiver]]
         sock.close()
 
 
+def _get_free_udp_port() -> int:
+    """Get a free UDP port by binding to port 0 and releasing."""
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock.bind(("127.0.0.1", 0))
+    port = sock.getsockname()[1]
+    sock.close()
+    return port
+
+
 @pytest.mark.asyncio
 async def test_udp_send_receive(
     yaml_config: str,
     run_compiled: RunCompiledFunction,
     api_client_connected: APIClientConnectedFactory,
 ) -> None:
-    """Test UDP component can send messages with multiple addresses configured."""
-    # Track log lines to verify dump_config output
+    """Test UDP component can send and receive messages."""
     log_lines: list[str] = []
+    receive_event = asyncio.Event()
 
     def on_log_line(line: str) -> None:
         log_lines.append(line)
+        if "Received UDP:" in line:
+            receive_event.set()
 
-    async with udp_listener() as (udp_port, receiver):
-        # Replace placeholders in the config
-        config = yaml_config.replace("UDP_LISTEN_PORT_PLACEHOLDER", str(udp_port + 1))
-        config = config.replace("UDP_BROADCAST_PORT_PLACEHOLDER", str(udp_port))
+    async with udp_listener() as (broadcast_port, receiver):
+        listen_port = _get_free_udp_port()
+        config = yaml_config.replace("UDP_LISTEN_PORT_PLACEHOLDER", str(listen_port))
+        config = config.replace("UDP_BROADCAST_PORT_PLACEHOLDER", str(broadcast_port))
 
         async with (
             run_compiled(config, line_callback=on_log_line),
@@ -169,3 +180,19 @@ async def test_udp_send_receive(
             assert "Address: 127.0.0.2" in log_text, (
                 f"Address 127.0.0.2 not found in dump_config. Log: {log_text[-2000:]}"
             )
+
+            # Test receiving a UDP packet (exercises on_receive with std::span)
+            test_payload = b"TEST_RECEIVE_UDP"
+            send_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+            try:
+                send_sock.sendto(test_payload, ("127.0.0.1", listen_port))
+            finally:
+                send_sock.close()
+
+            try:
+                await asyncio.wait_for(receive_event.wait(), timeout=5.0)
+            except TimeoutError:
+                pytest.fail(
+                    f"on_receive did not fire. Expected 'Received UDP:' in logs. "
+                    f"Last log lines: {log_lines[-20:]}"
+                )